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Volume 18, issue 3 | Copyright
Atmos. Chem. Phys., 18, 1977-1996, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Feb 2018

Research article | 12 Feb 2018

Nitrous acid formation in a snow-free wintertime polluted rural area

Catalina Tsai1, Max Spolaor1, Santo Fedele Colosimo1, Olga Pikelnaya1,2, Ross Cheung1, Eric Williams3, Jessica B. Gilman3, Brian M. Lerner3,a, Robert J. Zamora3, Carsten Warneke3, James M. Roberts3, Ravan Ahmadov3, Joost de Gouw4, Timothy Bates5, Patricia K. Quinn5, and Jochen Stutz1 Catalina Tsai et al.
  • 1Department of Atmospheric and Oceanic Sciences, University of California Los Angeles, Los Angeles, CA, USA
  • 2now at South-Coast Air Quality Management District, Diamond Bar, CA, USA
  • 3Earth System Research Laboratory, NOAA, Boulder, CO, USA
  • 4Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
  • 5Pacific Marine Environmental Laboratory, NOAA, Seattle, WA, USA
  • anow at: Aerodyne Research, Inc., Billerica, MA, USA

Abstract. Nitrous acid (HONO) photolysis is an important source of hydroxyl radicals (OH) in the lower atmosphere, in particular in winter when other OH sources are less efficient. The nighttime formation of HONO and its photolysis in the early morning have long been recognized as an important contributor to the OH budget in polluted environments. Over the past few decades it has become clear that the formation of HONO during the day is an even larger contributor to the OH budget and additionally provides a pathway to recycle NOx. Despite the recognition of this unidentified HONO daytime source, the precise chemical mechanism remains elusive. A number of mechanisms have been proposed, including gas-phase, aerosol, and ground surface processes, to explain the elevated levels of daytime HONO. To identify the likely HONO formation mechanisms in a wintertime polluted rural environment we present LP-DOAS observations of HONO, NO2, and O3 on three absorption paths that cover altitude intervals from 2 to 31, 45, and 68m above ground level (a.g.l.) during the UBWOS 2012 experiment in the Uintah Basin, Utah, USA. Daytime HONO mixing ratios in the 2–31m height interval were, on average, 78ppt, which is lower than HONO levels measured in most polluted urban environments with similar NO2 mixing ratios of 1–2ppb. HONO surface fluxes at 19ma.g.l., calculated using the HONO gradients from the LP-DOAS and measured eddy diffusivity coefficient, show clear upward fluxes. The hourly average vertical HONO flux during sunny days followed solar irradiance, with a maximum of (4.9±0.2) ×−2s−1 at noontime. A photostationary state analysis of the HONO budget shows that the surface flux closes the HONO budget, accounting for 63±32% of the unidentified HONO daytime source throughout the day and 90±30% near noontime. This is also supported by 1-D chemistry and transport model calculations that include the measured surface flux, thus clearly identifying chemistry at the ground as the missing daytime HONO source in this environment. Comparison between HONO surface flux, solar radiation, NO2 and HNO3 mixing ratios, and results from 1-D model runs suggest that, under high NOx conditions, HONO formation mechanisms related to solar radiation and NO2 mixing ratios, such as photo-enhanced conversion of NO2 on the ground, are most likely the source of daytime HONO. Under moderate to low NO2 conditions, photolysis of HNO3 on the ground seems to be the main source of HONO.

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Nitrous acid (HONO) photolysis is an important source of hydroxyl radicals (OH). Vertical HONO fluxes, observed in the snow-free, wintertime Uintah Basin, Utah, USA, show that chemical formation of HONO on the ground closes the HONO budget. Under high NOx conditions, HONO formation is most likely due to photo-enhanced conversion of NO2 on the ground. Under moderate to low NO2 conditions, photolysis of HNO3 on the ground seems to be the most likely source of HONO.
Nitrous acid (HONO) photolysis is an important source of hydroxyl radicals (OH). Vertical HONO...