Particulate and gaseous constituents of air pollution were measured in downtown Los Angeles during daylight hours (0730 to 1930 PDT) from August 1 through October 30, 1993. Atmospheric acids were quantified as strong acidity in PM2.5 (particles with aerodynamic diameters < 2.5 µm) and as gaseous nitric acid. PM2.5 mass, nitrate, chloride, sulfate and ammonium were also measured. Collocated measurements with a Harvard HEADS sampling system were acquired from October 9 through October 20, 1993. The following conclusions are drawn from these measurements.
* Strong acid concentrations in the particulate phase were generally low. For nearly all of the samples, hydrogen ion concentrations (derived from the difference in pH before and after sample extraction) were lower than 10 nmole/m3, and the highest value of ~60 nmole/m3 was accompanied by a high sulfate concentration ( > 40 µg/m3) which was more than 90 percent neutralized. Significant levels of aerosol strong acidity were not found during the study period.
* Nitric acid concentrations ranged from < 1 to 18 µg/m3. Nitric acid concentrations were moderately correlated with particulate nitrate levels, but they were often lower than particulate nitrate concentrations when the latter were at their highest levels.
* More than 50 percent of the average PM2.5 was composed of secondary aerosol, specifically sulfate, nitrate and ammonium, during the study period. The highest PM2.5 concentrations were measured during the fall and the highest sulfate, nitrate, and ammonium concentrations were also found during this period.
* Medvol and HEADS measurements were comparable, within +20 percent, for particulate nitrate and sulfate. Total nitrate (sum of particulate nitrate and nitric acid) concentrations were comparable below about 10 µg/m3, but Medvol total nitrate was less than half of HEADS total nitrate for several of the concentrations measured above this level. Since nitric acid concentrations for the Medvol are calculated from the difference between total nitrate and particulate nitrate, the Medvol showed nitric acid concentrations to be less than half of the corresponding HEADS nitric acid levels for many of the 12 collocated samples.
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