1Department of Chemical and Biochemical Engineering, University of Iowa, Iowa City, IA 52242, USA
2IIHR Hydroscience and Engineering, University of Iowa, Iowa City, IA 52242, USA
3Center for Global and Regional Environmental Research, University of Iowa, Iowa City, IA 52242, USA
4Wisconsin Department of Natural Resources (WDNR), Madison, WI 53707, USA
5Illinois State Water Survey, Champaign, IL 61820, USA
6US Atmospheric Research & Analysis Inc., Cary, NC 27513, USA
7Lake Michigan Air Directors Consortium, Rosemont, IL 60018, USA
8Department of Biostatistics, University of Iowa, Iowa City, IA 52242, USA
9Department of Atmospheric Sciences, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA
10Electric Power Research Institute, Palo Alto, CA 94304, USA
11University of Iowa Policy Center, Iowa City, IA 52242, USA
*now at: Doosan Heavy Industries & Construction Co., LTD, Seoul, Korea
Received: 04 Apr 2012 – Discussion started: 06 Jun 2012
Abstract. An overview of the LADCO (Lake Michigan Air Directors Consortium) Winter Nitrate Study (WNS) is presented. Sampling was conducted at ground level at an urban-rural pair of sites during January–March 2009 in eastern Wisconsin, toward the western edge of the US Great Lakes region. Areas surrounding these sites experience multiday episodes of wintertime PM2.5 pollution characterized by high fractions of ammonium nitrate in PM, low wind speeds, and air mass stagnation. Hourly surface monitoring of inorganic gases and aerosols supplemented long-term 24-h aerosol chemistry monitoring at these locations. The urban site (Milwaukee, WI) experienced 13 PM2.5 episodes, defined as periods where the seven-hour moving average PM2.5 concentration exceeded 27 μg m−3 for at least four consecutive hours. The rural site experienced seven episodes by the same metric, and all rural episodes coincided with urban episodes. Episodes were characterized by low pressure systems, shallow/stable boundary layer, light winds, and increased temperature and relative humidity relative to climatological mean conditions. They often occurred in the presence of regional snow cover at temperatures near freezing, when snow melt and sublimation could generate fog and strengthen the boundary layer inversion. Substantial contribution to nitrate production from nighttime chemistry of ozone and NO2 to N2O5 and nitric acid is likely and requires further investigation. Pollutant-specific urban excess during episode and non-episode conditions is presented. The largest remaining uncertainties in the conceptual model of the wintertime episodes are the variability from episode-to-episode in ammonia emissions, the balance of daytime and nighttime nitrate production, the relationship between ammonia controls, NOx controls and ammonium nitrate reductions, and the extent to which snow and fog are causal (either through meteorological or chemical processes) rather than just correlated with episodes because of similar synoptic meteorology.
Revised: 05 Oct 2012 – Accepted: 24 Oct 2012 – Published: 22 Nov 2012
Stanier, C., Singh, A., Adamski, W., Baek, J., Caughey, M., Carmichael, G., Edgerton, E., Kenski, D., Koerber, M., Oleson, J., Rohlf, T., Lee, S. R., Riemer, N., Shaw, S., Sousan, S., and Spak, S. N.: 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, Atmos. Chem. Phys., 12, 11037-11056, doi:10.5194/acp-12-11037-2012, 2012.