Wintertime Spatial Distribution of Ammonia and its Emission Sources in the Great Salt Lake Region

Abstract

<p><strong>Abstract.</strong> Ammonium-containing aerosols are a major component of winter time air pollution in many densely populated regions around the world. Especially in mountain basins, the formation of persistent cold air pool (PCAP) periods can enhance particulate matter with diameters less than 2.5 μm (PM<sub>2.5</sub>) to levels above air quality standards. Under these conditions, PM<sub>2.5</sub> in the Great Salt Lake Region of northern Utah has been shown to be primarily composed of ammonium nitrate, however, its formation processes and sources of its precursors are not fully understood. Hence, it is key to understand the emission sources of its gas-phase precursor, ammonia (NH<sub>3</sub>). To investigate the formation of ammonium nitrate, a suite of trace gases and aerosol composition were sampled from the NOAA Twin Otter aircraft during the Utah Winter Fine Particulate Study (UWFPS) in January and February 2017. NH<sub>3</sub> was measured using a Quantum Cascade Tunable Infrared Laser Differential Absorption Spectrometer (QC-TILDAS), while aerosol composition, including particulate ammonium (pNH<sub>4</sub>), was measured with an aerosol mass spectrometer (AMS). The origin of the sampled air masses was investigated using the Stochastic Time-Inverted Lagrangian Transport (STILT) model and combined with an NH<sub>3</sub> emission inventory to obtain model-predicted NH<sub>x</sub> (= NH<sub>3</sub> + pNH<sub>4</sub>) enhancements. Comparison of these NH<sub>x</sub> enhancements with measured NH<sub>x</sub> from the Twin Otter shows that modelled values are a factor of 1.6 to 4.4 lower for the three major valleys in the region. Among these, the underestimation is largest for Cache Valley, an area with intensive agricultural activities. We find that one explanation for the underestimation of wintertime emissions may be the seasonality factors applied to NH<sub>3</sub> emissions from livestock. An investigation of inter-valley exchange revealed that transport of NH<sub>3</sub> between major valleys was limited and PM<sub>2.5</sub> in Salt Lake Valley (the most densely populated area in Utah) was not significantly impacted by NH<sub>3</sub> from the agricultural areas in Cache Valley. We found that in Salt Lake Valley around two thirds of NH<sub>x</sub> originated within the valley, while about 30 % originated from mobile source and 60 % from area source emissions in the region. For Cache Valley, a large fraction of NO<sub>x</sub> potentially leading to PM<sub>2.5</sub> formation may not be locally emitted but mixed in from other counties.</p>

The Role of Coarse Aerosol Particles as a Sink of HNO<sub>3</sub> in Wintertime Pollution Events in the Salt Lake Valley

“…CC BY 4.0 License. using a QC-TILDAS instrument (Aerodyne Inc, MA, USA) and are described in Moravek et al, 2019. Based on aircraft data (Franchin et al, 2018) and AIM-IC measurements at the ground site, the sum of nitrate in the gas phase and particle phases, or total NO3 (TNO3) per m 3 of ambient air, is less than the sum of ammonia in the gas phase and particle phases, total NHx per m 3 of ambient air.…”

“…Aircraft measurements during the 2017 Utah Winter Fine Particulate Study (UWFPS) provided the first detailed vertically-resolved chemical composition of these PCAP episodes in the SLV. Analysis presented in McDuffie et al (2019) confirms that the formation of NH4NO3 during these pollution events is largely limited by HNO3, but with more frequent periods approaching NH3-limited conditions in the upper boundary layer over time (McDuffie et al, 2019). Regimes of both NH3-limited and HNO3-limited conditions are also consistent with an aerosol thermodynamic model sensitivity study of the same data by Franchin et al (2018), who found simulations of non-refractory PM1 mass were sensitive to reductions in both total ammonium and total nitrate (Franchin et al, 2018).…”

“…Regimes of both NH3-limited and HNO3-limited conditions are also consistent with an aerosol thermodynamic model sensitivity study of the same data by Franchin et al (2018), who found simulations of non-refractory PM1 mass were sensitive to reductions in both total ammonium and total nitrate (Franchin et al, 2018). Box model studies of the odd oxygen budget during UWFPS by Womack et al (2019) assumed that NH4NO3 formation during PCAP episodes is HNO3-limited, and found full episodes tend to be NOx-saturated.…”

“…The balance of factors influencing HNO3 formation are still being understood. Modelling results from both McDuffie et al (2019) and Womack et al (2019) predict that up to ~ 50 % of HNO3 production is from the heterogeneous N2O5 pathway in the residual layer. This is in line with ground-based measurements of N2O5 by Baasandorj et al (2017) highlighting the importance of nighttime pathway (Baasandorj et al, 2017).…”

Review of amt-2019-193 by Moravek et al.

“…Printer-friendly version Discussion paper I recommend the authors attend to a few comments in revising the manuscript: Page 2, Line 5: In addition to Europe, the Great Salt Lake Region also experienced an ammonium salts rich condition, as suggested by another paper of the authors (Moravek et al, 2019). This is also the case in North China (Li et al, 2019).…”