Overview of the LADCO winter nitrate study: hourly ammonia, nitric acid and PM2.5 composition at an urban and rural site pair during PM2.5 episodes in the US Great Lakes region

摘要

An overview of the LADCO (Lake Michigan Air Directors Consortium) WinterNitrate Study (WNS) is presented. Sampling was conducted at ground level atan urban-rural pair of sites during January–March 2009 in eastern Wisconsin,toward the western edge of the US Great Lakes region. Areas surrounding thesesites experience multiday episodes of wintertime PM pollutioncharacterized by high fractions of ammonium nitrate in PM, low wind speeds,and air mass stagnation. Hourly surface monitoring of inorganic gases andaerosols supplemented long-term 24-h aerosol chemistry monitoring at theselocations. The urban site (Milwaukee, WI) experienced 13 PM episodes,defined as periods where the seven-hour moving average PMconcentration exceeded 27 μg m for at least fourconsecutive hours. The rural site experienced seven episodes by the samemetric, and all rural episodes coincided with urban episodes. Episodes werecharacterized by low pressure systems, shallow/stable boundary layer, lightwinds, and increased temperature and relative humidity relative toclimatological mean conditions. They often occurred in the presence ofregional snow cover at temperatures near freezing, when snow melt andsublimation could generate fog and strengthen the boundary layer inversion.Substantial contribution to nitrate production from nighttime chemistry ofozone and NO to NO and nitric acid is likely and requires furtherinvestigation. Pollutant-specific urban excess during episode and non-episodeconditions is presented. The largest remaining uncertainties in theconceptual model of the wintertime episodes are the variability fromepisode-to-episode in ammonia emissions, the balance of daytime and nighttimenitrate production, the relationship between ammonia controls,NO controls and ammonium nitrate reductions, and the extent towhich snow and fog are causal (either through meteorological or chemicalprocesses) rather than just correlated with episodes because of similarsynoptic meteorology.