Weekly cycles in fine particulate nitrate

Atmospheric responses to changes in emissions are a complex but central issue in control strategy design for pollutants such as ozone and particulate matter. Here, we investigate fine particle nitrate response to weekly cycles in emissions, which includes a large decrease in diesel NOx emissions among other changes. Nitrate concentrations were measured at 10-min time resolution for a year or longer at four US urban sites: Fresno and Claremont in California, St. Louis, and Pittsburgh. Weekly minima in nitrate concentrations were observed at Fresno, Claremont, and St. Louis, with mean reductions of 21–29% below weekly average values on Sundays or Mondays. The day of week with lowest nitrate varied with site and season. No significant day-of-week variations in nitrate were observed at Pittsburgh. Analysis of ammonium and sulfate measurements at Pittsburgh indicates that weekend sulfate reductions observed at this site during spring/summer months do not increase ammonia availability, but rather lead to more complete neutralization of S(VI). Fine particle nitrate measurements at Claremont were resolved into three size ranges (0.07–0.45, 0.45–1.0, and 1.0–2.5 μm); similar weekly reductions were seen for each size range.