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 well by the model. 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 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. 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 emission. Vertical transport was found to be the dominant source of HONO aloft

Implementation and refinement of a surface model for heterogeneous HONO formation in a 3-D chemical transport model

“…Using a 1D chemistry and transport model, Wong et al (2011) showed that this mechanism, together with an accurate description of vertical mixing, is able to successfully explain the nocturnal HONO observations in Houston. This mechanism also seems to depend on the amount of water adsorbed on the surface, i.e.…”

“…the relative humidity, as pointed out by Stutz et al (2004). Vertical nighttime profile measurements (e.g., Kleffmann et al, 2003;Wong et al, 2011;VandenBoer et al, 2013) suggest that NO 2 to HONO conversion on the ground is the dominant source of nocturnal HONO.…”

“…A number of field studies (e.g., Kleffmann et al, 2003;Wong et al, 2011Wong et al, , 2012Wong et al, , 2013VandenBoer et al, 2013;Villena et al, 2011) and direct surface flux experiments (e.g., Ren et al, 2011;Zhang et al, 2012a) have measured large vertical gradients in HONO concentrations, with higher concentrations near the surface. This suggests the existence of surface sources of HONO, such as direct HONO emissions from combustion sources and heterogeneous reactions on ground surfaces.…”

Occurrence of atmospheric nitrous acid in the urban area of Beijing (China)

“…However, this mechanism is still under discussion and does not significantly seem to contribute to the atmospheric HONO formation (Crowley and Carl, 1997;Carr et al, 2009;Amedro et al, 2011;Sörgel et al, 2011;Wong et al, 2011). Recently, Bejan et al (2006) and Kleffmann (2007) proposed the gas-phase HONO production during the photolysis of aromatic compounds containing ortho-nitrophenols and a noontime HONO production of 100 ppt h −1 was estimated for urban conditions.…”

“…contributes to significant HONO amounts under conditions of elevated NO concentrations, particularly during daytime Volkamer et al, 2010;Wong et al, 2011), but this process competes with photolysis. Most recently Li et al (2008) showed that HONO can be formed upon the gas-phase reaction of photolytically excited NO 2 with water:…”