One of the fundamental challenges for current chemistry-climate models is the representation of the complex lifecycle of semi-volatile organic compounds, which are emitted from the biosphere and anthropogenic activities, and are constantly evolving due to their oxidative chemistry, conversion into secondary organic aerosols (SOA) and interactions with clouds (removal). This is especially the case now that the production of sulfate aerosols has been substantially decreasing, and semi-volatile organic aerosols are becoming the dominant aerosol component for climate forcing. The recent intercomparison study by Tsigaridis et al. (2014) of more than 30 state-of-the-art global chemistry-climate models including CESM1 showed that estimates of the SOA annual production rate are highly uncertain and vary among models by an order of magnitude, which is an issue that needs to be urgently addressed.
In the past year, ACOM scientists have worked intensively on integrating insights from the process- and regional-model studies into the CESM model and estimating the effects on organic aerosol global budgets and lifetime. Typically global models include highly simplified (and often ad hoc and incomplete) approaches to model organic aerosol lifecycle. The ACOM scientists have developed new constraints on both the formation and removal processes of semi-volatile organic gases and aerosols including: (i) updated MOZART gas-phase mechanism to include additional precursors species for organic chemistry, (ii) new production rates corrected for chamber wall losses based on process modeling with GECKO-A (at NCAR) and the Statistical Oxidation Model (SOM, UC Davis), (iii) the production of organics from the emissions of long-chain semi-volatile and intermediate volatility organic compounds from additional anthropogenic sources that were missing in the current emission inventories, (iv) the effect of water-solubility of condensable organic gases and their wet and dry removal, and (v) removal of organics by photo-fragmentation reactions.
The updated CESM simulations have been performed for the 1960-2005 time period, and the results compared against an extensive dataset of ground, aircraft and satellite measurements of organic aerosols to assess the extent to which these improved representations of organic aerosol formation and removal processes are consistent with observed characteristics of their distribution. The updated model presents a more dynamic picture of the lifecycle of atmospheric SOA, with higher production rates and faster sinks than in the traditional model, resulting in a very different vertical distribution of organic aerosols. This work has allowed reconciling for the first time the observed vertical profiles of OA with the predicted ones allowing for a much higher OA concentrations near the surface without creating large overpredictions in the upper troposphere.