INSIGHT INTO GLOBAL GROUND-LEVEL AIR QUALITY USING SATELLITES, MODELING AND IN SITU MEASUREMENTS
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Ground-level air quality depends on the ambient concentration of atmospheric aerosols and trace gases. We applied information on aerosols and trace gases gathered from satellite remote sensing, in situ observations, and atmospheric chemistry modelling to improve estimates of air quality. We inferred fine particulate matter (PM2.5) chemical composition at 0.1 degree x 0.1 degree spatial resolution for 2004-2008 by combining aerosol optical depth retrieved from the MODIS and MISR satellite instruments, with coincident profile and composition information from the GEOS-Chem global chemical transport model. Evaluation of the satellite-model PM2.5 composition dataset with North American in situ measurements indicated significant spatial agreement. We found that global population-weighted PM2.5 concentrations were dominated by particulate organic mass (11.9 ± 7.3 microgram per cubic meter), secondary inorganic aerosol (11.1 ± 5.0 microgram per cubic meter), and mineral dust (11.1 ± 7.9 microgram per cubic meter). Secondary inorganic PM2.5 concentrations exceeded 30 microgram per cubic meter over East China. Sensitivity simulations suggested that population-weighted ambient PM2.5 from biofuel burning (11 microgram per cubic meter) could be almost as large as from fossil fuel combustion sources (17 microgram per cubic meter). We developed a simple method to derive an estimate of the spatially and seasonally resolved global, lower tropospheric, ratio between organic mass (OM) and organic carbon (OC). We used the Aerosol Mass Spectrometer-measured organic aerosol data, and the ground-level nitrogen dioxide concentrations derived from the OMI satellite instrument, to develop the OM/OC estimate. The global OM/OC ratio ranged from 1.3 to 2.1 microgram/microgram Carbon, with distinct spatial variation between urban and rural regions. The seasonal OM/OC ratio had a summer maximum and a winter minimum over regions dominated by combustion emissions. We assessed the sensitivity of chemical transport models to the duration of the chemical and transport operators used to calculate the mass continuity equation. Increasing the transport timestep increased the concentrations of emitted species, and the production of ozone. Increasing the chemical timestep increased hydroxyl radical and chemical feedbacks. The simulation error from changing spatial resolution exceeds that from changing temporal resolution.