Seasonal dependence of aerosol composition in Philadelphia: sources, trends, and meteorological impacts

Philadelphia is the fifth largest city in the United States, and sits in the middle of the highly urbanized northeast corridor. Aerosol concentrations have been steadily decreasing and Philadelphia is currently in compliance with annual PM_2.5 air quality standards, but annual average concentrations remain close to the annual threshold. To understand the relative changes in composition of the aerosol by season, aerosol mass spectrometry measurements were made in three separate seasons. Concentrations of sulfate, nitrate, chloride, ammonium, and organic species were measured with high time resolution and subsequent analysis of organic aerosol mass spectral matrix by positive matrix factorization allowed the separation of source contributions to organic aerosol. Strong seasonal dependence in aerosol composition was observed with inorganics species (Sulfate, Nitrate, Ammonium) dominating the wintertime aerosol, and organic species, mostly secondary in nature, contributing two thirds of the submicron summertime aerosol mass. Strong dependence on meteorology was observed with aerosol concentrations highest with southwesterly winds and low to moderate windspeeds consistent with regional contributions from the I-95 corridor (see Figure).