P. J. Adams

Interactions between tropospheric chemistry and aerosols in a unified general circulation model

[1]xa0A unified tropospheric chemistry-aerosol model has been developed within the Goddard Institute for Space Studies general circulation model (GCM). The model includes a detailed simulation of tropospheric ozone-NOx-hydrocarbon chemistry as well as aerosols and aerosol precursors. Predicted aerosol species include sulfate, nitrate, ammonium, black carbon, primary organic carbon, and secondary organic carbon. The partitioning of ammonia and nitrate between gas and aerosol phases is determined by on-line thermodynamic equilibrium, and the formation of secondary organic aerosols is based on equilibrium partitioning and experimentally determined parameters. Two-way coupling between aerosols and chemistry provides consistent chemical fields for aerosol dynamics and aerosol mass for heterogeneous processes and calculations of gas-phase photolysis rates. Although the current version of the unified model does not include a prognostic treatment of mineral dust, we include its effects on photolysis and heterogeneous processes by using three-dimensional off-line fields. We also simulate sulfate and nitrate aerosols that are associated with mineral dust based on currently available chemical understanding. Considering both mineral dust uptake of HNO3 and wet scavenging of HNO3 on ice leads to closer agreement between predicted gas-phase HNO3 concentrations and measurements than in previous global chemical transport model simulations, especially in the middle to upper troposphere. As a result of the coupling between chemistry and aerosols, global burdens of both gas-phase and aerosol species are predicted to respond nonlinearly to changing emissions of NOx, NH3, and sulfur.

Chapter 18 Formation and cycling of aerosols in the global troposphere

Abstract Aerosols are formed, evolve, and are eventually removed within the general circulation of the atmosphere. The characteristic time of many of the microphysical aerosol processes is days up to several weeks, hence longer than the residence time of the aerosol within a typical atmospheric compartment (e.g. the marine boundary layer, the free troposphere, etc.). Hence, to understand aerosol properties, one cannot confine the discussion to such compartments, but one needs to view aerosol microphysical phenomena within the context of atmospheric dynamics that connects those compartments. This paper attempts to present an integrated microphysical and dynamical picture of the global tropospheric aerosol system. It does so by reviewing the microphysical processes and those elements of the general circulation that determine the size distribution and chemical composition of the aerosol, and by implementing both types of processes in a diagnostic model, in a 3-D global Chemical Transport Model, and in a General Circulation Model. Initial results are presented regarding the formation, transformation, and cycling of aerosols within the global troposphere.