Emission inventories can track changes in emissions and removals through changing activity levels or changing emission rates, or both. The way in which such changes are included in methodologies can have a significant impact on time series consistency.
National statistics typically will account for significant changes in activity levels. For example, fuel switching from coal to natural gas in electricity generation will be reflected in the national fuel consumption statistics. Further disaggregation of activity data can provide more transparency to indicate specifically where the change in activity is occurring. This approach is relevant when changes are taking place in one or more subcategories, but not throughout the entire category. To maintain time series consistency, the same level of disaggregation into subcategories should so far as possible be used for the entire time series, even if the change began recently.
Research may indicate that the average rate of emissions/removals per unit of activity has changed over the time series. In some cases, the factors leading to a technological change may also make it possible to use a higher tier method. For example, an aluminium plant manager who introduces measures to reduce the frequency and intensity of anode effects may also collect plant-specific parameters that can be used to estimate a new emission factor, This new factor might not be appropriate for estimating emissions for earlier years in the time series, before the technological change occurred. In these cases it is good practice to use the updated emission factor or other estimation parameters or data to reflect these changes. Since a general assumption is that emission factors or other estimation parameters do not change over time unless otherwise indicated, countries should clearly document the reason for using different factors or parameters in the time series. This is particularly important if sampling or surveying occurs periodically and emission factors or estimation parameters for years in between are interpolated rather than measured.
Capture, destruction, or combustion of emissions
Larger point sources such as chemical manufacturing facilities or power plants might generate emissions but prevent them from being released to the atmosphere through capture and storage (e.g., CO2), destruction (e.g., HFC-23) or combustion (e.g., CH4). These activities do not necessarily change the average emissions generated per unit of activity, and therefore it is not good practice to use different emission factors for different years. Instead, the inventory compiler should estimate total emissions generated and emissions reduced separately, and then subtract reductions from the total generation to arrive at an estimate for total emissions to the atmosphere.
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