Water‐soluble nitrogen at the New Hampshire sea coast: HNO3, aerosols, precipitation, and fog

Abstract

An intensive sampling program was carried out from May 1994 through November 1997 on the shore of the Gulf of Maine in New Castle, New Hampshire. Daily (24 hour averages) samples of bulk aerosol and gas phase HNO3, precipitation, and 20 aerosol size distributions were obtained. Particulate NH4+ and gas phase HNO3 were the dominant water‐soluble nitrogen species in the atmosphere. There was a summer peak in the mixing ratios of both of these species. Daily mixing ratios of HNO3 and all aerosol species were highly variable, yet the annual averages tended to be similar from one year to the next. The concentrations of all the inorganic species we measured in precipitation were generally higher than those of two National Acid Deposition Program (NADP) coastal sites. In particular, the annual volume‐weighted means for NO3− (22–27 μmol/L) and NH4+ (11–17 μmol/L) were found to be 20%–60% and 40%–90% higher, respectively, than those reported from Cape Cod, Massachusetts. Nitrate was the dominant inorganic nitrogen ion in precipitation at New Castle. In autumn, concentrations of continentally derived species in precipitation decreased substantially while sea salts increased. There was insufficient NH3 to fully neutralize HNO3 and H2SO4 in aerosols and precipitation. The overall atmospheric chemistry in this region was heavily dominated by anthropogenic pollution products. The samples collected were used in conjunction with 1000 hPa streamlines to classify sampled air masses according to their surface level transport and chemistry. Eight characteristic groups were defined; of these the three primary groups were polluted continental, ″clean″ continental, and marine. Highly variable mixing ratios of HNO3 and aerosol species were observed within each group from day to day, yet each group had a unique average chemical signature. On average, the HNO3 and aerosol mixing ratios observed in 1995 were roughly a factor of 2 lower than seen for the groups in other years. Overall, mixed conditions occurred in 42% of the samples, continental species were dominant in 37%, and marine species were dominant in 21%. Rain occurred frequently under sea‐salt‐dominant conditions; about 47% of the days classified as such had rain events. Fog chemistry and average aerosol chemical size distributions were evaluated based on which species dominated their chemical signatures, marine, continental, or a relatively even mixture of the two. Particulate NO3− was associated with sea‐salt Na+ in the coarse aerosol fraction peaking at approximately 4 μm in diameter. There was also a distinct secondary peak in the submicron fraction observed in air dominated by continental aerosols. Particulate NH4+ was associated with non‐sea‐salt SO42− (nss‐SO42−), with the bulk of the NH4+ present on particles in the 0.43–1.1 μm diameter range. Although nss‐SO42− was primarily found in the submicron size range, a substantial fraction (≥25%) was found in the supermicron range for all three cases. Chloride was depleted on average 25% in aerosols and 13% in precipitation with respect to sea‐salt aerosols, with the deficit greatest for particles in the 1.1–3.3 μm and 9.0–25 μm diameter ranges.