NASA - National Aeronautics and Space Administration

We investigate and assess how well a global chemical transport model (GEOS-Chem) simulates submicron aerosol mass concentrations in the remote troposphere. The simulated speciated aerosol (organic aerosol (OA), black carbon, sulfate, nitrate, and ammonium) mass concentrations are evaluated against airborne observations made during all four seasons of the NASA Atmospheric Tomography Mission (ATom) deployments over the remote Pacific and Atlantic Oceans. Such measurements over pristine environments offer fresh insights into the spatial (Northern [NH] and Southern Hemispheres [SH], Atlantic, and Pacific Oceans) and temporal (all seasons) variability in aerosol composition and lifetime, away from continental sources. The model captures the dominance of fine OA and sulfate aerosol mass concentrations in all seasons. There is a high bias across all species in the ATom-2 (NH winter) simulations; implementing recent updates to the wet scavenging parameterization improves our simulations, eliminating the large ATom-2 (NH winter) bias, improving the ATom-1 (NH summer) and ATom-3 (NH fall) simulations, but producing a model underestimate in aerosol mass concentrations for the ATom-4 (NH spring) simulations. Following the wet scavenging updates, simulated global annual mean aerosol lifetimes vary from 1.9 to 4.0 days, depending on species. Aerosol lifetimes in each hemisphere vary by season, and are longest for carbonaceous aerosol during the southern hemispheric fire season. The updated wet scavenging parameterization brings simulated concentrations closer to observations and reduces global aerosol lifetime for all species, indicating the sensitivity of global aerosol lifetime and burden to wet removal processes. Plain Language Summary Aerosols are a key constituent of the atmosphere. Their lifetime in the atmosphere controls their global distribution and the resulting air quality and climate impacts. We use a global model (GEOS-Chem) to simulate their mass concentrations. To evaluate the model, we compare simulations against airborne measurements over the remote Pacific and Atlantic oceans made in all four seasons during the NASA Atmospheric Tomography Mission (ATom). The model generally reproduces organic and sulfate aerosols well, yet it over-estimates all species in the Northern Hemispheric (NH) winter season. An updated wet deposition method, which provides more effective aerosol removal, reduced the overestimation in this season, improved the simulation in NH summer and fall as well, but underestimated the concentrations in NH spring. With the model update, aerosol lifetimes vary from 1.9 to 4.0 days and by species and seasons, with organic aerosols and black carbon having the longest lifetime during the Southern Hemispheric fire season. The updated wet deposition scheme reduces these lifetimes significantly, demonstrating the sensitivity of aerosol lifetime and concentration to the wet removal process.