What emission sources are expected to be included in APIS "background" concentration and deposition values?

APIS concentration and deposition data is updated annually as a three-year average to account for variation in precipitation and temperature between years. When considering whether a specific emission source would be included in this, it is important to understand when the emission source began operation or was given permission. Emission sources could be included both via the measurement network and also the spatial mapping we do with the national model as that includes emissions from the National Atmospheric Emissions Inventory (NAEI) https://naei.beis.gov.uk/data  (this is done yearly). Typically, emission sources are considered to be in APIS background if they were operational by 31 Dec of the mid year within the three-year average dataset.