Winter measurements of trace gas and aerosol composition at a rural site in southern ontario

Abstract

This paper reports the results of continuous measurements of concentrations of trace gas and aerosol species at Powassan, Ontario, a rural location in southern Ontario, from 20 January to 24 February 1984. The measurements included aerosol H + , NH 4 +, Na +, Ca 2+ , NO 3 −, SO 4 2− and Cl −, gaseous SO 2, NO, NO′ y ; ( = NO + NO 2 + PAN + HNO 3), HNO 3, PAN, and O 3. Average values of concentrations for key species during the project were: SO 2, 7.3 ppb; NO y , 7.5 ppb; HNO 3, 0.85 ppb; O 3, 33 ppb; NH 4 + 1.5 ppb; NO 3 −, 0.4 ppb; and SO 4 2−, 0.9 ppb. Concentrations of primary pollutants (e.g. SO 2) were typically much higher, and concentrations of secondary species (e.g. SO 4 2−) typically lower, than observed at this location in summer. However, clear-air t- NO 3 − SO 4 2− ratios averaged 5–10 times higher in winter than in summer which suggests that HNO 3 is a more important source of atmospheric acidity, relative to SO 4 2− aerosol, in winter than in summer. Pollutant concentrations were highly variable; back trajectory calculations indicate that periods of high concentrations of both primary and secondary species were typically associated with air-mass back trajectories from the southern sectors while periods of low concentrations of secondary species were associated with back trajectories from the north. Comparison of these measurements with those at other locations suggests that concentrations at Powassan were characteristic of those prevailing over a much larger, possibly regional, area.