Articles | Volume 12, issue 22
Atmos. Chem. Phys., 12, 11037–11056, 2012
Atmos. Chem. Phys., 12, 11037–11056, 2012

Research article 22 Nov 2012

Research article | 22 Nov 2012

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

C. Stanier1,2,3, A. Singh1,2,3, W. Adamski4, J. Baek2,3, M. Caughey5, G. Carmichael1,3, E. Edgerton6, D. Kenski7, M. Koerber7, J. Oleson8, T. Rohlf1, S. R. Lee2,*, N. Riemer9, S. Shaw10, S. Sousan1,2,3, and S. N. Spak3,11 C. Stanier et al.
  • 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

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.

Final-revised paper