Effect of Nitrate Aerosols on Indirect Forcing As Modeled By CAM with MOSAIC

According to IPCC AR5, nitrate aerosols are predicted to increase steadily through 2100 because of the reduction in sulfate aerosols. Over some regions that are heavily polluted by ammonia and NOx, burdens of nitrate aerosol are already comparable to those of sulfate aerosols. Despite the importance of nitrate aerosols, only a few models in CMIP5 include nitrate aerosols. Even fewer models examine the indirect forcing caused by nitrate aerosols, even though they are highly hygroscopic. In this study, we employ CAM model coupled with MOSAIC aerosol module to simulate nitrate aerosols and assess their indirect forcings. In this version of CAM, aerosol gas-particle partition is treated by MOSAIC, while the other processes, such as dry/wet deposition, coagulation, are still treated by CAM.

In order to evaluate the indirect forcings of nitrate aerosols, we design three experiment cases. In the first two cases, MOSAIC aerosol module is turned on, but nitrate aerosols are assigned with high and extreme low hygroscopicities, respectively. In the third case, MOSAIC aerosol module is turned off. In this case, no nitrate aerosols but additional sulfate aerosols are formed. The cloud radiative forcings modeled by three cases are compared. Modeled nitrate aerosols will be thoroughly compared with several ground-based observations, such as EAnet over East Asia, IMPROVE over North America, and EMEP over Europe, as well as with aircraft in-situ measurements.