Importance of global aerosol modeling including secondary organic aerosol formed from monoterpene

A global three-dimensional aerosol transport-radiation model, coupled to an atmospheric general circulation model (AGCM), has been extended to improve the model process for organic aerosols, particularly secondary organic aerosols (SOA), and to estimate SOA contributions to direct and indirect radiative effects. Because the SOA formation process is complicated and unknown, the results in different model simulations include large differences. In this work, we simulate SOA production assuming various parameterizations of (1) primary organic aerosols (POA) mass concentrations, (2) oxidant species concentrations, and (3) volatile organic compound (VOC) concentrations in the SOA formation through gas-to-particle conversion governed by equilibrium partitioning of monoterpene oxidation products. Comparisons of results from observations, other models, and our simulations with/without the SOA partitioning theory lead to some findings of the influence of SOA on the radiation and cloud fields. First, the SOA number concentrations control cloud droplet effective radii near water cloud tops in the tropics and can affect the estimation of the aerosol indirect radiative effect. Second, SOA simulation results strongly depend on POA concentrations and emission data, so that disregarding this dependence may lead to a significant underestimation of the aerosol radiative effect because most of other studies assume that the SOA production level in the preindustrial era is same as in the current level. The global annual mean production of SOA formed from monoterpene is evaluated in this study as 6.74 Tg a^-1, and the global annual mean radiative forcings of the direct and indirect effects by SOA from monoterpene are calculated to be -0.01 and -0.19 W m^-2, respectively.