HR: 09:20h
AN: A51G-05    [Abstracts]
TI: Global Modeling of Nitrate and Ammonium: Implications on Direct and Indirect Aerosol Forcing
AU: * Feng, Y
EM: yafeng@ucsd.edu
AF: University of California, San Diego Scripps Institution of Oceanography Center for Atmospheric Sciences, 9500 Gilman Dr. # 0221 , La Jolla, CA 92093-0221 United States
AU: Penner, J
EM: penner@umich.edu
AF: University of Michigan Dept. of Atmospheric, Oceanic, and Space Sciences, 2455 Hayward, Ann Arbor, MI 48109-2143 United States
AU: Chen, Y
EM: ychenz@umich.edu
AF: University of Michigan Dept. of Atmospheric, Oceanic, and Space Sciences, 2455 Hayward, Ann Arbor, MI 48109-2143 United States
AB: Global radiative forcing of nitrate and ammonium aerosols has mostly been estimated from aerosol concentrations calculated at thermodynamic equilibrium or using approximate treatments for their uptake by aerosols. In this study, a more accurate hybrid dynamical approach (DYN) was used to simulate the uptake of nitrate and ammonium by aerosols and the interaction with tropospheric reactive nitrogen chemistry in a threedimensional global aerosol and chemistry model, IMPACT, which also treats sulfate, sea salt and mineral dust aerosol. 43% of the global annual average nitrate aerosol burden, 0.16 TgN, and 92% of the global annual average ammonium aerosol burden, 0.29 TgN, exist in the fine mode (D<1.25μm) that scatters most efficiently. Results from an equilibrium calculation differ significantly from those of DYN since the fraction of finemode nitrate to total nitrate (gas plus aerosol) is 9.8%, compared to 13% in DYN. Our results suggest that the estimates of aerosol forcing from equilibrium concentrations will be underestimated. We also show that two common approaches used to treat nitrate and ammonium in aerosol in global models, including the first-order gas-to-particle approximation based on uptake coefficients (UPTAKE) and a hybrid method that combines the former with an equilibrium model (HYB), significantly overpredict the nitrate uptake by aerosols especially that by coarse particles, resulting in total nitrate aerosol burdens higher than that in DYN by +106% and +47%, respectively. Thus, nitrate aerosol in the coarse mode calculated by HYB is 0.18 Tg N, a factor of 2 more than that in DYN (0.086 Tg N). Excessive formation of the coarse-mode nitrate in HYB leads to near surface nitrate concentrations in the fine mode lower than that in DYN by up to 50% over continents. In addition, the presence of nitrate on aerosol is shown to increase cloud droplet number concentration by 10% at selected continental and marine sites. Cloud droplet number concentration is about 5% underpredicted by other treatments for nitrate aerosol compared to that of DYN. These results suggest the importance of using the more accurate hybrid dynamical method in the estimates of both direct and indirect aerosol forcing.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0365 Troposphere: composition and chemistry
DE: 1610 Atmosphere (0315, 0325)
DE: 3337 Global climate models (1626, 4928)
DE: 3394 Instruments and techniques
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005