Eighth Conference on Atmospheric Chemistry


Direct and Indirect Effects of Nitrate and Ammonium Aerosol: A Column Model Study

Yan Feng, SIO/Univ. Of California, La Jolla, CA; and J. E. Penner and Y. Chen

Direct radiative effect of nitrate and ammonium aerosols has mostly been studied based on aerosol concentrations calculated at thermodynamic equilibrium or using approximate treatments for their uptake by aerosols. In a column model study, we examined the impact on the estimated aerosol direct forcing due to differences in nitrate and ammonium aerosol calculated by these methods, compared to that of a more accurate hybrid dynamical approach (DYN). In addition, indirect radiative effect of nitrate and ammonium aerosol (reducing particle surface tension and hence elevating aerosol activation to cloud droplet) was studied for the first time using an air parcel model. Column results show that the thermodynamic equilibrium assumption significantly overpredicts nitrate aerosol concentration on coarse particles especially at the initial stage of gas-to-particle conversion or when solid ammonium nitrate is formed on sulfate aerosol; as a result, nitrate direct forcing is underestimated. Methods based on the measured HNO3 uptake coefficients (0.1) by aerosols overpredict the nitrate partitioning into the aerosol phase, and they also give very different aerosol size distributions (hence direct forcing). The presence of nitrate on aerosol increase cloud droplet number concentration by 10% at selected columns including continental and marine sites. Cloud droplet number concentration is about 5% underpredicted by other treatments for nitrate aerosol compared to that of DYN. The calculated indirect forcing of nitrate aerosol is 1/10 of that of sulfate aerosol at a European site in July. These results suggest the importance of using the more accurate hybrid dynamical method in the estimates of aerosol direct forcing. And the full consideration of aerosol chemical composition including hygroscopic components like nitrate and ammonium is also important in the calculation of aerosol indirect forcing. .

Session 1, AerosolsóRadiative Impacts and Visibility Reduction
Monday, 30 January 2006, 9:00 AM-11:45 AM, A408

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