Explaining the Excess Nitrate Observed in PM2.5 during Wintertime Conditions in Idaho’s Treasure Valley

GEORGE R. MWANIKI (1), Chelsea Rosenkrance (1), Shelley N. Pressley (1), H. William Wallace (1), B. Thomas Jobson (1), and Timothy M. VanReken (1)

(1) Washington State University, Pullman

     Abstract Number: 316
     Last modified: May 6, 2010

Abstract
The Treasure Valley extends from the low-lying areas of southeast Oregon into southern Idaho; the valley is home to significant agricultural activity as well as the three largest cities in Idaho. The valley generally experiences its highest PM2.5 concentrations during winter periods, when the weather is characterized by low temperatures and high relative humidities. Occasionally the PM2.5 problem is aggravated by stagnation events that prevent atmospheric mixing and lead to a buildup of pollutant concentrations. In an effort to better understand the major contributors to PM during wintertime stagnation conditions, the Idaho Department of Environmental Quality commissioned the Treasure Valley PM2.5 Precursor Study.

During the two-month study, the meteorological conditions, trace gas concentrations, and aerosol physical and chemical conditions within the valley were characterized in detail. The study period was characterized by low temperatures and high relative humidity, and included a stagnation event that spanned from January 15-28, 2009. Samples of PM2.5 were collected using a Particle Into Liquid Sampler (PILS) and subsequently analyzed using ion chromatographs and a total organic carbon analyzer. In these samples we observed strong variations in the particulate nitrate concentration, including periods where the nitrate significantly exceeded the available ammonium. These variations have been found to be dependent on the relative humidity and the concentration of water-soluble organic compounds (WSOC). From these results, we conclude that the excess nitrate is driven by three factors: 1) the heterogeneous conversion of NO2 to N2O5 and then HNO3; 2) the role of organic aerosol components in inhibiting N2O5 heterogeneous hydrolysis; and 3) the shift in HNO3 partitioning toward the condensed phase during cold conditions.