Inverse modeling and mapping US air quality influences of inorganic PM _2.5 precursor emissions using the adjoint of GEOS-Chem

Influences of specific sources of inorganic PM _2.5 on peak and ambient aerosol concentrations in the US are evaluated using a combination of inverse modeling and sensitivity analysis. First, sulfate and nitrate aerosol measurements from the IMPROVE network are assimilated using the four-dimensional variational (4D-Var) method into the GEOS-Chem chemical transport model in order to constrain emissions estimates in four separate month-long inversions (one per season). Of the precursor emissions, these observations primarily constrain ammonia (NH ₃). While the net result is a decrease in estimated US~NH ₃ emissions relative to the original inventory, there is considerable variability in adjustments made to NH ₃ emissions in different locations, seasons and source sectors, such as focused decreases in the midwest during July, broad decreases throughout the US~in January, increases in eastern coastal areas in April, and an effective redistribution of emissions from natural to anthropogenic sources. Implementing these constrained emissions, the adjoint model is applied to quantify the influences of emissions on representative PM _2.5 air quality metrics within the US. The resulting sensitivity maps display a wide range of spatial, sectoral and seasonal variability in the susceptibility of the air quality metrics to absolute emissions changes and the effectiveness of incremental emissions controls of specific source sectors. NH ₃ emissions near sources of sulfur oxides (SO _x) are estimated to most influence peak inorganic PM _2.5 levels in the East; thus, the most effective controls of NH ₃ emissions are often disjoint from locations of peak NH ₃ emissions. Controls of emissions from industrial sectors of SO _x and NO _x are estimated to be more effective than surface emissions, and changes to NH ₃ emissions in regions dominated by natural sources are disproportionately more effective than regions dominated by anthropogenic sources. NO _x controls are most effective in northern states in October; in January, SO _x controls may be counterproductive. When considering ambient inorganic PM _2.5 concentrations, intercontinental influences are small, though transboundary influences within North America are significant, with SO _x emissions from surface sources in Mexico contributing almost a fourth of the total influence from this sector.