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Wintertime ammonium nitrate (NH 4 NO 3) pollution events burden urban mountain basins around the globe. In the Salt Lake Valley of Utah in the United States, such pollution events are often driven by the formation of persistent cold-air pools (PCAPs) that trap emissions near the surface for several consecutive days. As a result, secondary pollutants including fine particulate matter less than 2.5 µ m in diameter (PM 2.5), largely in the form of NH 4 NO 3 , build up during these events and lead to severe haze. As part of an extensive measurement campaign to understand the chemical processes underlying PM 2.5 formation, the 2017 Utah Winter Fine Particulate Study, water-soluble trace gases and PM 2.5 constituents were continuously monitored using the ambient ion monitoring ion chromatograph (AIM-IC) system at the University of Utah campus. Gas-phase NH 3 , HNO 3 , HCl, and SO 2 along with particulate NH 4+ , Na + , K + , Mg 2+ , Ca 2+ , NO 3- , Cl - , and SO 42- were measured from 21 January to 21 February 2017. During the two PCAP events captured, the fine particulate matter was dominated by secondary NH 4 NO 3. The comparison of total nitrate (HNO 3 + PM 2.5 NO 3-) and total NH x (NH 3 + PM 2.5 NH 4+) showed NH x was in excess during both pollution events. However, chemical composition analysis of the snowpack during the first PCAP event revealed that the total concentration of deposited NO 3- was nearly 3 times greater than that of deposited NH 4+. Daily snow composition measurements showed a strong correlation between NO 3- and Ca 2+ in the snowpack. The presence of non-volatile salts (Na + , Ca 2+ , and Mg 2+), which are frequently associated with coarse-mode dust, was also detected in PM 2.5 by the AIM-IC during the two PCAP events, accounting for roughly 5 % of total mass loading. The presence of a significant particle mass and surface area in the coarse mode during the first PCAP event was indicated by size-resolved particle measurements from an aerodynamic particle sizer. Taken together, these observations imply that atmospheric measurements of the gas-phase and fine-mode particle nitrate may not represent the total burden of nitrate in the atmosphere, implying a potentially significant role for uptake by coarse-mode dust. Using the NO 3- : NH 4+ ratio observed in the snowpack to estimate the proportion of atmospheric nitrate present in the coarse mode, we estimate that the amount of secondary NH 4 NO 3 could double in the absence of the coarse-mode sink. The underestimation of total nitrate indicates an incomplete account of the total oxidant production during PCAP events. The ability of coarse particles to permanently remove HNO 3 and influence PM 2.5 formation is discussed using information about particle composition and size distribution. [ABSTRACT FROM AUTHOR]

Wintertime ammonium nitrate (NH4NO3) pollution events burden urban mountain basins around the globe. In the Salt Lake Valley of Utah in the United States, such pollution events are often driven by the formation of persistent cold air pools (PCAP) that trap emissions near the surface for several consecutive days. As a result, secondary pollutants including fine particulate matter less than 2.5 μm in diameter (PM2.5), largely in the form of NH4NO3, build up during these events and lead to severe haze. As part of an extensive measurement campaign to understand the chemical processes underlying PM2.5 formation, the 2017 Utah Winter Fine Particulate Study, water-soluble trace gases and PM2.5 constituents were continuously monitored using the Ambient Ion Monitoring Ion Chromatograph system (AIM-IC) at the University of Utah campus. Gas phase NH3, HNO3, HCl and SO2 along with particulate NH4+, Na+, K+, Mg2+, Ca2+, NO3-, Cl-, and SO42- were measured from January 21 to February 21, 2017. During the two PCAP events captured, the fine particulate matter was dominated by secondary NH4NO3. The comparison of total nitrate (HNO3 + PM2.5 NO3-) and total NHx (NH3 + PM2.5 NH4+) showed NHx was in excess during both pollution events. However, chemical composition analysis of the snowpack during the first PCAP event revealed that the total concentration of deposited NO3- was nearly three times greater than that of deposited NH4+. Daily snow composition measurements showed a strong correlation between NO3- and Ca2+ in the snowpack. The presence of non-volatile salts (Na+, Ca2+, and Mg2+), which are frequently associated with coarse mode dust, was also detected in PM2.5 by the AIM-IC during the two PCAP events, accounting for roughly 5 % of total mass loading. The presence of a significant particle mass and surface area in the coarse mode during the first PCAP event was indicated by size-resolved particle measurements from an Aerodynamic Particle Sizer. Taken together, these observations imply that atmospheric measurements of the gas phase and fine mode particle nitrate may not represent the total burden of nitrate in the atmosphere, implying a potentially significant role for uptake by coarse mode dust. Using the NO3- : NH4+ ratio observed in the snowpack to estimate the proportion of atmospheric nitrate present in the coarse mode, we estimate that the amount of secondary NH4NO3 could double in the absence of the coarse mode sink. The underestimation of total nitrate indicates an incomplete account of the total oxidant production during PCAP events. The ability of coarse particles to permanently remove HNO3 and influence PM2.5 formation is discussed using information about particle composition and size distribution. [ABSTRACT FROM AUTHOR]

Ammonium-containing aerosols are a major component of wintertime air pollution in many densely populated regions around the world. Especially in mountain basins, the formation of persistent cold-air pools (PCAPs) can enhance particulate matter with diameters less than 2.5 µ m (PM 2.5) to levels above air quality standards. Under these conditions, PM 2.5 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 understanding the emission sources of its gas phase precursor, ammonia (NH3). 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. NH3 was measured using a quantum cascade tunable infrared laser differential absorption spectrometer (QC-TILDAS), while aerosol composition, including particulate ammonium (pNH4), 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 NH3 emission inventory to obtain model-predicted NHx (=NH3+pNH4) enhancements. Enhancements represent the increase in NH3 mixing ratios within the last 24 h due to emissions within the model footprint. Comparison of these NHx enhancements with measured NHx 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 NH3 emissions from livestock. An investigation of inter-valley exchange revealed that transport of NH3 between major valleys was limited and PM 2.5 in Salt Lake Valley (the most densely populated area in Utah) was not significantly impacted by NH3 from the agricultural areas in Cache Valley. We found that in Salt Lake Valley around two thirds of NHx originated within the valley, while about 30 % originated from mobile sources and 60 % from area source emissions in the region. For Cache Valley, a large fraction of NOx potentially leading to PM 2.5 formation may not be locally emitted but mixed in from other counties. [ABSTRACT FROM AUTHOR]

Measurements of the surface–atmosphere exchange of ammonia (NH3) are necessary to study the emission and deposition processes of NH3 from managed and natural ecosystems. The eddy covariance technique, which is the most direct method for trace gas exchange measurements at the ecosystem level, requires trace gas detection at a fast sample frequency and high precision. In the past, the major limitation for measuring NH3 eddy covariance fluxes has been the slow time response of NH3 measurements due to NH3 adsorption on instrument surfaces. While high-frequency attenuation correction methods are used, large uncertainties in these corrections still exist, which are mainly due to the lack of understanding of the processes that govern the time response. We measured NH3 fluxes over a corn crop field using a quantum cascade laser spectrometer (QCL) that enables measurements of NH3 at a 10 Hz measurement frequency. The 5-month measurement period covered a large range of environmental conditions that included both periods of NH3 emission and deposition and allowed us to investigate the time response controlling parameters under field conditions. Without high-frequency loss correction, the median daytime NH3 flux was 8.59 ng m -2 s -1 during emission and - 19.87 ng m -2 s -1 during deposition periods, with a median daytime random flux error of 1.61 ng m -2 s -1. The overall median flux detection limit was 2.15 ng m -2 s -1 , leading to only 11.6 % of valid flux data below the detection limit. From the flux attenuation analysis, we determined a median flux loss of 17 % using the ogive method. No correlations of the flux loss with environmental or analyser parameters (such as humidity or inlet ageing) were found, which was attributed to the uncertainties in the ogive method. Therefore, we propose a new method that simulates the flux loss by using the analyser time response that is determined frequently over the course of the measurement campaign. A correction that uses as a function of the horizontal wind speed and the time response is formulated which accounts for surface ageing and contamination over the course of the experiment. Using this method, the median flux loss was calculated to be 46 %, which was substantially higher than with the ogive method. [ABSTRACT FROM AUTHOR]

Mountain basins in Northern Utah, including the Salt Lake Valley (SLV), suffer from wintertime air pollution events associated with stagnant atmospheric conditions. During these events, fine particulate matter concentrations (PM 2.5) can exceed national ambient air quality standards. Previous studies in the SLV have found that PM 2.5 is primarily composed of ammonium nitrate (NH4NO3), formed from the condensation of gas-phase ammonia (NH3) and nitric acid (HNO3). Additional studies in several western basins, including the SLV, have suggested that production of HNO3 from nocturnal heterogeneous N2O5 uptake is the dominant source of NH4NO3 during winter. The rate of this process, however, remains poorly quantified, in part due to limited vertical measurements above the surface, where this chemistry is most active. The 2017 Utah Winter Fine Particulate Study (UWFPS) provided the first aircraft measurements of detailed chemical composition during wintertime pollution events in the SLV. Coupled with ground-based observations, analyses of day- and nighttime research flights confirm that PM 2.5 during wintertime pollution events is principally composed of NH4NO3 , limited by HNO3. Here, observations and box model analyses assess the contribution of N2O5 uptake to nitrate aerosol during pollution events using the NO3- production rate, N2O5 heterogeneous uptake coefficient (γ(N2O5)), and production yield of ClNO2 (φ(ClNO2)), which had medians of 1.6 µ g m -3 h -1 , 0.076, and 0.220, respectively. While fit values of γ(N2O5) may be biased high by a potential under-measurement in aerosol surface area, other fit quantities are unaffected. Lastly, additional model simulations suggest nocturnal N2O5 uptake produces between 2.4 and 3.9 µ g m -3 of nitrate per day when considering the possible effects of dilution. This nocturnal production is sufficient to account for 52 %–85 % of the daily observed surface-level buildup of aerosol nitrate, though accurate quantification is dependent on modeled dilution, mixing processes, and photochemistry. [ABSTRACT FROM AUTHOR]

Summertime Arctic aerosol size distributions are strongly controlled by natural regional emissions. Within this context, we use a chemical transport model with size-resolved aerosol microphysics (GEOS-Chem-TOMAS) to interpret measurements of aerosol size distributions from the Canadian Arctic Archipelago during the summer of 2016, as part of the "NETwork on Climate and Aerosols: Addressing key uncertainties in Remote Canadian Environments" (NETCARE) project. Our simulations suggest that condensation of secondary organic aerosol (SOA) from precursor vapors emitted in the Arctic and near Arctic marine (ice-free seawater) regions plays a key role in particle growth events that shape the aerosol size distributions observed at Alert (82.5 ∘ N, 62.3 ∘ W), Eureka (80.1 ∘ N, 86.4 ∘ W), and along a NETCARE ship track within the Archipelago. We refer to this SOA as Arctic marine SOA (AMSOA) to reflect the Arctic marine-based and likely biogenic sources for the precursors of the condensing organic vapors. AMSOA from a simulated flux (500 µgm-2day-1 , north of 50 ∘ N) of precursor vapors (with an assumed yield of unity) reduces the summertime particle size distribution model–observation mean fractional error 2- to 4-fold, relative to a simulation without this AMSOA. Particle growth due to the condensable organic vapor flux contributes strongly (30 %–50 %) to the simulated summertime-mean number of particles with diameters larger than 20 nm in the study region. This growth couples with ternary particle nucleation (sulfuric acid, ammonia, and water vapor) and biogenic sulfate condensation to account for more than 90 % of this simulated particle number, which represents a strong biogenic influence. The simulated fit to summertime size-distribution observations is further improved at Eureka and for the ship track by scaling up the nucleation rate by a factor of 100 to account for other particle precursors such as gas-phase iodine and/or amines and/or fragmenting primary particles that could be missing from our simulations. Additionally, the fits to the observed size distributions and total aerosol number concentrations for particles larger than 4 nm improve with the assumption that the AMSOA contains semi-volatile species: the model–observation mean fractional error is reduced 2- to 3-fold for the Alert and ship track size distributions. AMSOA accounts for about half of the simulated particle surface area and volume distributions in the summertime Canadian Arctic Archipelago, with climate-relevant simulated summertime pan-Arctic-mean top-of-the-atmosphere aerosol direct (-0.04 Wm-2) and cloud-albedo indirect (-0.4 Wm-2) radiative effects, which due to uncertainties are viewed as an order of magnitude estimate. Future work should focus on further understanding summertime Arctic sources of AMSOA. [ABSTRACT FROM AUTHOR]

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 µ g m -3 during clean periods to over 100 µ g 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 µ g 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 µ g 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 (HNO3) and ammonia (NH3) 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 NH3 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 % µ g 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 nitrate conditions. In the Salt Lake Valley, both HNO3 and NH3 concentrations controlled aerosol formation. [ABSTRACT FROM AUTHOR]

Summertime Arctic aerosol size distributions are strongly controlled by natural regional emissions. Within this context, we use a chemical transport model with size-resolved aerosol microphysics (GEOS-Chem-TOMAS) to interpret measurements of aerosol size distributions from the Canadian Arctic Archipelago during the summer of 2016, as part of the NETwork on Climate and Aerosols: addressing key uncertainties in Remote Canadian Environments (NETCARE). Our simulations suggest that condensation of secondary organic aerosol (SOA) from precursor vapors emitted in the Arctic and near Arctic marine (open ocean and coastal) regions plays a key role in particle growth events that shape the aerosol size distributions observed at Alert (82.5° N, 62.3° W), Eureka (80.1° N, 86.4° W), and along a NETCARE ship track within the Archipelago. We refer to this SOA as Arctic marine SOA (Arctic MSOA) to reflect the Arctic marine-based and likely biogenic sources for the precursors of the condensing organic vapors. Arctic MSOA from a simulated flux (500 μg m-2 d-1, north of 50° N) of precursor vapors (assumed yield of unity) reduces the summertime particle size distribution model-observation mean fractional error by 2- to 4-fold, relative to a simulation without this Arctic MSOA. Particle growth due to the condensable organic vapor flux contributes strongly (30-50 %) to the simulated summertime-mean number of particles with diameters larger than 20 nm in the study region, and couples with ternary particle nucleation (sulfuric acid, ammonia, and water vapor) and biogenic sulfate condensation to account for more than 90 % of this simulated particle number, a strong biogenic influence. The simulated fit to summertime size-distribution observations is further improved at Eureka and for the ship track by scaling up the nucleation rate by a factor of 100 to account for other particle precursors such as gas-phase iodine and/or amines and/or fragmenting primary particles that could be missing from our simulations. Additionally, the fits to observed size distributions and total aerosol number concentrations for particles larger than 4 nm improve with the assumption that the Arctic MSOA contains semi-volatile species; reducing model-observation mean fractional error by 2- to 3-fold for the Alert and ship track size distributions. Arctic MSOA accounts for more than half of the simulated total particulate organic matter mass concentrations in the summertime Canadian Arctic Archipelago, and this Arctic MSOA has strong simulated summertime pan-Arctic-mean top-of-the-atmosphere aerosol direct (-0.04 W m-2) and cloud-albedo indirect (-0.4 W m-2) radiative effects. Future work should focus on further understanding summertime Arctic sources of Arctic MSOA. [ABSTRACT FROM AUTHOR]