Towards an improved representation of organic aerosol (OA) in the remote troposphere: Chemical removal and aging of OA during the ATom mission as a function of photochemical age

Submicron aerosols in the remote free troposphere (FT) originate mostly from long-range transport from distant biogenic, anthropogenic, and biomass burning sources. Very limited local secondary production in this part of the atmosphere heightens the sensitivity of aerosol concentrations to slow removal processes. As of yet, few studies with an advanced aerosol payload have targeted the remote FT. Current global models exhibit a one to three-order-of-magnitude diversity in predicting aerosol concentrations in these regions of the atmosphere, particularly when trying to model organic aerosol (OA), which, together with sulfate, is the most prevalent type of non-refractory aerosol in the remote FT.

We have acquired a global dataset of submicron aerosol concentration and composition over the remote Atlantic and Pacific Oceans as part of NASA's Atmospheric Tomography (ATom) aircraft mission. We find that OA is an important fraction of the aerosol burden in the remote FT, with OA/Sulfate ratios in many regions comparable to the ones observed over the continents (0.4-0.6)., However, OA exhibits on average a much higher carbon oxidation state than in continental airmasses (OS_c up to +1 compared to -1 over the continents), much higher than assumed in most models. This also suggests a moderately hygroscopic aerosol. However, in the cleanest/most remote parts of the global FT, sulfate predominates. This is not captured by current global models and suggests an additional chemical removal of OA (and possibly additional sources of sulfate).

Using several different hydrocarbon-ratio based photochemical clocks in combination with back trajectories to infer the age of the airmasses sampled during ATom, we estimate that the lifetime of OA in the remote FT is on the order of 10 days. This is significantly shorter than the FT lifetime assuming just wet and dry deposition as the primary loss mechanisms, and suggests a chemical removal mechanism. This provides a key constraint for modeling of OA in the FT, based solely on measurements. Heterogeneous oxidation by OH is likely to play a major role, with a possible additional contribution from photolysis. The likelihood of these different chemical removal mechanisms will be discussed and their potential implementation in global models such as GEOS, GEOS-Chem and CESM explored.