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Nitrous Acid Formation and Chemistry over Soil and Snow in a Polluted Rural Area

  • Author(s): Tsai, Catalina Juiyi
  • Advisor(s): Stutz, Jochen
  • et al.
Abstract

Despite of decades of research, the formation mechanism of daytime HONO is still not fully understood and little is known about the daytime behavior of HONO over soil and snow surfaces in polluted rural areas. HONO measurements in two regions heavily affected by oil and gas drilling activities (in Wyoming and Utah) show high levels of daytime HONO (above 500 pptv) during the wintertime. However, questions were raised about the reliability of wintertime HONO observations as recent studies show that many in-situ HONO measurement methods suffer from pernitric acid (PNA) interferences that overestimate HONO concentrations.

The UCLA Long-Path (LP) Differential Optical Absorption Spectroscopy (DOAS) instrument measured HONO and HONO gradients in the Uintah Basin (UB), a region highly impacted by oil and gas activities. The UCLA LP-DOAS system measured HONO on three light paths covering height intervals from 2 m to 31, 45, and 68 m above ground level (agl). Due to disparate meteorological conditions in 2012 and 2014, HONO was measured over soil in 2012 and over snow in 2014.

HONO mixing ratios averaged 74 ppt in 2012. These levels are lower than HONO measured in polluted urban areas but comparable to levels measured in rural environments. Distinct daytime negative vertical gradients of HONO were observed in 2012, with higher levels of HONO measured on the lower than upper light paths, an indication that HONO had a surface source in 2012. HONO vertical profiles were retrieved using a novel least square minimization approach, in which the average mixing ratios of HONO along the light paths were simulated using a height dependent exponential function. The HONO vertical profiles, together with NOAA micrometeorological data were used to calculate HONO fluxes at 19 m agl. Sunny days hourly average HONO fluxes show that they follow the same trend as solar irradiance, with a maximum of (1.7�0.3) x1010 molec. cm-2 s-1 at noontime, supporting the idea of a photolytic surface HONO source in 2012.

Analysis of HONO flux and photolytic surface HONO sources show that under conditions of high NO2 levels, conversion of NO2 on ground surfaces is most likely the source of daytime HONO. In contrast, on days with moderate NO2 levels, photolysis of surface-adsorbed HNO3 and nocturnal uptake of HONO followed by daytime acid displacement might play an important role; however, additional measurements are needed to confirm this theory. Comparison of the missing source of HONO (Punknwon) and HONO surface flux rate shows that the HONO surface flux rate accounts for (63�32%) of the Punknown throughout the day indicating that photolytic surface sources of HONO were the dominant sources of HONO in 2012.

HONO mixing ratios measured in 2014 were on average 96 ppt. Comparison of DOAS HONO measurements with four in-situ instruments shows that the DOAS system consistently measured the lowest HONO levels. Preliminary analysis of PNA interferences shows that some in-situ measurements might overestimate HONO concentrations in the presence of PNA (average maximum concentration of 120 ppt measured around noontime), therefore, HONO DOAS were considered the reference measurements in the 2014 study.

2014 HONO vertical profiles also show higher HONO mixing ratios near the surface and a rapid decay of HONO mixing ratios with height. The sunny days hourly average HONO fluxes were also positive with a maximum HONO flux near noontime of (1.1�0.7)x1010 molec. cm-2 s-1. The sunny days hourly average HONO fluxes also follow a similar temporal trend as solar irradiance, further supporting the hypothesis of a photolytic surface HONO source in 2014.

Due to insufficient data, we were not able to determine which photolytic surface HONO formation pathway dominated in 2014. However, we were able to determine that the measured 2014 HONO was most probably formed at the top of the snowpack and not in the firn. This result is supported by the surprisingly similar HONO fluxes measured in 2012 and 2014, which indicate that the HONO formation mechanisms during these two years were probably very similar.

Comparison of the missing source of HONO (Punknwon) and HONO surface flux rate shows that HONO surface flux rate accounts for (54�% 15) of the Punknown in the early morning and noontime, suggesting that HONO sources at noon and early morning are light dependent and are produced on the snow surface.

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