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

Abstract

Airborne and ground-based measurements of aerosol concentrations, chemical composition, and gas-phase precursors were obtained in three valleys in northern Utah (USA). The measurements were part of the Utah Winter Fine Particulate Study (UWFPS) that took place in January-February 2017. Total aerosol mass concentrations of PM 1 were measured from a Twin Otter aircraft, with an aerosol mass spectrometer (AMS). PM 1 concentrations ranged from less than 2 µg m −3 during clean periods to over 100 µg m −3 during the most polluted episodes, consistent with PM 2.5 total mass concentrations measured concurrently at ground sites. Across the entire region, increases in total aerosol mass above ∼ 2 µg m −3 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 µg m −3 , with 74 ± 5 % (average ± standard deviation) ammonium nitrate, 18 ± 3 % organic material, 6 ± 3 % ammonium sulfate, and 2±2 % 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 3 ) and ammonia (NH 3 ) during the pollution episodes revealed that in the Cache and Utah valleys, partitioning of inorganic semi-volatiles to the aerosol phase was usually limited by the amount of gas-phase nitric acid, with NH 3 being in excess. The inorganic species were compared with the ISORROPIA thermodynamic model. Total inorganic aerosol mass concentrations were calculated for various decreases in total nitrate and total ammonium. For pollution episodes, our simulations of a 50 % decrease in total nitrate lead to a 46 ± 3 % decrease in total PM 1 mass. A simulated 50 % decrease in total ammonium leads to a 36 ± 17 % µg m −3 decrease in total PM 1 mass, over the entire area of the study. Despite some differences among locations, our results showed a higher sensitivity to decreasing nitric acid concentrations and the importance of ammonia at the lowest total Published by Copernicus Publications on behalf of the European Geosciences Union. 17260A. Franchin et al.: Airborne and ground-based observations of ammonium-nitrate-dominated aerosols nitrate conditions. In the Salt Lake Valley, both HNO 3 and NH 3 concentrations controlled aerosol formation.

An Odd Oxygen Framework for Wintertime Ammonium Nitrate Aerosol Pollution in Urban Areas: NOx and VOC Control as Mitigation Strategies

“…The model predicts this value to be 1.07 in the SLV (Figure S5), so HO x is near its peak efficiency for HNO 3 production, as over 99% of HO x radicals terminate in HNO 3 , rather than an alkyl nitrate or HO x self‐reaction. This observation is also consistent with the low observed aerosol‐phase organic nitrates (Franchin et al, ). NO x serves to quench the HO x ‐NO x cycle, and therefore, reductions in NO x increase the HO x chain length and drive more efficient production of O 3 and HNO 3 .…”

“…The model did not attempt to replicate the partitioning of HNO 3 between the gas and aerosol phase and simply treated the two as the lumped sum HNO 3 + pNO 3 − . Franchin et al () showed this partitioning to be >92% to the aerosol phase during UWFPS PCAP events. The UU site is located at slightly higher elevation on the side of the valley wall and may sample from either the NBL or RL at night at different times during the PCAP (see Text S3.1).…”

“…The Utah Winter Fine Particulate Study (UWFPS) was a 4‐week field campaign in the SLV region during the winter of 2016–2017 designed to observe the chemical processes that form ammonium nitrate aerosol during PCAPs (Baasandorj et al, ; Bares et al, ; Franchin et al, ). Ground sites made continuous surface‐level measurements of trace gases (including O 3 , NO x , and VOCs) and the chemical composition of PM 2.5 , while a Twin Otter light aircraft made periodic measurements aloft with a similar payload.…”

“…The SLV in northern Utah regularly experiences episodes of PM 2.5 pollution (Silcox et al, ), exacerbated by the onset of persistent cold air pools (PCAPs; high‐pressure systems that cap colder air within the basin), which prevent the vertical mixing of pollutants away from the surface (Whiteman et al, ). The trapped gas‐phase pollutants, including NO x and VOCs from anthropogenic sources, undergo complex chemical oxidation reactions, forming nitric acid (HNO 3 ) that reacts with gas‐phase ammonia (NH 3 ) to form ammonium nitrate aerosol (NH 4 NO 3 ), which comprises ~75% of the PM 2.5 composition in the SLV during pollution episodes (Baasandorj et al, ; Franchin et al, ; Kuprov et al, ). …”

“…Therefore, the formation rate of pNO 3 − is mainly limited by the chemical formation rate of nitric acid (HNO 3 ) from oxidation of NO x , which is emitted from fossil fuel combustion. This HNO 3 limitation has led typical western U.S. control strategies to focus on emission reductions of NO x rather than NH 3 (Franchin et al, ; Kelly et al, ; Pusede et al, ). We demonstrate in this study that ammonium nitrate aerosol pollution may be treated in the same manner as O 3 pollution through a common parameter, O x,total , and may be most effectively reduced in the SLV by initially controlling VOC emissions rather than controlling emissions of NH 3 or NO x .…”