Modeling nitrous acid and its impact on ozone and hydroxyl radical during the Texas Air Quality Study 2006

Abstract

Nitrous acid (HONO) mixing ratios for the Houston metropolitan area were simulated with the Community Multiscale Air Quality (CMAQ) model for an episode during the Texas Air Quality Study (TexAQS) II in August/September 2006 and compared to in-situ MC/IC (mist-chamber/ion chromatograph) and long path DOAS (Differential Optical Absorption Spectroscopy) measurements at three different altitudes. Several HONO sources were accounted for in simulations, such as gas phase formation, direct emissions, nitrogen dioxide (NO<sub>2</sub><sup>*</sup>) hydrolysis, photo-induced formation from excited NO<sub>2</sub><sup>*</sup> and photo-induced conversion of NO<sub>2</sub> into HONO on surfaces covered with organic materials. Compared to the gas-phase HONO formation there was about a tenfold increase in HONO mixing ratios when additional HONO sources were taken into account, which improved the correlation between modeled and measured values. Concentrations of HONO simulated with only gas phase chemistry did not change with altitude, while measured HONO concentrations decrease with height. A trend of decreasing HONO concentration with altitude was well captured with CMAQ predicted concentrations when heterogeneous chemistry and photolytic sources of HONO were taken into account. Heterogeneous HONO production mainly accelerated morning ozone formation, albeit slightly. Also HONO formation from excited NO<sub>2</sub> only slightly affected HONO and ozone (O<sub>3</sub>) concentrations. Photo-induced conversion of NO<sub>2</sub> into HONO on surfaces covered with organic materials turned out to be a strong source of daytime HONO. Since HONO immediately photo-dissociates during daytime its ambient mixing ratios were only marginally altered (up to 0.5 ppbv), but significant increase in the hydroxyl radical (OH) and ozone concentration was obtained. In contrast to heterogeneous HONO formation that mainly accelerated morning ozone formation, inclusion of photo-induced surface chemistry influenced ozone throughout the day

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

“…Measurements taken during the 2006 Texas Air Quality Study (TexAQS) and the 2009 SHARP campaign showed strong vertical HONO concentration gradients during the day with higher values near the ground, which modeled homogeneous chemistry did not predict (e.g., Czader et al, 2012;Wong et al, 2012). Wong et al (2012) hypothesized that the missing HONO formation pathways are photolytic and occur on or near the ground.…”

“…Several recent modeling studies with CMAQ (e.g., Sarwar et al, 2008;Czader et al, 2012;Gonçalves et al, 2012;Zhang et al, 2012b) and WRF-Chem (e.g., Li et al, 2010Li et al, , 2011Tang et al, 2015) have considered additional mechanisms for HONO formation, and have found improvement in model performance, although the degree of improvement is variable depending upon the approach used. The Supplemental Section provides a brief review of the methods and results from some of these modeling studies.…”

“…These high daytime levels also cannot be explained by the homogeneous pathways for HONO formation currently included in photochemical models. Modeling studies with the U.S. EPA Community Multiscale Air Quality (CMAQ) model (e.g., Sarwar et al, 2008;Czader et al, 2012) and the Weather Research and Forecasting model with chemistry (WRFChem) (e.g., Li et al, 2010Li et al, , 2011 have also shown consistently large (order of magnitude) under-predictions of observed HONO levels when only the gas-phase HONO formation pathway is included.…”

Nitrous acid (HONO) in a polluted subtropical atmosphere: Seasonal variability, direct vehicle emissions and heterogeneous production at ground surface

“…Another area of active ongoing research has considered elevated daytime HONO concentrations, which cannot be explained by the known homogenous reaction between NO and OH and the sources just mentioned (Acker and M€ oller, 2007;Czader et al, 2012Czader et al, , 2013Li et al, 2012;Qin et al, 2009;Sorgel et al, 2011;Su et al, 2008b;VandenBoer et al, 2013). Several potential photo-enhanced sources have been proposed, including light-dependent heterogeneous reactions of NO 2 on aerosol and ground surfaces, photolysis of HNO 3 absorbed on surfaces, the reaction of excited NO 2 with water vapors and the reaction of NO 2 with HO 2 *H 2 O (Kleffmann, 2007;Li et al, 2008Li et al, , 2014Monge et al, 2010;Wong et al, 2012;Zhou et al, 2011).…”

Overview of the SHARP campaign: Motivation, design, and major outcomes

“…On the other hand, early modeling studies of the Houston airshed that assumed either direct emissions of formaldehyde from industrial flares [ Vizuete et al ., ] or secondary formation of HONO on surfaces, such as heterogeneous conversion of NO 2 on soot particles [ Lei et al ., ], resulted in significant additions to peak ozone. This has since been confirmed by more contemporary modeling studies [ Olaguer , ; Czader et al ., ]. The regulatory implication of this finding was that the simulated efficacy of control strategies in the Houston SIP was likely to be underestimated in the absence of additional sources of radical precursors, both primary and secondary.…”