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




                                     PN 1610
                            ISBN 978-1-77202-063-2 PDF




© Canadian Council of Ministers of the Environment, 2020
                                           TABLE OF CONTENTS

BACKGROUND ON AIR QUALITY MANAGEMENT SYSTEM ................................. 1
1.0      INTRODUCTION ................................................................................................. 2
2.0     CANADIAN AMBIENT AIR QUALITY STANDARDS FOR SULPHUR
        DIOXIDE. ............................................................................................................. 2
      2.1 Basis for the Statistical Form of the Standards ..................................................... 5
3.0      SULPHUR 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 SULPHUR DIOXIDE METRIC VALUES............................ 10
      5.1 Calculation of 1-hour Metric Values ................................................................... 10
      5.2 Calculation of Annual Metric Values .................................................................. 11
      5.3 Data Completeness Criteria and Exceptions ........................................................ 12
      5.4 Decimal Places and Number Rounding Rules ..................................................... 14
6.0      TRANSBOUNDARY FLOWS AND EXCEPTIONAL EVENTS ........................... 16
7.0      COMMUNICATION AND REPORTING ............................................................. 16
8.0      SUMMARY OF GUIDANCE ............................................................................... 17
9.0      REFERENCES ..................................................................................................... 18
APPENDIX A – EXAMPLE CALCULATION OF THE 1-HOUR CANADIAN
    AMBIENT AIR QUALITY STANDARDS METRIC VALUE ............................... 19
APPENDIX B – THE PERCENTILE RANKING APPROACH ...................................... 22
                                                           TABLES

Table 2-1: CAAQS for sulphur dioxide .......................................................................... 3
Table 3-1: Management levels for sulphur dioxide ......................................................... 7
Table 5-1: Example of four highest SO 2 Dmax 1-hour in decreasing order ..................... 11
Table 5-2: The 99 th percentile as a function of available data ......................................... 11
Table 5-3: Data completeness and exceptions criteria .................................................... 13
Table 5-4: Decimal place and rounding rules ................................................................. 15


                                                          FIGURES

Figure 4-1: The NAPS SO 2 monitoring stations for 2018 ................................................ 9
Figure 8-1: Outline for determining the achievement status of a SO 2 CAAQS ............... 17


                                                      TEXT BOXES

Text Box 1: Example for calculating the SO 2 1-hour standard metric value ..................... 5
Text Box 2: Example for evaluating the potential of an exceedance ............................... 14
Text Box 3: Example for rounding the 1-hour standard metric value to a whole number..
.................................................................................................................................... 16
BACKGROUND ON 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 sulphur
dioxide (SO 2 ) and information on the procedures, methodologies and criteria for
determining whether the CAAQS for SO 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 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 sulphur dioxide (SO 2 ). Its main purpose is to:
    • present the CAAQS and management levels for SO 2
    • provide guidance on the SO 2 monitors and monitoring stations to use for reporting
        on the achievement status of the SO 2 CAAQS
    • provide the procedures for calculating the concentrations to use for direct
        comparison to the SO 2 standards (called metric values)
    • provide the procedures for determining whether an SO 2 CAAQS is achieved or
        exceeded at monitoring stations and in air zones.

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


2.0 CANADIAN AMBIENT                      AIR      QUALITY         STANDARDS            FOR
   SULPHUR 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 SO 2 for 2020 and 2025, listed in Table 2-1. The federal
government established these CAAQS as ambient air quality objectives on October 28,



                                                                                              2
2017 pursuant to sections 54 and 55 of the Canadian Environmental Protection Act, 1999 2.
The 2020 CAAQS came into effect on October 29, 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: CAAQS for sulphur dioxide
                                  Standard
     Averaging                 (numerical value)                               Statistical form
       time 3                                                                  of the standard
                            2020                 2025

                                                                 The 3-year average of the annual 99 th
       1-hour             70 ppb*               65 ppb           percentile of the SO 2 daily-maximum 1-hour
                                                                 average concentrations.

                                                                 The arithmetic average over a single
       1-year
                          5.0 ppb               4.0 ppb          calendar year of all SO 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 SO 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 SO 2 1-hour average concentration is denoted by “SO 2 1-hour”
and the daily maximum of the SO 2 1-hour as the “SO 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 SO 2 standard of 70 ppb (for 2020) applies to SO 2 1-hour. The statistical form of the
standard means that the concentration to use to determine whether 70 ppb was exceeded at
a station is the 3-year average of the annual 99 th percentile of the SO 2 Dmax 1-hour
measured at the station. In a year with complete data (see section 5.3), the 99 th percentile
is the fourth highest SO 2 Dmax 1-hour. The annual SO 2 standard applies to the average of
all SO 2 1-hour measured at a station over a single calendar year.


2
  Canada Gazette Part 1, Volume 151, no. 43, October 28, 2017. http://www.gazette.gc.ca/rp-pr/p1/2017/2017-10-
28/html/index-eng.html.
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 value” or simply “metric
value” 4. An SO 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 SO 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 SO 2 CAAQS
is achieved in an air zone if the metric values at all SO 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 SO 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 SO 2 CAAQS
metric value in an air zone with two monitoring stations. Since the 3-year period for this
example is 2018-20, the 2020 standard of 70 ppb applies. For this example, the SO 2 1-hour
standard is achieved at Station A (since the metric value of 44 ppb is less than the standard
of 70 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 99th percentiles, the metric
values and for number rounding. These are addressed in section 5.




4
 It should be noted that the annual highest 1-hour average concentration and a single annual 99 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 SO 2 1-hour standard metric value
                                     Annual 99 th percentile of the SO 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 99 th                   1-hour SO 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 SO 2 1-hour CAAQS, higher health impacts are associated with the annual highest
SO 1-hour. However, data analyses indicate that the annual highest varies more from year

5
  For example, the average SO 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
to year than the annual 99th percentile, likely because the annual highest is more sensitive
to meteorological conditions 6. In a year with complete data the 99th percentile is the fourth
highest SO 2 Dmax 1-hour and, as such, it is representative of the higher SO 2
concentrations. Therefore, given that the 99 th percentile varies less, and given that it is also
representative of the higher SO 2 concentrations, a 99th percentile form was adopted for the
1-hour CAAQS. In addition, a 99 th percentile form also aligns with the SO 2 standard of the
United States, which is a desirable feature since it allows direct comparison of SO 2 air
quality between the two countries. The final form as a 3-year average of the 99 th percentile
was selected as a mean to further reduce the variability between years.

For the annual SO 2 CAAQS, all SO 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 SO 2 1-
hour are typically not overtly influenced by meteorological conditions. As such, a single
year average of all SO 2 1-hour measured in the year was deemed appropriate for the annual
CAAQS.


3.0      SULPHUR DIOXIDE MANAGEMENT LEVELS

AQMS includes an AZMF, which provides guidance to jurisdictions on the 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 SO 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 SO 2 CAAQS metric values discussed in section 2 are also used
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.




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
Table 3-1: Management levels for sulphur dioxide

                                        SO 2 1-hour                                 SO 2 annual
       Management
          level
                                 2020                 2025                 2020                   2025


      Red                     > 70 ppb             > 65 ppb                > 5.0               > 4.0 ppb


      Orange                51 to 70 ppb         51 to 65 ppb         3.1 to 5.0 ppb         3.1 to 4.0 ppb


      Yellow                            31 to 50 ppb                               2.1 to 3.0 ppb


      Green                              ≤ 30 ppb                                      ≤ 2.0




4.0         MONITORS AND REPORTING STATIONS

This section provides guidance on the SO 2 monitors to use for reporting on the achievement
status of the SO 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 SO 2 CAAQS should:

      1. measure SO 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: National Air
         Pollution Surveillance Program (CCME 2019c)

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
(ECCC) 7. Metro Vancouver and Ville de Montréal also contribute to the NAPS Program.



7
    A NAPS Program Memorandum of Understanding (MOU) describes the roles and responsibilities of each jurisdiction.

                                                                                                                  7
4.2 CAAQS Reporting Stations

Provinces and territories are responsible for designating the SO 2 CAAQS reporting
stations. Ideally, all CAAQS reporting stations should be planned to be operational for the
long-term.

In Canada, the major sources of SO 2 are either industrial in nature or electricity generation
utilities which use fossil fuels like coal. For most of these sources SO 2 is released from tall
stacks. SO 2 emissions released from tall stacks can occasionally affect areas located tens
of kilometers away from the stacks depending on the meteorological and topographical
conditions. Typically, however, ambient SO 2 concentrations are highest in the vicinity of
the stacks, with the overall highest SO 2 1-hour mostly occurring within a distance of five
to ten times the effective stack height.

From a population health perspective, this points to the need to have SO 2 CAAQS reporting
stations in communities with major sources of SO 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 SO 2 or acid deposition.

Provincial and territorial jurisdictions may determine that ambient monitoring for SO 2 is
not required in an air zone, but ideally each air zone with a major source of SO 2 should
have at least one SO 2 CAAQS-reporting station, preferably located near a populated area.
While the annual emission quantity of SO 2 is one criterion to consider in defining what
constitutes a “major” source, other criteria include, for example, the proximity of the source
to populations and sensitive ecosystems. In 2018, SO 2 was measured across Canada on an
hourly basis at 149 NAPS stations 8 (Figure 4-1) and a NAPS station was present in many
communities having major sources of SO 2 . As a start, the NAPS SO 2 stations should be
used as SO 2 CAAQS-reporting stations.




8
 The SO 2 monitors at NAPS stations are all designated as FRM by the EPA and they all meet the Ambient Air Monitoring
and Quality Assurance and Quality Control Guidelines: National Air Pollution Surveillance Program (CCME 2019c).

                                                                                                                   8
Figure 4-1: The NAPS SO 2 monitoring stations for 2018




                                                                                                      149 NAPS stations



To augment spatial coverage, provinces and territories could also use additional stations
that they deem appropriate, as long as the SO 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 9
     3. stations owned by third parties.

Provinces and territories are encouraged to use any additional stations wherever a source
of SO 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 SO 2 CAAQS reporting
unless the station is near a populated area or a sensitive ecosystem. Under NAPS, a fence-

9
  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.

                                                                                                                    9
line monitoring station is defined as: a station that is located within or on the 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.

The Canadian Air and Precipitation Monitoring Network (CAPMoN) of ECCC measures
24-hour average ambient SO 2 concentrations using filter-packs at some stations located
mostly in undeveloped areas across Canada. While these SO 2 data cannot be used for
reporting on the SO 2 CAAQS (because of the hourly data requirement), they could be used
as supplemental information to describe the SO 2 air quality in air zones.


5.0    CALCULATION OF SULPHUR DIOXIDE METRIC VALUES

This section provides guidance on the procedures for calculating the SO 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 SO 2 1-hour
average concentration is denoted by “SO 2 1-hour” and the daily maximum of the SO 2 1-
hour as the “SO 2 Dmax 1-hour”.

SO 2 CAAQS metric values can be calculated for any SO 2 monitoring station. However, for
reporting on SO 2 CAAQS achievement status and management level only stations
designated by provinces and territories as SO 2 CAAQS reporting stations should be used.


5.1 Calculation of 1-hour Metric Values

The 1-hour SO 2 CAAQS metric value at a monitoring station is the average of three annual
99 th percentile values over three consecutive years and is calculated as per equation 5.1.
         1-hour metric value Y1-Y3 = (99P Y1 + 99P Y2 + 99P Y3 ) ÷ 3 (equation 5.1)

In this equation, 99P Y1 , 99P Y2 and 99 Y3 represent the annual 99th percentiles of the SO 2
Dmax 1-hour for the consecutive calendar years Y1, Y2 and Y3 respectively.

An annual 99th percentile is to be obtained based on the following three main steps:

Step 1: Select the SO 2 Dmax 1-hour for each day.



                                                                                             10
Step 2: Select the four highest SO 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.

Table 5-1: Example of four highest SO 2 Dmax 1-hour in decreasing order
                    SO 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


Step 3: Use Table 5-2 to obtain the value of the annual 99 th percentile depending on the
        number of SO 2 Dmax 1-hour available in the year.

Table 5-2: The 99th percentile as a function of available data
    Number of SO 2 Dmax 1-hour
                                                      99 th percentile value
      available in a year (N DM )

              1 to 100                            The highest SO 2 Dmax 1-hour

             101 to 200                                  Second highest

             201 to 300                                   Third highest

             301 to 366                                   Fourth highest


Table 5-2 is from the percentile ranking approach and it is discussed in greater detail in
Appendix B. Under this approach, the value of the annual 99th percentile of the SO 2 Dmax
1-hour is one of the four highest SO 2 Dmax 1-hour measured in the year depending on the
number (N DM ) of available SO 2 Dmax 1-hour. For example, for N DM = 345 the 99 th
percentile value is the fourth highest SO 2 Dmax 1-hour; for the example in Table 5-1, this
would be 63.2 ppb.

There are different methods for obtaining a 99th 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.


5.2 Calculation of Annual Metric Values

The annual metric value at a monitoring station is the average of all SO 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)

                                                                                         11
In this equation, C i is the SO 2 1-hour for the “i th ” hour in the year. “N 1h ” is the number of
SO 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 SO 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 99 th percentile (75% data
completeness) was not satisfied and the 99th percentile of the available SO 2 Dmax 1-hour
is 100 ppb. Since this 99 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.




                                                                                                 12
   Table 5-3: Data completeness and exceptions criteria
                                                                                   Exceptions to the data completeness
                                                                                                  criteria
           Parameter                    Data completeness criteria                  (This parameter will always be considered
           (Column 1)                               (Column 2)                       in the calculation of metric values if the
                                                                                         following conditions are satisfied)
                                                                                                      (Column 3)


                                      At least 18 of the 24 (75%) SO 2
                                                                                   The SO 2 Dmax 1-hour exceeds the
SO 2 Dmax 1-hour                      1-hour are available in the
                                                                                   standard.
                                      day 10.

                                      The SO 2 Dmax 1-hour are
                                      available for at least:
                                                                                   The 99 th percentile based on the
               th
Annual 99 percentile                   1. 75% of the days in a year;
                                                                                   available SO 2 Dmax 1-hour exceeds
of the SO 2 Dmax 1-hour                   and
                                                                                   the standard.
                                       2. 60% of the days in each
                                          calendar quarter*.

                                      Two of the possible three
1-hour metric value                   annual 99 th percentiles are                                  No exceptions
                                      available 11.

                                      1. At least 75% of the SO 2 1-               1. At least 50% of the SO 2 1-hour are
                                         hour are available in the                    available in each calendar quarter;
                                         year; and                                    and
Annual metric value
                                      2. at least 60% of the SO 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 99th percentiles of the SO 2 Dmax 1-hour are available.
   However, for cases when a 1-hour metric value is based on only two annual 99 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.

   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

   10
      The data completeness and other information for the SO 2 1-hour are provided in the Ambient Air Monitoring and
   Quality Assurance and Quality Control Guidelines: National Air Pollution Surveillance Program (CCME 2019c).
   11
        If only two annual 99 th percentiles are available, the divisor in equation 5.1 will be 2 instead of the indicated 3.

                                                                                                                                13
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 SO 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 99 th percentile of the SO 2 Dmax 1-hour are satisfied for 2018 and 2019 but not for 2020.
 For 2020 the 99 th percentile based on the available SO 2 Dmax 1-hour is 90.1 ppb and since it
 is greater than the 2020 standard it is retained for the calculation of the metric value. The 99 th
 percentiles for 2018 and 2019 are 66.6 and 65.5 ppb respectively. The 1-hour metric value at
 the station is the average of the three annual 99 th percentiles or 74 ppb. Since the metric value
 exceeds the standard, the jurisdiction opts to evaluate if the 99 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 SO 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 SO 2 Dmax 1-hour for these two months and finds that in the previous
 five years there were up to four days per year where the SO 2 Dmax 1-hour was greater than
 the standard. The jurisdiction next conducts analyses of the 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 SO 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 SO 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 Number Rounding Rules

Calculated values, such as the 1-hour average concentrations 12 and the CAAQS metric
values can be reported to various decimal places. Table 5-4 specifies the number of decimal
places that 1-hour average concentrations and metric values are to be reported to, together
with the rules for rounding the values to the specified decimal places.




12
  For most SO 2 monitors, the 1-hour average concentrations correspond to the average of concentrations measured
over shorter time periods.

                                                                                                             14
 The SO 2 1-hour are to be reported to one decimal place. The SO 2 CAAQS metric values
 are to be reported to the same number of decimal places as their corresponding standard,
 which is a whole number for the 1-hour standard and one decimal place for the annual
 standard. It should be noted that the SO 2 Dmax 1-hour and their 99 th percentiles are not
 calculated values but are instead directly obtained from the SO 2 1-hour. As such, the SO 2
 Dmax 1-hour and their 99 th percentiles are also reported to one decimal place. Text Box 3
 below 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
  SO 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 SO 2 1-                                                (65.44 is rounded downward to 65.4 ppb).
       hour)
                                                       The rounded number is then the SO 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 99 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 SO 2 Dmax 1-hour and their 99 th percentile are also reported to one decimal place since they are directly obtained
from the SO 2 1-hour.




                                                                                                                       15
Text Box 3: Example for rounding the 1-hour standard metric value to a whole
      number
 The annual 99 th percentiles of the SO 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
province and territory 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 SO 2 CAAQS metric values at each SO 2 CAAQS-reporting station
   2. the SO 2 CAAQS achievement status for each CAAQS-reporting station
   3. the SO 2 CAAQS achievement status for the air zone.



                                                                                              16
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 SO 2 CAAQS-reporting station, the air zone
report would clarify that the SO 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 SO 2 CAAQS
achievement status is summarized schematically in Figure 8-1.


Figure 8-1: Outline for determining the achievement status of a SO 2 CAAQS



Calculate the SO2 CAAQS metric value
at each SO2 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 the Guidance Document on Air
CAAQS and management level for the air                     Zone Management to assign the
                zone.                                              management level.




                                                                                             17
9.0    REFERENCES

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.

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): 483-489.
        www.tandfonline.com/doi/abs/10.1080/00022470.1961.10468029.

U.S. EPA (United States Environmental Protection Agency). 2016. Reference and Equivalent Methods Used
       to Measure National Ambient Air Quality Standards (NAAQS) Criteria Air Pollutants. Volume I,
       EPA/600/R-16/139, June 2016.
       cfpub.epa.gov/si/si_public_file_download.cfm?p_download_id=528565&Lab=NERL.




                                                                                                  18
APPENDIX A – EXAMPLE CALCULATION OF THE 1-HOUR
CANADIAN AMBIENT AIR QUALITY STANDARDS METRIC VALUE

This Appendix provides an example of the procedures for calculating the SO 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 = (99P 2018 + 99P 2019 + 99P 2020 ) ÷ 3

The 99 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 99P 2018 .

Step 1: Select the SO 2 Dmax 1-hour for each day.

The SO 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 SO 2 1-
hour was not satisfied. However, since the SO 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.




                                                                                          19
Table A-1: Selecting the SO 2 Dmax 1-hour per day
                     Hour of   SO 2 1-hour     SO 2 Dmax 1-    # of available
          Day
                     the day     (ppb)          hour (ppb)    SO 2 Dmax 1-hour
        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



                                                                                 20
Step 2: Select the four highest SO 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 SO 2 Dmax 1-hour in the year, the four highest were selected and common
values were repeated as often as they occurred (Table A-2).

Table A-2: The four highest SO 2 Dmax 1-hour in 2018 ranked in decreasing
      order
           SO 2 Dmax 1-hour (ppb)                   Rank                  Date measured

                    125.7                          Highest                  1-Jan-2018
                    125.7                      Second highest               15-Feb-2018
                    72.5                        Third highest               3-Mar-2018
                    70.9                       Fourth highest               5-Jan-2018


Step 3: Use Table 5-2 to determine the annual 99 th percentile value depending on the
        number of SO 2 Dmax 1-hour available in the year.

For this example, it is assumed that there are 300 SO 2 Dmax 1-hour in 2018. With N DM =
300, the 99th percentile is the value of the third highest SO 2 Dmax 1-hour (Table 5-2), or
72.5 ppb. Therefore, 99P 2018 = 72.5 ppb.

Calculation of the SO 2 1-hour CAAQS Metric Value

The same three steps were repeated for each of 2019 and 2020 and the obtained 99 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 63 ppb as shown below.

         1-hour CAAQS metric value 2018-2020   = (99P 2018 + 99P 2019 + 99P 2020 ) ÷ 3

                                               = (72.5 + 60.5 + 55.9) ÷ 3

                                               = 62.96667

                                               = 63 ppb




                                                                                          21
APPENDIX B – THE PERCENTILE RANKING APPROACH

This Appendix explains the derivation of Table 5-2.

In the percentile ranking approach, the 99th percentile of the SO 2 Dmax 1-hour is the Kth
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 ×0.99)

“Truncated” means that the value of the product (N DM x0.99) is converted to a whole
number by discarding (not rounding) the decimal part of the value. If the value of the
product N DM ×0.99 is a whole number, no truncation is needed.

For example, with N DM = 355:

                                  K th highest =   N DM − Truncated (N DM ×0.99)

                                              =    355 – Truncated (355×0.99)

                                              =    355 – Truncated (351.45)

                                              =    355 – 351

                                              =    Fourth highest


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




                                                                                              22
