Overview of the LADCO winter nitrate study: hourly ammonia, nitric acid and PM<sub>2.5</sub> composition at an urban and rural site pair during PM<sub>2.5</sub> episodes in the US Great Lakes region

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 PM_2.5 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 PM_2.5 episodes, defined as periods where the seven-hour moving average PM_2.5 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, NO_x 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

Composition and sources of fine particulate matter across urban and rural sites in the Midwestern United States

“…It is well-established that winter conditions (including long nights, lower temperatures and boundary layer height, and higher humidity) promote the formation of secondary NH 4 NO 3 . 43,44 The summertime minimum may be partially enhanced to negative sampling artifacts whereby ammonium nitrate volatilizes as ammonia and nitric acid from Teflon filters during periods of elevated temperatures. 45 These seasonal variations of NH 4 + and NO 3 − are consistent with a greater trend across the US.…”

“…5b) when low temperatures and longer nights promote the formation of secondary NH 4 NO 3 . 43, 44 This factor accounted for 17–24% of PM 2.5 mass at the monitoring sites with maximum contributions occurring in winter (Table 2). A minor rural enhancement (1.03) in the secondary nitrate PMF factor was detected in PMF model results, but is not consistent with the urban enhancement in ambient nitrate concentrations.…”

A comparison between 2010 and 2006 air quality and meteorological conditions, and emissions and boundary conditions used in simulations of the AQMEII-2 North American domain

“…Particulate NH 4 increased at all but one site and increases in NO 3 are closely correlated with the increases in NH 4 (r 2 ¼ 0.84) although the regression fit shows a 2.3 mg/m 3 increase in NO 3 for each 1 mg/ m 3 increase in NH 4 , well below the 3.4:1 NO 3 /NH 4 molar mass ratio for NH 4 NO 3 . Stanier et al (2012) studied winter NO 3 episodes in Wisconsin and identified a relationship between elevated NO 3 , snow cover and near freezing temperatures which promote fog formation and stabilization of the boundary layer. While the causality chain resulting in the winter NO 3 episodes remains uncertain, it is interesting to note that the 2009e2010 winter was exceptionally snowy in many parts of the eastern U.S., including the Midwest (NCDC, 2010), thus suggesting the conditions found by Stanier et al to be favorable for elevated NO 3 concentrations may have been more prevalent in 2010.…”

Nitrogen, Aerosol Composition, and Halogens on a Tall Tower (NACHTT): Overview of a wintertime air chemistry field study in the front range urban corridor of Colorado

“…Although a number of important issues are specific to polluted wintertime environments, detailed wintertime intensive field studies are less common. Accumulation of aerosol mass in excess of National Ambient Air Quality Standards (NAAQS, currently 35 µg m –3 for PM 2.5, particulate matter (PM) less than 2.5 µm diameter, 24 h average), particularly ammonium nitrate aerosols, tends to be a winter phenomenon due to the temperature‐dependent phase partitioning of nitric acid and ammonia [ Chen et al ., ; Chen et al ., ; Fischer and Talbot , ; Katzman et al ., ; Mathur et al ., ; Schaap et al ., ; Stanier et al ., ; Tolocka et al ., ; Yu et al ., ]. Air quality models often fail to accurately predict nitrate aerosol, due to the complexity of both the chemistry and the shallow boundary layers typically associated with these episodes.…”