Air Pollution in the Northeastern United States: Elucidating Drivers of PM2.5 and O3 Events

Fine mode particulate matter (PM_2.5) and tropospheric ozone (O₃) exhibit large temporal variability during boreal summer in the northeastern United States. We examine here the causes of this variability and, in particular, the occurrence of high pollutant events. We show that the vast majority of PM_2.5 and O₃ events occur on the same day. Moreover, daily regionally-averaged temperatures, derived from Modern-Era Retrospective Analysis for Research and Applications (MERRA) meteorological data, and SO₂ and NO_x point source emissions totals for facilities in our focus region demonstrate remarkable correlation, indicating that high temperatures are responsible for increased emissions, presumably due to spikes in power demand. Our analysis indicates that most PM_2.5 and O₃ events coincide on days that also have extreme temperatures and SO₂ and NO_x emissions.

The relative role of meteorology versus emissions on regional pollution events is unknown, but the aforementioned correlations suggest that both emissions and meteorology are key drivers of PM_2.5 and O₃ events. Meteorology drives regional pollution events through temperature-induced changes in power demand and prevailing synoptic conditions such as stagnation, and emissions drive PM_2.5 and O₃ events through primary emissions and secondary aerosol formation from precursors. We analyze synoptic-scale flow patterns during cases when (1) high temperatures, emissions, and pollutants coincide as well as cases with (2) low temperatures and emissions but high pollutants and (3) high temperatures and emissions but low pollutants, and we examine whether transport from outside the focus region or ventilation can explicitly explain differences in PM_2.5 and O₃ events. Finally, we use the Global Modeling Initiative modular 3-D chemistry and transport model driven by MERRA meteorology forced with variable emissions to isolate the impact of emissions variability on regional O₃ events and thereby deduce the relative role of meteorology versus emissions on regional pollution events.