Heterogeneous Chemistry: Understanding Aerosol/Oxidant Interactions
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 three-dimensional 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<1.25 {micro}m) that scatters most efficiently. Results from an equilibrium calculation differ significantly from those of DYN since the fraction of fine-mode 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, near-surface HNO{sub 3} and NO{sub x} concentrations are underpredicted by HYB by up to 90% and 5%, respectively. UPTAKE overpredicts the NO{sub x} burden by 56% and near-surface NO{sub x} concentrations by a factor of 2-5. These results suggest the importance of using the more accurate hybrid dynamical method in the estimates of both aerosol forcing and tropospheric ozone chemistry.
- Research Organization:
- Univ. of Michigan, Ann Arbor, MI (United States)
- Sponsoring Organization:
- US Department of Energy/ ACQ Chicago Operations Office
- DOE Contract Number:
- FG02-98ER62584
- OSTI ID:
- 859102
- Report Number(s):
- Final Technical Report; TRN: US200721%%626
- Country of Publication:
- United States
- Language:
- English
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