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

Abstract

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 NO x . 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, NO 2 , and O 3 on three absorption paths that cover altitude intervals from 2 to 31, 45, and 68 m above ground level (a.g.l.) 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 NO 2 mixing ratios of 1-2 ppb. HONO surface fluxes at 19 m a.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) × 10 10 molec. cm −2 s −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, NO 2 and HNO 3 mixing ratios, and results from 1-D model runs suggest that, under high NO x conditions, HONO formation mechanisms related to solar radiation and NO 2 mixing ratios, such as photo-enhanced conversion of NO 2 on the ground, are most likely the source of daytime HONO. Under moderate to low NO 2 conditions, photolysis of HNO 3 on the ground seems to be the main source of HONO.

Microbial mechanisms and ecosystem flux estimation for aerobic NO y emissions from deciduous forest soils

“…Another emerging question is the role of nitrifying microbes in the production of HONO, which is a major source of atmospheric OH and NO (28). Vertical gradients of HONO have been observed with the highest concentrations at ground level (29)(30)(31)(32), indicating that HONO production may be a function of biotic and/ or abiotic soil processes. Most studies have implicated abiotic mechanisms associated with NO x (NO and NO 2 ) chemistry as the primary driver of HONO production.…”

Review of - Role of HONO in O3 formation during KORUS-AQ

“…2. Given the focus of this work on the use of ANN to model nocturnal HONO chemistry, the Introduction and results should do a much better review and contrast to the existing capabilities of other models such as CMAQ (Czader et al, 2012;Diao et al, 2016;Gonçalves et al, 2012) or custom models Tsai et al, 2018;Vogel et al, 2003;Wong et al, 2012). The Authors should review this literature to aid in preparations for the re-running of their ANN.…”

“…Line 304: The Authors conclude here that aerosol surfaces are important, but their lack of awareness of literature reports on the role of the ground surface relative to aerosol surfaces being far more important suggests that they need to reconsider their interpretation of the data after more thoroughly reviewing the literature (see references given in Major Comments above). All vertically-resolved HONO measurements have shown that the ground is a major sink for HONO, as well as dew and available surface water (He et al, 2006;Rubio et al, 2009;Stutz et al, 2004;Tsai et al, 2018;VandenBoer et al, 2015;Wojtal et al, 2011).…”

Review of: Role of dew water in HONO distribution

“…The Authors should revisit this. It would be better to show that typical aerosol uptake coefficients used by (Tsai et al, 2018;Wong et al, 2012) fail to produce the observed quantities of HONO and to quantify the fraction that aerosol conversion represents so it is clear that aerosol conversion is trivial.…”