Aerosol Properties and Snow Chemistry During the Uintah Basin Wintertime Ozone Study

Ozone mixing ratios in the Uintah Basin of Utah frequently exceed air quality standards during the winter months when the ground is snow covered. Aerosol chemical, physical and optical properties were measured during the Uintah Basin Wintertime Ozone study in January and February 2012, 2013 and 2014 to assess the role of aerosols and snow chemistry in the formation of ozone. The three years provide a sharp contrast. There was no snow in 2012, abundant snow in 2013 and partial snow cover in 2014. Snow increases the surface albedo (enhancing atmospheric photochemistry) and the stability in the lowest 100-200 m of the atmosphere (reducing the height of the mixed layer). As a result, non-soil aerosol PM 2.5 concentrations 12.6 m agl were three times higher in 2013 than 2012 (3.5 ± 2.1 μg m-3 vs 11.3 ± 4.4 μg m-3). The higher aerosol concentrations and larger mean diameters resulted in enhanced aerosol light scattering. The mean PM 2.5 aerosol surface area was 4.5 times higher in 2013 than 2012 (270 μm2 cm-3 vs 60 μm2 cm-3), increasing the potential for heterogeneous reactions. 2014 values were intermediate.

Snow chemistry and optical properties were measured during the 2013 and 2014 studies. Nitrate and nitrite concentrations in the surface snow increased with time between snowfalls increasing the acidity of the surface snow. Deposition of soil dust during a period of atmospheric turbulent mixing, mid-experiment in 2013, increased aerosol light absorption in the snow. Dust concentrations in the snow in 2014 were much higher than 2013 due to the areas exposed dust next to the snow. Although the snow was a potential source of reactive nitrogen to the atmosphere, the high concentrations of dust likely inhibited the photochemical reactions.