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. Health impact analyses are increasingly tapping the broad spatial coverage ofsatellite aerosol optical depth (AOD) products to estimate human exposure tofine particulate matter (PM 2.5 ). We use a forward geophysical approachto derive ground-level PM 2.5 distributions from satellite AOD at1 km 2 resolution for 2011 over the northeastern US by applyingrelationships between surface PM 2.5 and column AOD (calculated offlinefrom speciated mass distributions) from a regional air quality model (CMAQ; 12×12  km 2 horizontal resolution). Seasonal averagesatellite-derived PM 2.5 reveals more spatial detail and best capturesobserved surface PM 2.5 levels during summer. At the daily scale,however, satellite-derived PM 2.5 is not only subject to measurementuncertainties from satellite instruments, but more importantly touncertainties in the relationship between surface PM 2.5 and column AOD.Using 11 ground-based AOD measurements within 10 km of surface PM 2.5 monitors, we show that uncertainties in modeledPM 2.5∕AOD can explain more than 70 % of the spatial andtemporal variance in the total uncertainty in daily satellite-derivedPM 2.5 evaluated at PM 2.5 monitors. This finding implies that asuccessful geophysical approach to deriving daily PM 2.5 from satelliteAOD requires model skill at capturing day-to-day variations inPM 2.5∕AOD relationships. Overall, we estimate thatuncertainties in the modeled PM 2.5∕AOD lead to an error of11  µ g m −3 in daily satellite-derived PM 2.5 , anduncertainties in satellite AOD lead to an error of 8  µ g m −3 .Using multi-platform ground, airborne, and radiosonde measurements, we showthat uncertainties of modeled PM 2.5∕AOD are mainly driven bymodel uncertainties in aerosol column mass and speciation, while modelrepresentation of relative humidity and aerosol vertical profile shapecontributes some systematic biases. The parameterization of aerosol opticalproperties, which determines the mass extinction efficiency, also contributesto random uncertainty, with the size distribution being the largest source ofuncertainty and hygroscopicity of inorganic salt the second largest. Futureefforts to reduce uncertainty in geophysical approaches to derive surfacePM 2.5 from satellite AOD would thus benefit from improving modelrepresentation of aerosol vertical distribution and aerosol opticalproperties, to narrow uncertainty in satellite-derived PM 2.5 .

atmospheric chemistry and physics

Assessing uncertainties of a geophysical approach to estimate surface fine particulate matter distributions from satellite-observed aerosol optical depth