No abstract

Nitryl chloride (ClNO2) is formed in urban areas from the multiphase reaction of dinitrogen pentoxide (N2O5) on chloride-containing surfaces. ClNO2 undergoes photolysis to produce atomic chlorine (Cl•), a strong atmospheric oxidant. While previous ClNO2 studies have focused on atmospheric particulate chloride, the saline snowpack in locations impacted by sea spray and road salt usage represents an additional, potentially large, source of ClNO2. Here, we present the first modeling study to explore the production of ClNO2 from the inland urban snowpack. The coupled snowpack-atmospheric one-dimensional model is constrained to and evaluated by an array of ambient measurements in Ann Arbor, Michigan, during February 2016. The model predicts strong N2O5 deposition onto the snowpack, with ClNO2 formation and release to the atmosphere at low temperatures (<∼260 K). However, at higher temperatures (>∼270 K), the ClNO2 yield is low (e.g., 10%), with ClNO2 undergoing hydrolysis on the snow grains, making the snowpack a net sink for ClNO2. These results motivate measurements to quantify ClNO2 production from the urban snowpack because of potential broader impacts on atmospheric composition and air quality.

A fundamental understanding of the processes that control Antarctic aerosols is necessary in determining the aerosol impacts on climate-relevant processes from Antarctic ice cores to clouds. The first in situ observational online composition measurements by an aerosol mass spectrometer (AMS) of Antarctic aerosols were only recently performed during the Two-Season Ozone Depletion and Interaction with Aerosols Campaign (2ODIAC). 2ODIAC was deployed to sea ice on the Ross Sea near McMurdo Station over two field seasons: austral spring-summer 2014 and winter-spring 2015. The results presented here focus on the overall trends in aerosol composition primarily as functions of air masses and local meteorological conditions. The results suggest that the impact of long-range air mass back trajectories on either the absolute or relative concentrations of the aerosol constituents measured by (and inferred from) an AMS at a coastal location is small relative to the impact of local meteorology. However, when the data are parsed by wind speed, two observations become clear. First, a critical wind speed is required to loft snow from the surface, which, in turn, increases particle counts in all measured size bins. Second, elevated wind speeds showed increased aerosol chloride and sodium. Further inspection of the AMS data shows that the increased chloride concentrations have more of a "fast-vaporizing" nature than chloride measured at low wind speed. Also presented are the Cl : Na ratios of snow samples and aerosol filter samples, as measured by ion chromatogra-phy, as well as non-chloride aerosol constituents measured by the AMS. Additionally, submicron aerosol iodine and bromine concentrations as functions of wind speed are also presented. The results presented here suggest that aerosol composition in coastal Antarctica is a strong function of wind speed and that the mechanisms determining aerosol composition are likely linked to blowing snow.

The acidity of atmospheric aerosols is a critical property that affects the chemistry and composition of the atmosphere. Many key multiphase chemical reactions are pH-dependent, impacting processes like secondary organic aerosol formation, and need to be understood at a single particle level due to differences in particle-to-particle composition that impact both climate and health. However, the analytical challenge of measuring aerosol acidity in individual particles has limited pH measurements for fine (<2.5 μm) and coarse (2.5–10 μm) particles. This has led to a reliance on indirect methods or thermodynamic modeling, which focus on average, not individual, particle pH. Thus, new approaches are needed to probe single particle pH. In this study, a novel method for pH measurement was explored using degradation of a pH-sensitive polymer, poly­(ε-caprolactone), to determine the acidity of individual submicron particles. Submicron particles of known pH (0 or 6) were deposited on a polymer film (21–25 nm thick) and allowed to react. Particles were then rinsed off, and the degradation of the polymer was characterized using atomic force microscopy and Raman microspectroscopy. After degradation, holes in the PCL films exposed to pH 0 were observed, and the loss of the carbonyl stretch was monitored at 1723 cm–1. As particle size decreased, polymer degradation increased, indicating an increase in aerosol acidity at smaller particle diameters. This study describes a new approach to determine individual particle acidity and is a step toward addressing a key measurement gap related to our understanding of atmospheric aerosol impacts on climate and health.

Abstract. A fundamental understanding of the processes that control Antarctic aerosols is necessary in determining thepresented here focus on the overall trends in aerosol composition primarily as functions of air masses and local meteorological conditions. The results suggest that air mass back trajectories have little impact on either the absolute or relative concentrations of the aerosol constituents measured by (and inferred from) an AMS at a coastal location. However, when the data is parsed 20 by wind speed, two observations become clear. First, a critical wind speed is required to loft snow from the surface, which, in turn, increases particle counts in all measured size bins. Second, this lofted (blowing) snow significantly increases both aerosol chloride and sodium. Further inspection of the AMS data shows that the increased chloride concentrations have distinctive signatures that differ from chloride measured at low wind speed. Also presented are the Cl:Na ratios of snow samples, aerosol filter samples, and non-chloride aerosol constituents measured by the AMS. Additionally, submicron aerosol iodine and 25 bromine concentrations as functions of wind speed are also presented. The results presented here suggest that aerosol composition in coastal Antarctica is a strong function of wind speed and that the mechanisms determining aerosol composition are likely linked to blowing snow.