Canada’s Air Pollutant Emissions Inventory Report: chapter 4
Data quality control
Quality control for the inventory takes place in two phases. In Phase 1, quality control is performed on the most recently submitted National Pollutant Release Inventory (NPRI) facility-reported data, prior to inclusion of the data in the Air Pollutant Emissions Inventory (APEI). A summary of the process for the APEI is presented in Section 4.1.
Phase 2 of the quality control occurs after the facility-reported data and in-house estimates are compiled and reconciled to form the APEI. During Phase 2, emissions are verified on the basis of established criteria (a description of this process is provided in Section 4.2).
4.1 Phase 1: Emission data from facilities
The quality control process involves a system of documented activities and procedures performed by a dedicated team to identify data outliers, inconsistencies, missing data, inaccuracies and errors. It includes communications with facilities to resolve identified issues. The quality control process can be adapted so that category-specific or sector-specific quality control procedures are applied, as appropriate.
An essential part of the quality control exercise is to identify missing NPRI facility reports/reporters and the assessment of new reports/reporters, to ensure that the correct data are captured.
The identification of outliers (i.e. reports that significantly depart from comparable NPRI facility-reported data) is of critical importance to ensure the usability of the NPRI facility-reported data. Identification, facility follow-up and resolution of such issues are conducted at the earliest stage of the quality control review.
Potential outliers are defined as any NPRI facility report that:
- has a large year-over-year change, and/or
- contributes an unrealistically high proportion of the total reported quantity of an air pollutant in the current or previous reporting year
The quality control review includes analysis of:
- the impact of first-year reporting
- substances that are no longer reported
- substance reports with a large change in contribution/impact on the reported total
- substance reports with identical reported quantities of an air pollutant within a five-year period
- substance reports with significant variation over a five-year period
- facilities assigned to incorrect subsectors
In the past, a common reporting error related to APEI pollutant reporting was the misreporting of the different-sized fractions of particulate matter (PM). Starting in 2013, data input checks have been implemented in the online data collection, which reduced the frequency of this type of error. Additional quality control checks were performed in 2018 on outstanding issues of particulate matter emissions.
Quality control checks are also performed on facility information. These checks include the verification of reported North American Industry Classification System (NAICS) codes, facility identification numbers and geographical information (i.e. city, province, address and latitude/longitude).
The quality control team continues to follow up on the few remaining unresolved issues, and any updates to the data will be reflected in the next inventory edition.
4.2 Phase 2: Compiled Air Pollutant Emission Inventory
The objective of Phase 2 of the quality control process is to identify and verify inconsistencies in the APEI at the subsector level. A series of verification and quality control checks are undertaken on the in-house emission estimates of the current year to ensure quality, accuracy and consistency. The following are verified:
- activity data
- emission factors
- unit conversions
- emission calculations
Phase 2 of the quality control is carried out through the following measures for the compiled APEI:
- manual verification of the updated emissions data as they are entered in central APEI database
- comparison of the emissions to those of the previous year’s inventory and to the previous year’s trends
The inventory data is reviewed, and any significant changes from year to year are identified and explained. Additionally, any significant changes in recalculated emissions are identified and explained.
The reporting of substances by facilities to the NPRI remains the primary source of data collection on air pollutant emissions for Canada. Sectors with significant sources of facility-reported data (e.g. oil refineries, smelters) are well represented by emissions data from the NPRI.
The completeness of the APEI is assessed by the level of inclusion of all known, quantifiable sources of pollutant emissions in the provincial/territorial and national totals that are attributed to anthropogenic activities. Where NPRI facility-reported data does not provide for complete sector coverage, additional estimates are developed in-house by Environment and Climate Change Canada. An overall estimation of completeness in this case is related to the availability and reliability of activity data and compilation methodologies used for the in-house estimates.
The development of complementary in-house estimates is not required in sectors where NPRI facility data provides complete coverage of air pollutant emissions (e.g. pulp and paper). To produce a complete inventory of emissions, complementary in-house estimates are necessary for those sectors that have facilities not reporting to the NPRI because they do not meet the reporting threshold (e.g. upstream oil and gas industry, wood products facilities and foundries).
Other sources of air pollutants, such as residential fuel combustion, transportation or fires, are not subject to reporting to the NPRI, and coverage is assured solely through the calculation of in-house emission estimates for these sources.
Although all major sources of air pollutant emissions are included in the APEI, a number of sources are not included in the national inventory, such as the burning of agricultural wastes and demolition activities in the construction industry.
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