No abstract

[1] During summer 2002, soluble trace gases, the ionic composition of size-resolved aerosols, and meteorological conditions were measured from the National Oceanic and Atmospheric Administration ship Ronald H. Brown in coastal air along the eastern U.S. seaboard. Aerosol acidities were extrapolated from direct pH measurements in minimally diluted extracts and were also inferred from the measured phase partitioning and thermodynamic properties of HCl, HNO 3 , NH 3 , HCOOH, and CH 3 COOH. Median pHs for 0.75-through 25-mm geometric mean diameter (GMD) size fractions based on direct measurements (2.6-3.9) were higher than those inferred from HCl partitioning (1.7-3.3). Phase disequilibria caused negative deviations in median pHs inferred from HNO 3 partitioning with aerosol size fractions greater than 2.8-mm GMD; pHs inferred for smaller size fractions (median values of 1.9-3.0) were similar to those based on HCl (median values of 1.5-3.0). The pHs inferred from NH 3 partitioning were significantly lower than those estimated by other approaches; causes for this apparent bias are not known. The dominance of gas-phase HCOOH and CH 3 COOH was generally consistent with predicted phase partitioning with acidic aerosols. Typically large diel excursions in most gases implied corresponding variability in aerosol acidity. The pHs inferred from maximum and minimum mixing ratios of gases over each aerosol sampling interval suggested median 12-hour variations of $0.4-0.7 pH unit. Total acidity (H t = H + + undissociated acids) was greater than H + by 1-2 orders of magnitude in all size fractions; most H t was in the form of HSO 4 À .

Gasoline-powered motor vehicles are a major source of toxic air contaminants such as benzene. Emissions from light-duty vehicles were measured in a San Francisco area highway tunnel during summers 1991, 1994-1997, 1999, 2001, and 2004. Benzene emission rates decreased over this time period, with a large (54 +/- 5%) decrease observed between 1995 and 1996 when California phase 2 reformulated gasoline (RFG) was introduced. We attribute this one-year change in benzene mainly to RFG effects: 36% from lower aromatics in gasoline that led to a lower benzene mass fraction in vehicle emissions, 14% due to RFG effects on total nonmethane organic compound mass emissions, and the remaining 4% due to fleet turnover. Fleet turnover effects accumulate over longer time periods: between 1995 and 2004, fleet turnover led to a 32% reduction in the benzene emission rate. A approximately 4 microg m(-3) decrease in benzene concentrations was observed at a network of ambient air sampling sites in the San Francisco Bay area between the late 1980s and 2004. The largest decrease in annual average ambient benzene concentrations (1.5 +/- 0.7 microg m(-3) or 42 +/- 19%) was observed between 1995 and 1996. The reduction in ambient benzene between spring/summer months of 1995 and 1996 due to phase 2 RFG was larger (60 +/- 20%). Effects of fuel changes on benzene during fall/winter months are difficult to quantify because some wintertime fuel changes had already occurred prior to 1995.