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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
© Author(s) 2017. This work is distributed under
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Research article
04 Aug 2017
Review status
This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Atmospheric Chemistry and Physics (ACP) and is expected to appear here in due course.
Nitrous acid formation in a snow-free wintertime polluted rural area
Catalina Tsai1, Max Spolaor1, Santo Fedele Colosimo1, Olga Pikelnaya1,a, Ross Cheung1, Eric Williams2, Jessica B. Gilman2, Brian M. Lerner2,b, Robert J. Zamora2, Carsten Warneke2, James M. Roberts2, Ravan Ahmadov2, Joost de Gouw3, Timothy Bates4, Patricia K. Quinn4, and Jochen Stutz1 1Department of Atmospheric and Oceanic Sciences, University of California Los Angeles, Los Angeles, CA, USA
2Earth System Research Laboratory, NOAA, Boulder, CO, USA
3Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA
4Pacific Marine Environmental Laboratory, NOAA, Seattle, WA, USA
anow at: South-Coast Air Quality Management District, Diamond Bar, CA, USA
bnow 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 m to 31 m, 45 m, and 68 m agl during the UBWOS 2012 experiment in the Uintah Basin, Utah, USA. Daytime HONO mixing ratios in the 2–31 m height interval were, on average, 78 ppt, which is lower than HONO levels measured in most polluted urban environments with similar NO2 mixing ratios of 1–2 ppb. HONO surface fluxes at 16 m agl, 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) x 1010 molec. cm−2 s−1 at noontime. A photo-stationary 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 1D 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 1D 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.

Citation: Tsai, C., Spolaor, M., Colosimo, S. F., Pikelnaya, O., Cheung, R., Williams, E., Gilman, J. B., Lerner, B. M., Zamora, R. J., Warneke, C., Roberts, J. M., Ahmadov, R., de Gouw, J., Bates, T., Quinn, P. K., and Stutz, J.: Nitrous acid formation in a snow-free wintertime polluted rural area, Atmos. Chem. Phys. Discuss.,, in review, 2017.
Catalina Tsai et al.
Interactive discussionStatus: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version      Supplement - Supplement
RC1: 'Review of acp-2017-648', Anonymous Referee #1, 03 Sep 2017 Printer-friendly Version 
RC2: 'referee comment', Anonymous Referee #2, 13 Sep 2017 Printer-friendly Version Supplement 
AC1: 'Response to Reviewers Comments', Jochen Stutz, 17 Nov 2017 Printer-friendly Version Supplement 
Catalina Tsai et al.
Catalina Tsai et al.


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Short summary
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...