                 GUIDANCE DOCUMENT
            ON ACHIEVEMENT DETERMINATION
          FOR CANADIAN AMBIENT AIR QUALITY
           STANDARDS FOR NITROGEN DIOXIDE




                                     PN 1608
                            ISBN 978-1-77202-061-8 PDF




© Canadian Council of Ministers of the Environment, 2020
                                           TABLE OF CONTENTS
BACKGROUND ON THE AIR QUALITY MANAGEMENT SYSTEM ......................... 1
1.0      INTRODUCTION ................................................................................................. 2
2.0     CANADIAN AMBIENT AIR QUALITY STANDARDS FOR NITROGEN
        DIOXIDE .............................................................................................................. 2
      2.1 Basis for the Statistical Form of the Standards ..................................................... 5
3.0      NITROGEN DIOXIDE MANAGEMENT LEVELS ................................................ 6
4.0     MONITORS AND REPORTING STATIONS ........................................................ 7
      4.1 Requirements for Monitors .................................................................................. 7
      4.2 CAAQS Reporting Stations ................................................................................. 8
5.0     CALCULATION OF THE NO 2 METRIC VALUES .............................................. 11
      5.1 Calculation of 1-hour Metric Values ................................................................... 11
      5.2 Calculation of Annual Metric Values .................................................................. 13
      5.3 Data Completeness Criteria and Exceptions ........................................................ 13
      5.4 Decimal Places and Rounding Rules ................................................................... 15
6.0      TRANSBOUNDARY FLOWS AND EXCEPTIONAL EVENTS ........................... 17
7.0      COMMUNICATION AND REPORTING ............................................................. 17
8.0      SUMMARY OF GUIDANCE ............................................................................... 18
9.0      REFERENCES ..................................................................................................... 19
APPENDIX A – EXAMPLE CALCULATION OF THE 1-HOUR CANADIAN
    AMBIANT AIR QUALITY STANDARDS METRIC VALUE ............................... 20
APPENDIX B – THE PERCENTILE RANKING APPROACH ...................................... 23
                                                 TABLES

Table 2-1: The CAAQS for nitrogen dioxide .................................................................. 3
Table 3-1: Management levels for nitrogen dioxide ........................................................ 7
Table 5-1: Example of eighth highest NO 2 Dmax 1-hour in decreasing order ................. 12
Table 5-2: The 98 th percentile as a function of available data ......................................... 12
Table 5-3: Data completeness and exceptions criteria .................................................... 14
Table 5-4: Decimal place and rounding rules ................................................................. 16


                                                FIGURES

Figure 4-1: The NAPS NO 2 monitoring stations for 2018 .............................................. 10
Figure 8-1: Outline for determining the achievement status of a NO 2 CAAQS ............... 18


                                            TEXT BOXES

Text Box 1: Example for calculating the NO 2 1-hour CAAQS metric value .................... 5
Text Box 2: Example for evaluating the potential of an exceedance ............................... 15
Text Box 3: Example for rounding the 1-hour CAAQS metric value to a whole number . 17
BACKGROUND ON THE AIR QUALITY MANAGEMENT SYSTEM

Air quality is important for all Canadians and it affects many aspects of our lives and our
society, including human health, the natural environment, buildings and infrastructure,
crop production, and the economy. In Canada, air quality management is a responsibility
shared between federal, provincial, and territorial governments. Under the Canadian
Council of Ministers of the Environment (CCME), federal, provincial and territorial
governments are working collaboratively to improve air quality by implementing the Air
Quality Management System (AQMS) 1. Key elements of AQMS include:

    1. Air zones – geographical areas that are used to manage local air quality within the
       provinces and territories in which they are located.
    2. Airsheds – broad geographic areas that encompass a number of air zones and may
       cross provincial, territorial, and international boundaries. They provide a framework
       for inter-jurisdictional collaboration to address transboundary air quality issues.
    3. Canadian Ambient Air Quality Standards (CAAQS) – health and environmental-
       based air quality objectives to further protect human health and the environment and
       to provide the drivers for air quality improvement across the country.
    4. Air Zone Management Framework – a framework to manage air quality in air zones.
    5. Base-level industrial emissions requirements (BLIERs) – emission requirements
       that are intended to apply to major industrial sectors or equipment types to ensure
       that significant industrial sources achieve a good base-level of performance.
    6. Mobile Sources – work that builds on the existing range of federal, provincial and
       territorial initiatives aimed at reducing emissions from mobile sources.

In addition to being endorsed by CCME, the CAAQS have also been established as ambient
air quality objectives by the federal government under the Canadian Environmental
Protection Act, 1999.

This document provides information on the CAAQS and management levels for nitrogen
dioxide (NO 2 ) and information on the procedures, methodologies and criteria for
determining whether the CAAQS for NO 2 are achieved or exceeded at monitoring stations
and within air zones.




1
  Although Québec supports the general objectives of the AQMS, the province will not implement the System since the
System calls for federal industrial emission requirements that duplicate Québec regulations. However, Québec is
collaborating with jurisdictions on developing other elements of the system, notably air zones and airsheds.



                                                                                                                 1
1.0    INTRODUCTION

Under the Air Quality Management System (AQMS), provinces and territories have been
delineated into one or more air zones by the respective jurisdiction. Air zones provide a
defined area within which stakeholders, other interested parties and governments work
together to improve local air quality and maintain air pollutant concentrations below the
Canadian Ambient Air Quality Standards (CAAQS).

As part of AQMS, provinces and territories have agreed to regularly publish air zone
reports for each of their air zones in a timely fashion. These reports are key to the integrity
of the AQMS and will include information on the achievement status of the CAAQS and
their management levels. Achievement status means whether ambient concentrations of air
pollutants are less than or equal to the corresponding standard (CAAQS is achieved) or
greater than the standard (CAAQS is exceeded). To ensure that reporting on CAAQS
achievement status is comparable among provinces and territories, guidance is needed on
the monitoring, procedures and methodologies to use. This document outlines this guidance
for nitrogen dioxide (NO 2 ). Its main purpose is to:
    • present the CAAQS and management levels for NO 2
    • provide guidance on the NO 2 monitors and monitoring stations to use for reporting
        on the achievement status of the NO 2 CAAQS
    • provide the procedures for calculating the concentrations to use for direct
        comparison to the NO 2 standards (called metric values)
    • provide the procedures for determining whether an NO 2 CAAQS is achieved or
        exceeded at monitoring stations and in air zones.

More information on AQMS and guidance on its implementation is available on ccme.ca.


2.0 CANADIAN AMBIENT                      AIR      QUALITY         STANDARDS            FOR
   NITROGEN DIOXIDE

CAAQS are health and environmental-based air quality objectives to further protect human
health and the environment and to provide the drivers for air quality improvement across
Canada. All CAAQS consist of three inter-related elements:

   1. an averaging time
   2. a concentration “standard” (or “numerical value”) associated with the averaging
      time
   3. the statistical form for the standard.

CCME has established CAAQS for NO 2 for 2020 and 2025, listed in Table 2-1. The federal
government established these CAAQS as ambient air quality objectives on December 9,



                                                                                              2
2017 pursuant to sections 54 and 55 of the Canadian Environmental Protection Act, 1999 2.
The 2020 CAAQS came into effect on December 10, 2017 and will remain in effect until
December 31, 2024. The 2025 CAAQS will become effective on January 1, 2025. The years
2020 and 2025 represent the years by which the associated standard should be achieved.
The intended use of all CAAQS is discussed in the CCME Guidance Document on Air Zone
Management (CCME 2019a).

Table 2-1: The CAAQS for nitrogen dioxide
                                  Standard
     Averaging                 (numerical value)                               Statistical form
       time 3                                                                  of the standard
                            2020                 2025

                                                                 The 3-year average of the annual 98 th
       1-hour             60 ppb*               42 ppb           percentile of the NO 2 daily-maximum 1-hour
                                                                 average concentrations.

                                                                 The arithmetic average over a single
       1-year
                         17.0 ppb              12.0 ppb          calendar year of all NO 2 1-hour average
      (annual)
                                                                 concentrations in the year.
* ppb-parts per billion by volume


As can be seen in Table 2-1, CAAQS for NO 2 have been established for 1-hour and 1-year
(or annual) averaging times. This takes into consideration that effects on health and the
environment can occur over both short (1-hour) and long-term (1-year) exposure times and
concentrations.

For ease of discussion, an NO 2 1-hour average concentration is denoted by “NO 2 1-hour”
and the daily maximum of the NO 2 1-hour as the “NO 2 Dmax 1-hour”.

The averaging times in Table 2-1 refer to the averaging period over which the
corresponding standard applies. The statistical form describes the calculation method for
the specific concentration that must be used for comparison to the standard to determine
whether the concentrations measured at a monitoring station exceed the standard. The 1-
hour NO 2 standard of 60 ppb (for 2020) applies to NO 2 1-hour. The statistical form of the
standard means that the concentration to use to determine whether 60 ppb was exceeded at
a monitoring station is the 3-year average of the annual 98 th percentile of the NO 2 Dmax
1-hour measured at the station. In a year with complete data (see section 5.3), the 98 th
percentile is the value of the eighth highest NO 2 Dmax 1-hour. The annual NO 2 standard
applies to the average of all NO 2 1-hour measured at a station over a single calendar year.


2
 Canada Gazette Part 1, Volume 151, December 9, 2017. http://www.gazette.gc.ca/rp-pr/p1/2017/2017-12-09/pdf/g1-
15149.pdf.
3
    This is referred to as “averaging period” in some jurisdictions.

                                                                                                               3
To simplify terminology, the concentrations measured at a monitoring station calculated in
the statistical form of a standard are referred to as “CAAQS metric values” or simply
“metric values” 4. An NO 2 CAAQS is achieved at a monitoring station if the corresponding
metric value is less than or equal to the standard; otherwise the standard is exceeded. An
NO 2 CAAQS is achieved in an air zone if the highest metric value in the air zone is less
than or equal to the standard; otherwise the standard is exceeded. In other words, an NO 2
CAAQS is achieved in an air zone if the metric values at all NO 2 CAAQS reporting stations
in the air zone are less than or equal to the standard.

For clarity, for the 1-hour standard a 3-year average is to be computed backward in time.
As such, the first formal achievement determination of the 1-hour standard for 2020 will
be based on metric values for the 3-year period from 2018 to 2020. For the 2025 standard
it will be based on metric values for the period 2023 to 2025. For the annual standard, the
first formal achievement determination will be based on the NO 2 1-hour measured in 2020
for the 2020 standard and measured in 2025 for the 2025 standard.

Text Box 1 provides a simplified example for the calculation of the 1-hour NO 2 CAAQS
metric values in an air zone with two monitoring stations. Since the 3-year period for this
example is 2018-20, the 2020 standard of 60 ppb applies. For this example, the NO 2 1-hour
standard is achieved at Station A (since the metric value of 44 ppb is less than the standard
of 60 ppb) and exceeded at Station B. Since the highest metric value in the air zone exceeds
the standard, the air zone does not achieve the 1-hour standard.

To ensure that reporting on CAAQS achievement is comparable across provinces and
territories, there are specific procedures to use for obtaining the 98 th percentiles, the metric
values and for number rounding. These are addressed in section 5.




4
 It should be noted that the annual maximum 1-hour average concentration and a single annual 98 th percentile of the
daily-maximum 1-hour average concentrations cannot be used to determine whether the 1-hour standard is achieved or
exceeded because neither of these correspond to the statistical form of the standard.

                                                                                                                  4
Text Box 1: Example for calculating the NO 2 1-hour CAAQS metric value
                                     Annual 98 th percentile of the NO 2 Dmax 1-hour

                                    2018                        2019                         2020

          Station A              39.4 ppb                     55.6 ppb                     38.0 ppb

          Station B              92.7 ppb                     85.6 ppb                     70.5 ppb



                          3-year average of the annual 98 th                   1-hour NO 2 CAAQS metric
                                     percentile                                   value for 2018-2020

    Station A         (39.4+55.6+38.0)÷3 = 133÷3 = 44.333 ppb                               44 ppb

    Station B        (92.7+85.6+70.5)÷3 = 248.8÷3 = 82.933 ppb                              83 ppb




2.1 Basis for the Statistical Form of the Standards

CAAQS are established to further protect the health of Canadians and their environment.
They are used to guide the air quality management actions to implement as part of the Air
Zone Management Framework (AZMF section 3). If ambient concentrations of air
pollutants exceed their corresponding standard, the AZMF calls for the implementation of
the most rigorous actions.

Exceedances of a standard, and generally variations in ambient concentrations from one
year to the next, are influenced not only by changes in the quantity of emissions of air
pollutants but also by variations in the prevailing meteorological conditions (see for
example, Turner 1961) 5. This implies that exceedances of a standard can at times be
influenced by the occurrence of meteorological conditions that are conducive to elevated
ambient concentrations of air pollutants even though emissions do not increase
substantially. As such, an air zone can shift in and out of achievement of a standard because
of variations in meteorological conditions rather than because of sustained changes in
emissions. To reduce this risk, the statistical form of a standard is established considering
not only the need to capture the associated health and environmental effects, but also the
need that it not be overly influenced by variations in meteorological conditions.

For the NO 2 1-hour CAAQS, studies indicate that the risk of adverse health impacts appears
linear throughout the range of concentrations experienced in Canada, with no clear signs

5
  For example, the average NO 2 concentrations measured at a monitor may be higher in a year where the wind transports
the plume from a nearby source more frequently towards the monitor and lower in a year where the plume is more
frequently transported away from the monitor.


                                                                                                                    5
of a threshold for such effects. While studies using various short-term averages of NO 2
(from 1 to 24 hour) demonstrate effects of similar public-health consequence, the majority
of studies utilize 1-hour averages and thus the greatest understanding of effects is
associated with this 1-hour timeframe. As the effects have been shown to increase almost
linearly with concentration throughout the range examined, the greatest risks are associated
with the annual highest 1-hour average concentration.

However, data analyses indicate that the annual highest varies more from year to year than
the annual 98 th percentile, likely because the annual highest is more sensitive to
meteorological conditions 6. In a year with complete data the 98 th percentile is the eight (8th )
highest NO 2 Dmax 1-hour and, as such, it is representative of the higher NO 2
concentrations. Therefore, given that the 98 th percentile varies less, and given that it is also
representative of the higher NO 2 concentrations, a 98 th percentile form was adopted for the
1-hour CAAQS. In addition, a 98th percentile form also aligns with the NO 2 standard of the
United States, which is a desirable feature since it allows direct comparison of NO 2 air
quality between the two countries. The final form as a 3-year average of the 98 th percentile
was selected as a mean to further reduce the variability between years.

For the annual NO 2 CAAQS, all NO 2 1-hour measured in a year (up to 8760 or 8784 in a
leap year) are considered. Data analyses indicate that annual averages based on all NO 2 1-
hour are typically not overtly influenced by meteorological conditions. As such, a single
year average of all 1-hour NO 2 measured in the year was deemed appropriate for the annual
CAAQS.


3.0      NITROGEN DIOXIDE MANAGEMENT LEVELS

AQMS includes an AZMF, which provides guidance to jurisdictions on the level of
monitoring, reporting and management actions to implement in air zones depending on the
prevailing concentrations of air pollutants. The framework includes four air quality
management categories, or levels, denoted by the colours green, yellow, orange, and red.
Each of these management levels is associated with a corresponding range of
concentrations of air pollutants that have been established concurrently and under the same
process as the corresponding CAAQS. The current NO 2 management levels are presented
in Table 3-1.

The concentrations in Table 3-1 have the same statistical form as the corresponding
CAAQS. Accordingly, the NO 2 CAAQS metric values discussed in section 2 are also used

6
  Substantial changes in emissions between years may also cause variability in the annual highest. However,
meteorological conditions likely have a more variable influence. For example, meteorological conditions may cause the
plume from a nearby source to directly impinge a monitor in one year (causing elevated 1-hour concentrations), and
never directly impinge the monitor in another year (resulting in lower concentrations).

                                                                                                                   6
for comparison to the management levels to determine the management level into which
the air zone falls. The procedures that provinces and territories use to assign management
levels to each of their air zones are discussed in CCME 2019a.

Table 3-1: Management levels for nitrogen dioxide

                               NO 2 1-hour                        NO 2 annual
      Management
         level
                        2020                 2025         2020                  2025


   Red                > 60 ppb          > 42 ppb       > 17.0 ppb          > 12.0 ppb


   Orange           32 to 60 ppb      32 to 42 ppb   7.1 to 17.0 ppb     7.1 to 12.0 ppb


   Yellow                      21 to 31 ppb                      2.1 to 7.0 ppb


   Green                         ≤ 20 ppb                          ≤ 2.0 ppb




4.0      MONITORS AND REPORTING STATIONS

This section provides guidance on the NO 2 monitors to use for reporting on the
achievement status of the NO 2 CAAQS. It also provides guidance on the location of
monitoring stations for reporting on the achievement status (CAAQS reporting stations).


4.1 Requirements for Monitors

The monitors for reporting on the achievement status of the NO 2 CAAQS should:

   1. measure NO 2 concentrations on an hourly basis
   2. be designated as either Federal Reference Method (FRM) or Federal Equivalent
      Method (FEM) by the United States Environmental Protection Agency (U.S. EPA
      2016)
   3. be subject to data validation procedures that meet (or exceed) the Ambient Air
      Monitoring and Quality Assurance and Quality Control Guidelines: NAPS Program
      (CCME 2019c).




                                                                                           7
The NAPS Program is a collaborative air quality monitoring network jointly operated and
maintained by the provinces and territories and Environment and Climate Change Canada.
Metro Vancouver and Ville de Montréal also contribute to the NAPS Program.


4.2 CAAQS Reporting Stations

Provinces and territories are responsible for designating the NO 2 CAAQS reporting
stations. Ideally, all CAAQS reporting stations should be planned to be operational for the
long-term.

Results of analyses of ambient NO 2 concentrations measured in urban and rural areas of
Canada indicate that concentrations are higher at monitoring stations located in urban
centers. The analyses indicate that concentrations are highest in urban centers with
population of 500,000 or more and lowest in communities with population less than 30,000.
Some of the overall highest concentrations were recorded near major urban roads located
in communities with population of 500,000 and more, where traffic emissions are an
important sources of nitrogen oxides (NO X, consisting mostly of nitric oxide and NO 2 ) 7.

Some high NO 2 concentrations were also measured at stations close to major point sources
of NO X . However, since many of these stations were also located near major roadways,
traffic emissions may also have influenced these concentrations. Generally, the relative
contributions from roadway traffic, other transportation sources (e.g. rail), and various
industrial sources will vary from station to station depending on the spatial distribution of
sources near the station.

In studies from Health Canada, it is discussed that NO 2 concentrations at all station types
(e.g. residential and traffic) exhibited a common pattern by season, with wintertime
maxima and summertime minima. The summertime minima is consistent with increased
mixing heights 8 and photochemical oxidation of NO 2 and decreased emissions from
residential heating compared with winter (Health Canada 2016).

Since NO 2 concentrations are higher in urban centers, from a population health perspective
urban centers of Canada should have one or more NO 2 CAAQS reporting stations
preferably located in an area that represents the community-wide exposure. Within urban
centers, monitors should be located close to major roadways where people live or work as
it is estimated that approximately one third of Canadians (approximately 10 million people)
live or work within 500 meters of highways or 100 meters from major urban roads (Brauer

7
  Close to 90% of the NO X released from combustion sources is nitric oxide (NO). Once in the air, NO rapidly converts
to NO 2 by reacting with other substances present in the air.
8
  The mixing height can be considered as being the height above ground up to which pollutants can disperse. The higher
the mixing height, the lower the concentrations of air pollutants can be.

                                                                                                                    8
et al. 2013). Monitoring stations could also be located in communities having large sources
of NO 2 .

From an environmental perspective, stations could also be located in air pollution-sensitive
ecosystems as priorities and resources allow. Sensitive ecosystems encompass national and
provincial parks, protected areas, areas of cultural or heritage value and areas that are or
may be susceptible to adverse effects from direct exposure to NO 2 , acid deposition or
eutrophication.

Provincial and territorial jurisdictions may determine that ambient monitoring for NO 2 is
not required in an air zone, but ideally, each air zone should have at least one NO 2 CAAQS-
reporting station, preferably located in the community with the largest population. In 2018,
NO 2 was monitored on an hourly basis at 192 NAPS stations (Figure 4-1), most of which
are located in populated centers and some of which are close to major urban roads. As a
start, the NAPS NO 2 stations should be used as NO 2 CAAQS-reporting stations 9.




9
 The NO 2 monitors at NAPS stations are all designated as either FRM or FEM by the EPA and they all meet the
Ambient Air Monitoring and Quality Assurance and Quality Control Guidelines: NAPS Program (CCME 2019c).

                                                                                                          9
Figure 4-1: The NAPS NO 2 monitoring stations for 2018




                                                                                              192 NAPS stations




To augment spatial coverage, provinces and territories could also use additional stations
that they deem appropriate, as long as the NO 2 monitors satisfy the requirements discussed
in section 4.1. These additional stations can include, for example:

     1. provincial and territorial non-NAPS stations
     2. stations owned by air zone organizations 10
     3. stations owned by third parties.

Provinces and territories are encouraged to use any additional stations wherever a major
source of NO 2 is near a populated area or a sensitive ecosystem. Stations located at or near
the property (fence) line of an industrial facility should not be used for NO 2 CAAQS
reporting unless the station is near a populated area or a sensitive ecosystem. Under NAPS,
a fence-line monitoring station is defined as: a station that is located within or on the

10
  These are not-for profit organizations with a multi-stakeholder membership and are established by some provinces
and territories to address air quality within the air zone. Some organizations operate their own monitoring stations.

                                                                                                                  10
property line of a facility or a station that is very near to a facility and in areas not used or
accessed by the public or with no nearby population of appreciable size. The determination
of what is considered “near” can be evaluated on a case-by-case basis by the reporting
jurisdiction. For example, what is considered near for emissions released at ground level
may be different than for emissions released from a tall stack.


5.0    CALCULATION OF THE NITROGEN DIOXIDE METRIC VALUES

This section provides guidance on the procedures for calculating the NO 2 CAAQS metric
values, the data completeness criteria and the number of decimal places that measured
concentrations and calculated values have to be reported. Appendix A provides an example
for the calculation of the 1-hour metric values. As mentioned in section 2, an NO 2 1-hour
average concentration is denoted by “NO 2 1-hour” and the daily maximum of the NO 2 1-
hour as the “NO 2 Dmax 1-hour”.

NO 2 CAAQS metric values can be calculated for any NO 2 monitoring station. However,
for reporting on NO 2 CAAQS achievement status and management level only stations
designated by provinces and territories as NO 2 CAAQS reporting stations should be used.


5.1 Calculation of 1-hour Metric Values

The 1-hour NO 2 CAAQS metric value at a monitoring station is the average of three
annual 98th percentile values over three consecutive years and is calculated as per equation
5.1.

        1-hour metric value Y1-Y3 = (98P Y1 + 98P Y2 + 98P Y3 ) ÷ 3 (equation 5.1)

In this equation, 98P Y1 , 98P Y2 and 98 Y3 represent the annual 98 th percentiles of the NO 2
Dmax 1-hour for the consecutive calendar years Y1, Y2 and Y3 respectively.

An annual 98th percentile is to be obtained based on the following three main steps:

Step 1: Select the NO 2 Dmax 1-hour for each day.

Step 2: Select the eight highest NO 2 Dmax 1-hour in the year and rank them in
        decreasing order of magnitude, repeating common values as often as they occur,
        as in the example in the Table 5-1.




                                                                                               11
Table 5-1: Example of eighth highest NO 2 Dmax 1-hour in decreasing order
                    NO 2 Dmax 1-
                                          Rank          Date measured
                     hour (ppb)
                         89.9             Highest         15-01-2018
                         76.4        Second highest       18-12-2018
                         76.4          Third highest      11-01-2018
                         63.2         Fourth highest      18-04-2018
                         57.1          Fifth highest      11-05-2018
                         45.9          Sixth highest      13-03-2018
                         44.3        Seventh highest      29-12-2018
                         40.1         Eighth highest      23-01-2018


Step 3: Use Table 5-2 to obtain the value of the annual 98 th percentile depending on the
        number of NO 2 Dmax 1-hour available in the year.

Table 5-2: The 98th percentile as a function of available data
          Number of NO 2 Dmax 1-hour
                                                       98 th percentile value
            available in a year (N DM )

                     1 to 50                      The highest NO 2 Dmax 1-hour

                    51 to 100                             Second highest

                   101 to 150                              Third highest

                   151 to 200                             Fourth highest

                   201 to 250                              Fifth highest

                   251 to 300                              Sixth highest

                   301 to 350                            Seventh highest

                   351 to 366                             Eighth highest


Table 5-2 is from the percentile ranking approach and it is discussed in detail in Appendix
B. Under this approach, the value of the annual 98th percentile of the NO 2 Dmax 1-hour is
one of the eight highest NO 2 Dmax 1-hour measured in the year depending on the number
(N DM ) of available NO 2 Dmax 1-hour. For example, for N DM = 355 the 98 th percentile
value is the eighth highest NO 2 Dmax 1-hour; for the example in Table 5-1, this would be
40.1 ppb.

There are different methods for obtaining a 98th percentile and each may give different
results. Off-the-shelf software and in-house written computer programs can be used only
if they always provide the same results as the percentile ranking approach.




                                                                                         12
5.2 Calculation of Annual Metric Values

The annual metric value at a monitoring station is the average of all NO 2 1-hour measured
in a single calendar year and is calculated as per equation 5.2.

            Annual metric value = (C 1 + C 2 + ∙∙∙ + C N1h ) ÷ N 1h (equation 5.2)

In this equation, C i is the NO 2 1-hour for the “i th ” hour in the year. “N 1h ” is the number of
NO 2 1-hour available in the year and it ranges from 1 up to 8760 (8784 in leap years). Note
that for the annual metric value the selection of the daily maximum 1-hour is not required.


5.3 Data Completeness Criteria and Exceptions

Generally, only data that meet completeness requirements should be used in the calculation
of the NO 2 metric values, and only metric values based on complete data should be used
for CAAQS reporting. Table 5-3 specifies the data completeness criteria under which
concentrations and metric values may be considered to be based on complete data.

There are exceptions to the data completeness criteria; these are indicated in column 3 of
Table 5-3. These exceptions ensure that potential exceedances of a standard are captured.
If a parameter in column 1 does not meet its completeness criteria in column 2 but meets
the exception criteria in column 3, it will then still be used for CAAQS reporting. For
example, at a given monitoring station criterion 1 for the annual 98 th percentile (75% data
completeness) was not satisfied and the 98th percentile of the available NO 2 Dmax 1-hour
is 100 ppb. Since this 98 th percentile exceeds the standard, it has to be used in the
calculation of the metric value at the station even though the completeness criterion was
not satisfied.




                                                                                                 13
Table 5-3: Data completeness and exceptions criteria
                                                                                         Exceptions to the data
                                                                                         completeness criteria
          Parameter                   Data completeness criteria                      (This parameter will always be
                                                                                  considered in the calculation of metric
          (Column 1)                             (Column 2)                        values if the following conditions are
                                                                                                  satisfied)
                                                                                                 (Column 3)


                                    At least 18 of the 24 (75%)
                                                                                The NO 2 Dmax 1-hour exceeds the
NO 2 Dmax 1-hour                    NO 2 1-hour are available in the
                                                                                standard.
                                    day 11.

                                    The NO 2 Dmax 1-hour are
                                    available for at least:
Annual 98 th percentile                                                         The 98 th percentile based on the
                                     1. 75% of the days in a year;
of the NO 2 Dmax 1-                                                             available NO 2 Dmax 1-hour exceeds
                                        and
hour                                                                            the standard.
                                     2. 60% of the days in each
                                        calendar quarter*.

                                    Two of the possible three
1-hour metric value                 annual 98 th percentiles are                               No exceptions
                                    available 12.


                                   1. at least 75% of the NO 2 1-               1. at least 50% of the NO 2 1-hour
                                      hour are available in the                    are available in each calendar
                                      year; and                                    quarter; and
Annual metric value
                                   2. at least 60% of the NO 2 1-               2. the annual average exceeds the
                                      hour are available in each                   standard.
                                      calendar quarter*.


* The calendar quarters (Q) are: Q1 - January 1 to March 31; Q2 - April 1 to June 30; Q3 - July 1 to September
30; Q4 - October 1 to December 31.



As indicated in Table 5-3, the 1-hour metric value is considered to be based on complete
data if two of the three annual 98th percentiles of the NO 2 Dmax 1-hour are available.
However, for cases when a 1-hour metric value is based on only two annual 98 th percentiles,
provinces and territories can nevertheless identify them as such in the air zone report. Cases
where one or more of the exceptions criteria listed above were applied can also be indicated
in the report. Section 7 provides suggestions on how to communicate CAAQS exceedances
based on incomplete data.


11
   The data completeness and other information for the NO 2 1-hour are provided in the Ambient Air Monitoring and
Quality Assurance and Quality Control Guidelines: NAPS Program (CCME 2019c).
12
     If only two annual 98 th percentiles are available, the divisor in equation 5.1 will be 2 instead of the indicated 3.

                                                                                                                             14
When a standard is exceeded under exceptions to the data completeness criteria, provinces
and territories could evaluate if an exceedance would also have occurred had the data been
complete. The evaluation may be important since the exceedance of a standard means that
the air zone could be managed at the Red management level. The result of the evaluation
could be used to inform the decision whether to manage at the Red management level under
exceedances associated with incomplete data. Text box 2 below provides an example of an
evaluation.

Text Box 2: Example for evaluating the potential of an exceedance
 An air zone has a single NO 2 CAAQS-reporting station and the achievement status for the 1-
 hour standard is evaluated for the 3-year period 2018 to 2020. The completeness criteria for
 the 98 th percentile of the NO 2 Dmax 1-hour are satisfied for 2018 and 2019 but not for 2020.
 For 2020 the 98 th percentile based on the available NO 2 Dmax 1-hour is 70.9 ppb and since it
 is greater than the 2020 standard it is retained for the calculation of the metric value. The 98 th
 percentiles for 2018 and 2019 are 65.6 and 53.8 ppb respectively. The 1-hour metric value at
 the station is the average of the three annual 98 th percentiles or 63 ppb. Since the metric value
 exceeds the 2020 standard, the jurisdiction opts to evaluate if the 98 th percentile in 2020 could
 also have been greater than the standard had the data been complete.

 For this, the jurisdiction first determines the time of year where the NO 2 Dmax 1-hour are
 missing in 2020 and it finds that they were missing in January and February. The jurisdiction
 next looks at the historical NO 2 Dmax 1-hour for these two months and finds that in the previous
 five years there were up to eight days per year where the NO 2 Dmax 1-hour was greater than
 the standard. The jurisdiction next conducts analyses of the general weather conditions in
 January and February 2020 and it concludes that they were mostly similar to those in the
 previous five years for the same two months. The jurisdiction also evaluates the emissions of
 NO 2 from sources known to influence the monitoring station. Since the emission information
 was not available on a monthly basis, the jurisdiction evaluates the annual emissions and it
 concludes that they were mostly similar to those in the previous five years.

 Considering all this information, the jurisdiction concludes that the missing 1-hour NO 2 in
 January and February 2020 likely included some concentrations which were greater than the
 standard and that, therefore, the metric value for 2018-2020 would also have exceeded the
 standard had the data been complete.




5.4 Decimal Places and Rounding Rules

Calculated values, such as the 1-hour average concentrations 13 and the CAAQS metric
values can be reported to various decimal places. Table 5-4 specifies the number of decimal



13
  For most NO 2 monitors, the 1-hour average concentrations correspond to the average of concentrations measured
over shorter time periods.

                                                                                                             15
 places that 1-hour average concentrations and metric values are to be reported, together
 with the rules for rounding the values to the specified decimal places.

 The NO 2 1-hour are to be reported to one decimal place. The NO 2 CAAQS metric values
 are to be reported to the same number of decimal places as their corresponding CAAQS,
 which is a whole number for the 1-hour CAAQS and one decimal place for the annual
 CAAQS. It should be noted that the NO 2 Dmax 1-hour and their 98 th percentiles are not
 calculated values but are instead directly obtained from the NO 2 1-hour. As such, the NO 2
 Dmax 1-hour and their 98 th percentiles are both to be reported to one decimal place. Text
 Box 3 provides an example for rounding the 1-hour CAAQS metric values to whole
 numbers.

 Table 5-4: Decimal place and rounding rules
                         Decimal place of
   Parameter                                                Rounding rule for the parameter
                            parameter

                                                 For the calculated average, first discard all numbers
                                                 after the second decimal. This results in a number
  NO 2 1-hour *
                                                 with two decimal places (a calculated average of
                                                 65.4599 ppb becomes 65.45 ppb). For the resulting
                                                 number, if its second decimal is:

                                                     1. ≥ 5, round upward to one decimal place (65.45
 Annual metric         One decimal place
                                                        ppb is rounded upward to 65.5 ppb)
    value
(annual average                                      2. ≤ 4, round downward to one decimal place
  of the NO 2 1-                                        (65.44 is rounded downward to 65.4 ppb).
       hour)
                                                 The rounded number is then the NO 2 1-hour or annual
                                                 metric value.

                                                 For the calculated 3-year average, first discard all
                                                 numbers after the first decimal. This results in a
                                                 number with one decimal place (a calculated 3-year
                                                 average of 65.4999 ppb becomes 65.4 ppb). For the
  1-hour metric                                  resulting number, if its decimal is:
      value
                       No decimal place
(average of three                                    1. ≥ 5, round upward to a whole number (65.5
                       (whole number)
   annual 98 th                                         ppb is rounded upward to 66 ppb)
   percentiles)
                                                     2. ≤ 4, round downward to a whole number (65.4
                                                        ppb is rounded downward to 65 ppb).

                                                 The rounded number is then the 1-hour metric value.

* The NO 2 Dmax 1-hour and their 98th percentile are also reported to one decimal place since they are
directly obtained from the NO 2 1-hour.




                                                                                                         16
Text Box 3: Example for rounding the 1-hour CAAQS metric value to a whole
      number
 The annual 98 th percentiles of the NO 2 Dmax 1-hour at a monitoring station for the years
 2018, 2019 and 2020 are 72.5, 60.5 and 55.9 ppb respectively. Their calculated 3-year
 average is 62.966…ppb*.

 According to Table 5-4, the 1-hour metric values are to be reported as a whole number (no
 decimal place) based on the specified rounding procedure.

 The procedure specifies to first discard all numbers after the first decimal place in the
 calculated 3-year average. Therefore, the calculated 3-year average of 62.966…ppb becomes
 62.9 ppb. For the resulting number, the rounding rule then specifies that if its decimal is:

      1. ≥ 5, round upward to a whole number

      2. ≤ 4, round downward to a whole number.

 Accordingly, 62.9 ppb is rounded upward to 63 ppb, which becomes the 1-hour metric value.

 *The three dots signify that the number 6 repeats indefinitely.




6.0     TRANSBOUNDARY FLOWS AND EXCEPTIONAL EVENTS

Transboundary flows (TF) and exceptional events (EE) are influences on concentrations
from sources over which a jurisdiction has little to no direct control and they are discussed
in the CCME Guidance Document on Transboundary Flows and Exceptional Events
(CCME 2019b). Under AQMS, provinces and territories can consider influences from TF
and EE on CAAQS exceedances and management levels and the procedures for doing so
are provided in CCME 2019b. Consideration of TF-EE can also be applied to exceedances
resulting from exceptions to the data completeness criteria discussed in section 5.3


7.0     COMMUNICATION AND REPORTING

Communicating with the Canadian public is an important component of AQMS. Each
jurisdiction will regularly publish reports on air quality containing information for each of
their air zones. With respect to CAAQS metric values, these reports should include the
following information:

   1. the NO 2 CAAQS metric values at each NO 2 CAAQS reporting station
   2. the NO 2 CAAQS achievement status for each CAAQS reporting station
   3. the NO 2 CAAQS achievement status for the air zone.



                                                                                              17
Including the metric values at each station will show the spatial variation in concentrations
and will also serve to show that an exceedance may be limited to only some areas instead
of the entire air zone. If an air zone has no NO 2 CAAQS-reporting station, the air zone
report would clarify that the NO 2 CAAQS achievement status could not be determined.
Exceedances of a standard under incomplete data may be identified as such in the air zone
reports and be accompanied with a cautionary note. Information may also be included in
regard to which concentration was based on the exceptions criteria outlined in Table 5-3.

The reporting requirements for air zone management levels are discussed in CCME 2019a.


8.0      SUMMARY OF GUIDANCE

The guidance provided in this document for the determination of the NO 2 CAAQS
achievement status is summarized schematically in Figure 8-1.

Figure 8-1: Outline for determining the achievement status of a NO 2 CAAQS

 Calculate the NO2 CAAQS metric value at
each NO2 CAAQS reporting station in the air
                   zone




                                                         Refer to the Guidance Document on
  Select the highest metric value in the air            Transboundary Flows and Exceptional
                   zone                                Events to consider influences from TF-EE
                                                                       (optional)




 Determine the achievement status of the               Refer to Guidance Document on Air Zone
 CAAQS and management level for the air                Management to assign the management
                 zone                                                   level




                                                                                              18
9.0     REFERENCES

Brauer M., Reynolds C. and Hystad P.. 2013. Traffic-related air pollution and health in Canada.
       Commentary, Can. Med. Assoc. J., December 10, 2013, 185(18).
       http://www.cmaj.ca/content/185/18/1557.

CCME (Canadian Council of Ministers of the Environment). 2019a. Guidance Document on Air Zone
     Management. CCME, 2019. www.ccme.ca.

CCME 2019b. Guidance Document on Transboundary Flows and Exceptional Events for Air Zone
     Management. CCME, 2019. www.ccme.ca.

CCME 2019c. Ambient Air Monitoring and Quality Assurance and Quality Control Guidelines: National
     Air Pollution Surveillance Program. CCME 2019. www.ccme.ca.

U.S. EPA (United States Environmental Protection Agency). 2010. Primary National Ambient Air Quality
       Standards for Nitrogen Dioxide; Final Rule. 40 CFR Parts 50 and 58. EPA, February 9, 2010.

Health Canada. 2016. Human Health Risk Assessment for Ambient Nitrogen Dioxide.
        https://www.canada.ca/en/health-canada/services/publications/healthy-living/human-health-risk-
        assessment-ambient-nitrogen-dioxide.html

Turner, D.B.. 1961. Relationships Between 24-hour Mean Air Quality Measurements and Meteorological
        Factors in Nashville, Tennessee. J. Air Pollut. Control Assoc., 11:10, 1961.
        https://www.tandfonline.com/doi/abs/10.1080/00022470.1961.10468029




                                                                                                     19
APPENDIX A – EXAMPLE CALCULATION OF THE 1-HOUR
CANADIAN AMBIANT AIR QUALITY STANDARDS METRIC VALUE

This appendix provides an example of the procedures for calculating the NO 2 1-hour
CAAQS metric value at a monitoring station for the three-year period 2018 to 2020.

Based on equation 5.1, the 1-hour metric value is:

         1-hour CAAQS metric value 2018-2020 = (98P 2018 + 98P 2019 + 98P 2020 ) ÷ 3

The 98 th percentiles for each of the years 2018 to 2020 are obtained by applying the three-
step procedure described in section 5.1 and an example is provided below for the 98P 2018 .


Step 1 – Select the NO 2 Dmax 1-hour for each day.

The NO 2 Dmax 1-hour was selected for each day in 2018 as shown by the fourth column
in Table A-1. For January 1, 2018 the data completeness criterion of at least eighteen NO 2
1-hour was not satisfied. However, since the NO 2 Dmax 1-hour for this day (125.7 ppb)
exceeds the standard, this maximum was retained as per the exception specified in Table
5-3.




                                                                                          20
Table A-1: Selecting the NO 2 Dmax 1-hour per day
                                NO 2 1-hour
                                                               # of available
                     Hour of     average        NO 2 Dmax 1-
          Day                                                  NO 2 Dmax 1-
                     the day   concentration     hour (ppb)
                                                                    hour
                                   (ppb)
        1-Jan-2018      1            2.1
        1-Jan-2018      2            3.9
        1-Jan-2018      3            4.1
        1-Jan-2018      4           10.1
        1-Jan-2018      5            4.4
        1-Jan-2018      6            4.1
        1-Jan-2018      7           12.5
        1-Jan-2018      8           55.5
        1-Jan-2018      9          125.7
        1-Jan-2018     10          100.9
        1-Jan-2018     11           40.4
        1-Jan-2018     12           10.4
        1-Jan-2018     13            8.3
        1-Jan-2018     14            4.5
        1-Jan-2018     15           55.6
        1-Jan-2018     16       Not available
        1-Jan-2018     17       Not available
        1-Jan-2018     18       Not available
        1-Jan-2018     19       Not available
        1-Jan-2018     20       Not available
        1-Jan-2018     21       Not available
        1-Jan-2018     22       Not available
        1-Jan-2018     23       Not available
        1-Jan-2018     24            4.5           125.7            16
            …          …             …
       31-Dec-2018      1            3.2
       31-Dec-2018      2            2.3
       31-Dec-2018      3            4.5
       31-Dec-2018      4            6.6
       31-Dec-2018      5           10.1
       31-Dec-2018      6            8.9
       31-Dec-2018      7            5.6
       31-Dec-2018      8           12.5
       31-Dec-2018      9           22.5
       31-Dec-2018     10           55.1
       31-Dec-2018     11           20.1
       31-Dec-2018     12           23.1
       31-Dec-2018     13            9.1
       31-Dec-2018     14            2.3
       31-Dec-2018     15            5.5
       31-Dec-2018     16            6.6
       31-Dec-2018     17            4.4
       31-Dec-2018     18            1.1
       31-Dec-2018     19            2.1
       31-Dec-2018     20            1.5
       31-Dec-2018     21       Not available
       31-Dec-2018     22       Not available
       31-Dec-2018     23            3.5
       31-Dec-2018     24            1.2            55.1            22

                                                                                21
Step 2 – Select the eight highest NO 2 Dmax 1-hour in the year and rank them in decreasing
order of magnitude and repeating common values as often as they occur.

From all the NO 2 Dmax 1-hour in the year, the eight highest were selected and common
values were repeated as often as they occurred (Table A-2).

Table A-2: The eight highest NO 2 Dmax 1-hour in 2018 ranked in decreasing
      order
           NO 2 Dmax 1-hour (ppb)                   Rank                  Date measured

                    125.7                          Highest                   1-01-2018
                    125.7                      Second highest               15-02-2018
                    72.5                        Third highest                3-03-2018
                    70.9                       Fourth highest                5-01-2018

                    65.6                        Fifth highest                6-03-2018

                    60.1                        Sixth highest               31-01-2018

                    55.3                       Seventh highest              17-05-2018

                    51.9                       Eighth highest               11-04-2018


Step 3 – Use Table 5-2 to determine the annual 98th percentile value depending on the
         number of NO 2 Dmax 1-hour available in the year.

For this example, it is assumed that there are 300 NO 2 Dmax 1-hour in 2018. With N DM =
300, the 98 th percentile is the value of the 6th highest NO 2 Dmax 1-hour (Table 5-2), or
60.1 ppb. Therefore, 98P 2018 = 60.1 ppb.

Calculation of the NO 2 1-hour CAAQS Metric Value

The same three steps were repeated for each of 2019 and 2020 and the obtained 98 th
percentiles are 60.5 and 55.9 ppb respectively. Using equation 5.1 and the rounding rule in
Table 5-4, the 1-hour metric value for the three-year period from 2018 to 2020 is calculated
to be 59 ppb as shown below.

         1-hour CAAQS metric value 2018-2020   = (98P 2018 + 98P 2019 + 98P 2020 ) ÷ 3

                                               = (60.1 + 60.5 + 55.9) ÷ 3

                                               = 58.8333

                                               = 59 ppb



                                                                                          22
APPENDIX B – THE PERCENTILE RANKING APPROACH

This section explains the derivation of Table 5-2.

In the percentile ranking approach, the 98 th percentile of the NO 2 Dmax 1-hour is the K th
highest concentration (C) in the decreasing ordered array (e.g. C 1 ≥ C 2 ≥ C 3 ∙∙∙≥ C n ). The
K th highest is obtained using the equation below for any number (N DM ) of the daily-
maximum 1-hour average concentrations:

                       K th highest = N DM − Truncated (N DM ×0.98)

“Truncated” means that the value of the product (N DM x0.98) is converted to a whole
number by discarding (not rounding) the decimal part of the value. If the value of the
product N DM ×0.98 is a whole number, no truncation is needed.

For example, with N DM = 355:

                                  K th highest =   N DM − Truncated (N DM ×0.98)

                                              =    355 – Truncated (355×0.98)

                                              =    355 – Truncated (347.9)

                                              =    355 – 347

                                              =    Eighth highest


Applying the equation for N DM from 1 to 366 provides the results of Table 5-2.




                                                                                             23
