Vertical profiles of nitrous acid in the nocturnal urban atmosphere of Houston, TX

Abstract

Nitrous acid (HONO) often plays an important role in tropospheric photochemistry as a major precursor of the hydroxyl radical (OH) in early morning hours and potentially during the day. However, the processes leading to formation of HONO and its vertical distribution at night, which can have a considerable impact on daytime ozone formation, are currently poorly characterized by observations and models. Long-path differential optical absorption spectroscopy (LP-DOAS) measurements of HONO during the 2006 TexAQS II Radical and Aerosol Measurement Project (TRAMP), near downtown Houston, TX, show nocturnal vertical profiles of HONO, with mixing ratios of up to 2.2 ppb near the surface and below 100 ppt aloft. Three nighttime periods of HONO, NO<sub>2</sub> and O<sub>3</sub> observations during TRAMP were used to perform model simulations of vertical mixing ratio profiles. By adjusting vertical mixing and NO<sub>x</sub> emissions the modeled NO<sub>2</sub> and O<sub>3</sub> mixing ratios showed very good agreement with the observations. <br><br> Using a simple conversion of NO<sub>2</sub> to HONO on the ground, direct HONO emissions, as well as HONO loss at the ground and on aerosol, the observed HONO profiles were reproduced by the model for 1–2 and 7–8 September in the nocturnal boundary layer (NBL). The unobserved increase of HONO to NO<sub>2</sub> ratio (HONO/NO<sub>2</sub>) with altitude that was simulated by the initial model runs was found to be due to HONO uptake being too small on aerosol and too large on the ground. Refined model runs, with adjusted HONO uptake coefficients, showed much better agreement of HONO and HONO/NO<sub>2</sub> for two typical nights, except during morning rush hour, when other HONO formation pathways are most likely active. One of the nights analyzed showed an increase of HONO mixing ratios together with decreasing NO<sub>2</sub> mixing ratios that the model was unable to reproduce, most likely due to the impact of weak precipitation during this night. <br><br> HONO formation and removal rates averaged over the lowest 300 m of the atmosphere showed that NO<sub>2</sub> to HONO conversion on the ground was the dominant source of HONO, followed by traffic emission. Aerosol did not play an important role in HONO formation. Although ground deposition was also a major removal pathway of HONO, net HONO production at the ground was the main source of HONO in our model studies. Sensitivity studies showed that in the stable NBL, net HONO production at the ground tends to increase with faster vertical mixing and stronger NO<sub>x</sub> emission. Vertical transport was found to be the dominant source of HONO aloft

Modeling of daytime HONO vertical gradients during SHARP 2009

“…The morning HONO peaks in the lower height intervals tended to be slightly overestimated by the model. This can be caused by the overestimated HONO/NO x emission ratio in the model, since traffic is a significant source of HONO during the early morning rush hour, as shown in Wong et al (2011). The decrease of HONO concentrations and vertical gradients due to photolysis and vertical mixing was reproduced well in the model.…”

“…While our model runs included 10-11 h before sunrise, we will only show the model results and observations of HONO, NO 2 and HONO/NO 2 ratio from 06:00 to 18:00 CST here. The ability of our model to describe nocturnal vertical profiles of NO 2 and HONO has been described in detail in Wong et al (2011), and will not be repeated here. While the model results of other trace gases have been compared to observations to ensure that we capture daytime chemistry, we will not show these comparisons here, as the vertical profiles of NO 2 and HONO are most relevant for this study.…”

“…We will refer to this parameterization of HONO chemistry as the base case here. It should be added that this model performed well for nocturnal conditions, as shown in Wong et al (2011). As previously mentioned, and also discussed below in more detail, the HONO formation mechanisms at night are insufficient to explain daytime HONO levels.…”

Quantification of the unknown HONO daytime source and its relation to NO<sub>2</sub>

“…Studies about HONO fluxes (Harrison and Kitto, 1994;Harrison et al, 1996;Stutz et al, 2002Stutz et al, , 2004 explained that measured HONO formation is a net process (pseudo steady state) of release and deposition (see also discussion in Vogel et al, 2003). A recent study by Wong et al (2011) provides detailed information about HONO formation and deposition in the Nocturnal Boundary Layer (NBL) by combining vertical gradient measurements with 1-D model calculations. According to their results the ground surface accounts for most (∼70 %) of the HONO formation by NO 2 conversion but also for most of the loss (∼70 %).…”

Evidence for a nitrous acid (HONO) reservoir at the ground surface in Bakersfield, CA, during CalNex 2010

“…The concentration of HONO (g) at night often reaches a steady state between production, vertical mixing, and a dry deposition sink of HONO (g) , which models reproduce by irreversible deposition/uptake to ground and aerosol surfaces [Czader et al, 2012;Wong et al, 2011]. The production of HONO (g) at night occurs dominantly by the reaction of NO 2 on wet surfaces (R3), although the mechanism relevant to the atmosphere likely differs from that derived in laboratory investigations [Barney and Finlayson-Pitts, 2000;Finlayson-Pitts, 2009;Finlayson-Pitts et al, 2003;Kamboures et al, 2008;Miller et al, 2009].…”