A modeling study of secondary organic aerosols using a general circulation model

A global 3-dimensional aerosol transport-radiation model coupled with CCSR/NIES/FRCGC Atmospheric General Circulation Model (AGCM), SPRINTARS, has been extended to simulate the distribution of tropospheric aerosols focusing on organic aerosols and especially secondary organic aerosols (SOA) and to estimate SOA contributions to direct and indirect effects.

A SOA formation process is very complicated and is scarcely known, so that the results of global simulations in past studies including SOA include large model dependences. In this work, a SOA production assumes to depend on (1) primary organic aerosols (POA) mass concentrations, (2) oxidant species (ozone (O3), hydroxyl radical (OH) and nitrate radical (NO3)) and concentrations, and (3) VOC (as SOA precursors) concentrations.

In this work, we made use of the SOA formation scheme and the predicted global and annual production of natural SOA is 6.73Tg/yr taking into account of terpene emission as SOA precursor through oxidations by O3, OH, and NO3. This value is smaller than 17.5Tg/yr in Takemura et al. (2005), which does not treat the physical-chemical SOA formation scheme.

Simulations including physical-chemical SOA formation suggest two points in estimating influences of aerosols to radiation and cloud field: First, OC number concentration mainly including SOA in Amazon and Congo basin determine cloud droplet effective radius near the top of water cloud and lead to wrong indirect effect estimation. Second, disregarding POA dependence in the SOA formation process may lead to a larger underestimation of the aerosol radiative forcing.