Airborne and ground-based observations of ammonium nitrate dominated aerosols in a shallow boundary layer during intense winter pollution episodes in northern Utah

  • Franchin A
  • Fibiger D
  • Goldberger L
  • et al.
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<p><strong>Abstract.</strong> Airborne and ground-based measurements of aerosol concentrations, chemical composition and gas phase precursors were obtained in three valleys in northern Utah (U.S.A.). The measurements were part of the Utah Winter Fine Particulate Study (UWFPS) that took place in January&amp;ndash;February, 2017. Total aerosol mass concentrations of PM<sub>1</sub> were measured from a Twin Otter aircraft, with an Aerosol Mass Spectrometer (AMS). PM<sub>1</sub> concentrations ranged from less than 2<span class="thinspace"></span>μg<span class="thinspace"></span>m<sup>&amp;minus;3</sup> during clean periods to over 100<span class="thinspace"></span>μg<span class="thinspace"></span>m<sup>&amp;minus;3</sup> during the most polluted episodes, consistent with PM<sub>2.5</sub> total mass concentrations measured concurrently at ground sites. Across the entire region, increases in total aerosol mass above ~<span class="thinspace"></span>2<span class="thinspace"></span>μg<span class="thinspace"></span>m<sup>&amp;minus;3</sup> were associated with increases in the ammonium nitrate mass fraction, clearly indicating that the highest aerosol mass loadings in the region were predominantly attributable to an increase in ammonium nitrate. The chemical composition was regionally homogenous for total aerosol mass concentrations above 17.5<span class="thinspace"></span>μg<span class="thinspace"></span>m<sup>&amp;minus;3</sup>, with 74<span class="thinspace"></span>±<span class="thinspace"></span>5<span class="thinspace"></span>% (average ± standard deviation) ammonium nitrate, 18<span class="thinspace"></span>±<span class="thinspace"></span>3<span class="thinspace"></span>% organic material, 6<span class="thinspace"></span>±<span class="thinspace"></span>3<span class="thinspace"></span>% ammonium sulfate, and 2<span class="thinspace"></span>±<span class="thinspace"></span>2<span class="thinspace"></span>% ammonium chloride. Vertical profiles of aerosol mass and volume in the region showed variable concentrations with height in the polluted boundary layer. Higher average mass concentrations were observed within the first few hundred meters above ground level in all three valleys during pollution episodes. Gas phase measurements of nitric acid (HNO<sub>3</sub>) and ammonia (NH<sub>3</sub>) during the pollution episodes revealed that in Cache and Utah Valley, partitioning of inorganic semi-volatiles to the aerosol phase was usually limited by the amount of gas phase nitric acid, with NH<sub>3</sub> being in excess. The inorganic species were compared with the ISORROPIA thermodynamic model. Total inorganic aerosol mass concentrations were calculated for various decreases of total nitrate and total ammonium. For pollution episodes, our simulations of a 50<span class="thinspace"></span>% decrease in total nitrate lead to a 46<span class="thinspace"></span>±<span class="thinspace"></span>3<span class="thinspace"></span>% decrease in total PM<sub>1</sub> mass. A simulated 50<span class="thinspace"></span>% decrease in total ammonium lead to a 36<span class="thinspace"></span>±<span class="thinspace"></span>17%<span class="thinspace"></span>µg<span class="thinspace"></span>m<sup>&amp;minus;3</sup> in total PM<sub>1</sub> mass, over the entire area of the study. Despite some differences among different locations, our results also showed a higher sensitivity to decreasing nitric acid concentrations and the importance of ammonia at the lowest total nitrate conditions. In the Salt Lake Valley, both HNO<sub>3</sub> and NH<sub>3</sub> concentrations controlled aerosol formation.</p>




Franchin, A., Fibiger, D. L., Goldberger, L., McDuffie, E. E., Moravek, A., Womack, C. C., … Middlebrook, A. M. (2018). Airborne and ground-based observations of ammonium nitrate dominated aerosols in a shallow boundary layer during intense winter pollution episodes in northern Utah. Atmospheric Chemistry and Physics Discussions, 1–32.

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