Responses of Secondary Inorganic PM2.5 to Precursor Gases in an Ammonia Abundant Area In North Carolina

Bin Cheng,Lingjuan Wang-Li

doi:10.4209/aaqr.2018.10.0384

Abstract

ABSTRACT Secondary inorganic fine particulate matter (iPM2.5) constitutes a significant amount of the atmospheric PM2.5. The formation of secondary iPM2.5 is characterized by thermodynamic equilibrium gas-particle partitioning of gaseous ammonia (NH3) and aerosol ammonium (NH4+). To develop effective strategies for controlling atmospheric PM2.5, it is essential to understand the responses of secondary iPM2.5 to different precursor gases. In southeastern North Carolina, the amount of NH3 is in excess to fully neutralize acidic gases (i.e., NH3-rich conditions). NH3-rich conditions are mainly attributed to the significant NH3 emissions in the region, especially from the large amounts of animal feeding operation (AFO). To gain a better understanding of the impact of NH3 on the formation of secondary iPM2.5 in this area, the responses of iPM2.5 to precursor gases under different ambient conditions were investigated based upon three-year monitoring data of the chemical components in iPM2.5, gaseous pollutants, and meteorological conditions. The gas ratio (GR) was used to assess the degree of neutralization via NH3, and ISORROPIA II model simulation was used to examine the responses of iPM2.5 to changes in the total NH3, the total sulfuric acid (H2SO4), and the total nitric acid (HNO3). It was discovered that under different ambient temperature and humidity conditions, the responses of iPM2.5 to precursor gases vary. In general, iPM2.5 responds nonlinearly to the total NH3 but linearly to the total H2SO4 and the total HNO3. In NH3-rich regions, iPM2.5 is not sensitive to changes in the total NH3, but it is very sensitive to changes in the total H2SO4 and/or the total HNO3. Reducing the total H2SO4, as opposed to the total HNO3 or the total NH3, leads to a significant reduction in iPM2.5 and is thus a more effective strategy for decreasing the concentration of iPM2.5. This research provides insight into controlling and regulating PM2.5 in NH3-rich regions.

Highlights

Particulate matter (PM) with an aerodynamic equivalent diameter less than or equal to 2.5 μm (i.e., PM2.5) is one of the six criteria air pollutants regulated under National Ambient Air Quality Standards (NAAQS) (U.S EPA, 2015a)
Concentrations are expressed in μg m–3 as equivalent concentrations; total HNO3 concentration is calculated based on 3.63% conversion percentage of NO2 to HNO3
The responses of iPM2.5 to changes in the total NH3, the total HNO3, and the total H2SO4 were simulated by ISORROPIA II based upon three-year measurements of the chemical components in iPM2.5 and gaseous pollutants as well as meteorological conditions

Summary

Introduction

Particulate matter (PM) with an aerodynamic equivalent diameter less than or equal to 2.5 μm (i.e., PM2.5) is one of the six criteria air pollutants regulated under National Ambient Air Quality Standards (NAAQS) (U.S EPA, 2015a). Various chemical components contribute to PM2.5 in different proportions, and the major chemical components of PM2.5 include ammonium (NH4+), sulfate (SO42–), nitrate (NO3–), organic carbon (OC), elemental carbon (EC), elements and other unknown components (Bell et al, 2007). The secondary inorganic PM2.5 (iPM2.5) is formed through chemical reactions between basic and acidic gases (e.g., ammonia [NH3], nitric acid [HNO3] and sulfuric acid [H2SO4]) (Hinds, 1998; Seinfeld and Pandis, 2006). The iPM2.5 mainly consists of NH4+ salts including ammonium nitrate (NH4NO3), ammonium sulfate ((NH4)2SO4), ammonium bisulfate (NH4HSO4) and ammonium chloride (NH4Cl) (Tanner et al, 1979; Tolocka et al, 2001; Walker et al, 2004; Li et al, 2012, 2014a). Ammonia is the major alkaline gas that may react with acidic gases to form iPM2.5 in ambient air, and this process is called gas-particle partitioning of NH3-NH4+. The neutralization degree of NH3 can be characterized by gas ratio (GR), which is in Eq (1) (Ansari and Pandis, 1998): GR (1)

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