Attribution of Tropospheric Ozone to Sources of NOx and VOC Precursor Emissions in a Global Chemistry-Climate Model (CESM1.2.2-CAM4-Chem) for the 2000-2018 Period

Tropospheric ozone (trop-O3) is a regional air pollutant and an important greenhouse gas. Major sources of trop-O3 are: transport from the stratosphere; and photochemical production within the troposphere involving reactions of ozone precursors: oxides of nitrogen (NO and NO2, collectively NOx) and volatile organic compounds (VOC), including methane.

Trop-O3 has an atmospheric lifetime of about a few weeks making it transportable over inter-continental distances. This makes ozone precursor emissions from one “source” region affect the ozone concentration at local and remote “receptor” regions, making it important for us to understand source-receptor relationships. These source-receptor relationships can be modelled using the source-attribution technique (also known as Tagging) where ozone molecules are tagged/labelled with their source identities allowing a direct attribution of sources in receptor regions, thereby, the relative contribution of various sources can be obtained.

Simulations using CESM 1.2.2- CAM4-Chem are performed for a global study of trop-O3 source attribution for the 2000-2018 period. Here, we modify the default chemical mechanism to output the ozone and its tags attributed to the source region/sector of its emitted precursors. For example, an NO molecule originating from biogenic source would be called NO_BIO, and all the other chemical species emanating from NO_BIO (NO2, NO3, O3 etc.) will hold the tag “BIO”.

Separate simulations are performed for tagging trop-O3 with its NOx and VOC precursor emission sources. We specify separate tag identities for emissions from anthropogenic, biogenic, biomass burning, and aircraft sources. Additional tags are specified for lightning NOx in the NOx-tagged simulation, and for methane in the VOC-tagged simulation. Here, all surface-based anthropogenic emissions hold tags representing the geographical location at which the emissions occur. Further, the ship-NOx emissions hold tags representing the ocean basin from which they are emitted.

The design of these simulations and several prominent results will be presented.