PM data analysis—a comparison of two urban areas:: Fresno and Atlanta

Urban speciated fine particulate data from the Speciation Trends Network from January 2001 to February 2002 were studied in both eastern and western locations of the United States. The seasonal variability of PM 2.5 mass, organic carbon, elemental carbon, sulfate ion, nitrate ion, and ammonium cation concentrations were analyzed. Their relationships with ozone and meteorology were also examined. The results reveal that differences in meteorology and emissions have a significant impact on the observed seasonality in species concentrations in Fresno and Atlanta. Based on a parallel analysis of regional PM 2.5 episodic events, this influence appears to be general and may typify the difference between eastern and western cities in the United States. In Atlanta, ozone, sulfate, and ammonium were high in the summer when temperatures and humidities were high, whereas organic carbon concentrations were relatively flat year-round. In Fresno, however, ozone concentrations were high but sulfate concentrations were very low even in the summer, whereas PM 2.5 concentrations were much higher in the winter and dominated by organics. Meteorologically, in Fresno, it was hot and dry in the summer but cool and humid in the winter. Organic carbon, nitrate, and ammonium ion concentrations were observed to be the highest in late fall and winter when the average relative humidity was the highest (above ∼60%). Much lower mixing heights and frequent stagnations in the winter in Fresno were the major factors influencing the observed high concentrations of various species. The wintertime organic aerosols in Fresno were predominately primary in origin. However, on some very high organic concentration days, up to 30% of the observed organic aerosols could be attributed to secondary organic aerosols (SOAs). These very high organic aerosol concentration days in the winter typically had mild temperatures, high humidities, low dilution rates, and an abundance of nitrate particles. These conditions were favorable for additional SOA formation through the acid catalyzed heterogeneous reactions at night on top of the already high primary organic emissions.