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

   https://doi.org/10.5194/acp-18-17259-2018  

   Franchin, Alessandro ; Fibiger, Dorothy L. ; Goldberger, Lexie ; McDuffie, Erin E. ; Moravek, Alexander ; Womack, Caroline C. ; Crosman, Erik T. ; Docherty, Kenneth S. ; Dube, William P. ; Hoch, Sebastian W. ; et al ( January 2018 , Atmospheric Chemistry and Physics)

   Abstract. Airborne and ground-based measurements of aerosol concentrations, chemicalcomposition, and gas-phase precursors were obtained in three valleys innorthern Utah (USA). The measurements were part of the Utah Winter FineParticulate Study (UWFPS) that took place in January–February 2017. Totalaerosol mass concentrations of PM₁ were measured from a Twin Otteraircraft, with an aerosol mass spectrometer (AMS). PM₁ concentrationsranged from less than 2µgm⁻³ during clean periods to over100µgm⁻³ during the most polluted episodes, consistent withPM_2.5 total mass concentrations measured concurrently at groundsites. Across the entire region, increases in total aerosol mass above∼2µgm⁻³ were associated with increases in theammonium nitrate mass fraction, clearly indicating that the highest aerosolmass loadings in the region were predominantly attributable to an increase inammonium nitrate. The chemical composition was regionally homogenous fortotal aerosol mass concentrations above 17.5µgm⁻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 regionshowed variable concentrations with height in the polluted boundary layer.Higher average mass concentrations were observed within themore »first few hundredmeters above ground level in all three valleys during pollution episodes. Gas-phase measurements of nitric acid (HNO₃) and ammonia (NH₃) duringthe pollution episodes revealed that in the Cache and Utah valleys, partitioningof inorganic semi-volatiles to the aerosol phase was usually limited by theamount of gas-phase nitric acid, with NH₃ being in excess. The inorganicspecies were compared with the ISORROPIA thermodynamic model. Total inorganicaerosol mass concentrations were calculated for various decreases in totalnitrate and total ammonium. For pollution episodes, our simulations of a50% decrease in total nitrate lead to a 46±3% decrease in totalPM₁ mass. A simulated 50% decrease in total ammonium leads to a36±17%µgm⁻³ decrease in total PM₁ mass, over the entirearea of the study. Despite some differences among locations, ourresults showed a higher sensitivity to decreasing nitric acid concentrationsand the importance of ammonia at the lowest total nitrate conditions. In theSalt Lake Valley, both HNO₃ and NH₃ concentrations controlledaerosol formation.

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2. The role of chlorine in global tropospheric chemistry

   Wang, X. ( January 2019 , Atmospheric chemistry and physics)

   We present a comprehensive simulation of tropospheric chlorine within the GEOS-Chem global 3-D model of oxidant–aerosol–halogen atmospheric chemistry. The simulation includes explicit accounting of chloride mobilization from sea salt aerosol by acid displacement of HCl and by other heterogeneous processes. Additional small sources of tropospheric chlorine (combustion, organochlorines, transport from stratosphere) are also included. Reactive gas-phase chlorine Cl*, including Cl, ClO, Cl2, BrCl, ICl, HOCl, ClNO3, ClNO2, and minor species, is produced by the HCl+OH reaction and by heterogeneous conversion of sea salt aerosol chloride to BrCl, ClNO2, Cl2, and ICl. The model successfully simulates the observed mixing ratios of HCl in marine air (highest at northern midlatitudes) and the associated HNO3 decrease from acid displacement. It captures the high ClNO2 mixing ratios observed in continental surface air at night and attributes the chlorine to HCl volatilized from sea salt aerosol and transported inland following uptake by fine aerosol. The model successfully simulates the vertical profiles of HCl measured from aircraft, where enhancements in the continental boundary layer can again be largely explained by transport inland of the marine source. It does not reproduce the boundary layer Cl2 mixing ratios measured in the WINTER aircraft campaign (1–5 ppt in the daytime,more »low at night); the model is too high at night, which could be due to uncertainty in the rate of the ClNO2+Cl− reaction, but we have no explanation for the high observed Cl2 in daytime. The global mean tropospheric concentration of Cl atoms in the model is 620 cm−3 and contributes 1.0 % of the global oxidation of methane, 20 % of ethane, 14 % of propane, and 4 % of methanol. Chlorine chemistry increases global mean tropospheric BrO by 85 %, mainly through the HOBr+Cl− reaction, and decreases global burdens of tropospheric ozone by 7 % and OH by 3 % through the associated bromine radical chemistry. ClNO2 chemistry drives increases in ozone of up to 8 ppb over polluted continents in winter.« less
3. African smoke particles act as cloud condensation nuclei in the wintertime tropical North Atlantic boundary layer over Barbados

   https://doi.org/10.17604/drt6-ys34  

   Royer, Haley ; Pöhlker, Mira ; Krueger, Ovid Oktavian ; Blades, Edmund ; Sealy, Peter ; Lata, Nurun Nahar ; Cheng, Zezhen ; China, Swarup ; Ault, Andrew ; Quinn, Patricia ; et al ( January 2023 )

   Abstract

   The number concentration and properties of aerosol particles serving as cloud condensation nuclei (CCN) are important for understanding cloud properties, including in the tropical Atlantic marine boundary layer (MBL), where marine cumulus clouds reflect incoming solar radiation and obscure the low-albedo ocean surface. Studies linking aerosol source, composition, and water uptake properties in this region have been conducted primarily during the summertime dust transport season, despite the region receiving a variety of aerosol particle types throughout the year. In this study, we compare size-resolved aerosol chemical composition data to the hygrocopicity parameter κ derived from size-resolved CCN measurements made during the Elucidating the Role of Clouds-Circulation Coupling in Climate (EUREC4A) and Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) campaigns from January to February 2020. We observed unexpected periods of wintertime long-range transport of African smoke and dust to Barbados. During these periods, the accumulation-mode aerosol particle and CCN Number concentrations as well as the proportions of dust and smoke particles increased, whereas average κ slightly decreased (κ = 0.46 +/- 0.10) from marine background conditions (κ = 0.52 +/- 0.09) when the particles were mostly composed of marine organics and sulfate. Size-resolved chemical analysis shows that smoke particles were the major contributor to the accumulation mode during long-range transport events, indicating that smoke is mainly responsible for the observed increase in CCN number concentrations. Earlier studies conducted at Barbados have mostly focused on the role of dust in CCN, but our results show that aerosol hygroscopicity and CCN number concentrations during wintertime long-range transport events over the tropical North Atlantic are also affected by African smoke. Our findings highlight the importance of African smoke for atmospheric processes and cloud formation over the Caribbean.
   

   Technical Information

   In the file “Dust_Mass_Conc_Royer2022” dust mass concentrations in grams per meter^3 are provided for each day of sampling. These data were used to generate Figure 2a in the manuscript. The file “Particle_Type_#fract_Royer2022” contains data obtained through CCSEM/EDX analysis and used to generate the temporal chemistry plot (Figure 4) provided in the manuscript. The data contains particle numbers for each particle type identified on stage 3 of the sampler, total particle numbers analyzed for the entire stage 3 sample, as well as particle number fractions in % values. In the file “Size-resolved_chem_Royer2022” we provide particle # and number fraction (%) values used to generate size-resolved chemistry plots in the manuscript (Figures 5a and 5b). The file includes all particle numbers and number fractions for sea salt, aged sea salt, dust+sea salt, dust, dust+smoke, smoke, sulfate, and organic particles in each size bin from 0.1 through 8.058 um during cumulative clean marine periods and CAT Event 1 as described in the manuscript. The file “K_at_0.16S_Royer2022” contains κ values calculated at 0.16% supersaturation (S) throughout the entire sampling period. These data were specifically used to generate the plot in Figure 7a. The file “CCN#_at_0.16S_Royer2022” contains cloud condensation nuclei (CCN) values calculated at 0.16% supersaturation (S) throughout the entire sampling period. These data were used to create the CCN portion of the plot in Figure 7b.
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4. Modeling the sources and transport processes during extreme ammonia episodes in the US Corn Belt

   https://doi.org/doi.org/10.1029/2019JD031207  

   Hu, C ; Griffis, T.J. ; Baker, J.M. ; Wood, J.D. ; Millet, D.B. ; Lee, X. ( January 2020 , Journal of geophysical research)

   Atmospheric ammonia (NH3) is the primary form of reactive nitrogen (Nr) and a precursor ofammonium (NH4+) aerosols. Ammonia has been linked to adverse impacts on human health, the loss ofecosystem biodiversity, and plays a key role in aerosol radiative forcing. The midwestern United States is themajor NH3source in North America because of dense livestock operations and the high use of syntheticnitrogen fertilizers. Here, we combine tall‐tower (100 m) observations in Minnesota and Weather Researchand Forecasting model coupled with Chemistry (WRF‐Chem) modeling to investigate high and low NH3emission episodes within the U.S. Corn Belt to improve our understanding of the distribution of emissionsources and transport processes. We examined observations and performed model simulations for cases inFebruary through November of 2017 and 2018. The results showed the following: (1) Peak emissions inNovember 2017 were enhanced by above‐normal air temperatures, implying aQ10(i.e., the change in NH3emissions for a temperature increase of 10°C) of 2.5 for emissions. (2) The intensive livestock emissionsrom northern Iowa, approximately 400 km away from the tall tower, accounted for 17.6% of theabundance in tall‐tower NH3mixing ratios. (3) Ammonia mixing ratios in the innermost domain 3frequently (i.e., 336 hr, 48% of November 2017) exceeded 5.3 ppb, an important air qualitymore »health standard.(4) In November 2017, simulated NH3net ecosystem exchange (the difference between NH3emissionsand dry deposition) accounted for 60–65% of gross NH3emissions for agricultural areas and was2.8–3.1 times the emissions of forested areas. (5) We estimated a mean annual NH3net ecosystem exchangeof 1.60 ± 0.06 nmol · m−2·s−1for agricultural lands and−0.07 ± 0.02 nmol · m−2·s−1for forested lands.These results imply that future warmer fall temperatures will enhance agricultural NH3emissions, increasethe frequency of dangerous NH3episodes, and enhance dry NH3deposition in adjacent forested lands.« less
5. Urban inland wintertime N₂O₅ and ClNO₂ influenced by snow-covered ground, air turbulence, and precipitation

   https://doi.org/10.5194/acp-22-2553-2022  

   Kulju, Kathryn D. ; McNamara, Stephen M. ; Chen, Qianjie ; Kenagy, Hannah S. ; Edebeli, Jacinta ; Fuentes, Jose D. ; Bertman, Steven B. ; Pratt, Kerri A. ( January 2022 , Atmospheric Chemistry and Physics)

   Abstract. The atmospheric multiphase reaction of dinitrogenpentoxide (N2O5) with chloride-containing aerosol particlesproduces nitryl chloride (ClNO2), which has been observed across theglobe. The photolysis of ClNO2 produces chlorine radicals and nitrogendioxide (NO2), which alter pollutant fates and air quality. However,the effects of local meteorology on near-surface ClNO2 production arenot yet well understood, as most observational and modeling studies focus onperiods of clear conditions. During a field campaign in Kalamazoo, Michigan,from January–February 2018, N2O5 and ClNO2 were measuredusing chemical ionization mass spectrometry, with simultaneous measurementsof atmospheric particulate matter and meteorological parameters. We examinethe impacts of atmospheric turbulence, precipitation (snow, rain) and fog,and ground cover (snow-covered and bare ground) on the abundances ofClNO2 and N2O5. N2O5 mole ratios were lowest duringperiods of lower turbulence and were not statistically significantlydifferent between snow-covered and bare ground. In contrast, ClNO2 moleratios were highest, on average, over snow-covered ground, due to salinesnowpack ClNO2 production. Both N2O5 and ClNO2 moleratios were lowest, on average, during rainfall and fog because ofscavenging, with N2O5 scavenging by fog droplets likelycontributing to observed increased particulate nitrate concentrations. Theseobservations, specifically those during active precipitation and withsnow-covered ground, highlight important processes, including N2O5and ClNO2 wet scavenging, fog nitrate production, and snowpackClNO2 production, that govern themore »variability in observed atmosphericchlorine and nitrogen chemistry and are missed when considering only clearconditions.« less