Geographical and Vertical Distribution of Organic Aerosol (OA) during ATom-1 and 2: Chemical Removal and Aging 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 production in this region heightens the sensitivity of aerosol concentrations to slow removal processes. As yet, few studies with an advanced aerosol payload have targeted the remote FT. Current global models exhibit a very large 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.

As part of NASA’s Atmospheric Tomography (ATom) aircraft mission, we have acquired a global dataset of organic aerosol (OA) concentration and composition over the remote Atlantic and Pacific Oceans from 0 to 12 km and from 65 S to 80 N for both Summer and Winter seasons. This dataset provides unique new constraints on the spatial distribution of OA and its contribution to the global aerosol background; of particular interest are the OA/Sulfate ratio and OA oxidation state that are critical for estimating the activity of cloud condensation nuclei (CCN) in the remote troposphere. We find that, except for the cleanest of the ATom-sampled airmasses, OA concentrations are comparable and often larger than sulfate. OA was highly oxidized, significantly more than over the continental FT, with O:C ratios often in excess of 1 (i.e. OA/OC >2.5).

Using several different hydrocarbon-ratio based photochemical clocks in combination with backtrajectories 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 such as heterogeneous oxidation or photolysis. This provides a key constraint for modeling of OA in the FT, based solely on measurements. The likelihood of different chemical removal mechanisms will be discussed and their potential implementation in global models such as GEOS-Chem explored.

The contributions of methanesulfonic acid (MSA) and particulate organic nitrates (pRONO₂) to total OA in the remote troposphere will be discussed as well.