Abstract
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 altitude ranges. Several HONO sources were accounted for in simulations, such as gas phase formation, direct emissions, nitrogen dioxide (NO2) hydrolysis, photo-induced formation from excited NO2 and photo-induced conversion of NO2 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 NO2 only slightly affected HONO and ozone (O3) concentrations. Photo-induced conversion of NO2 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.
Highlights
The importance of nitrous acid (HONO) in the chemistry of the atmosphere stems from its photo-dissociation that serves as a significant source of hydroxyl radical (OH) (Lammel and Cape, 1996; Alicke et al, 2002, 2003; Kleffmann et al, 2005)
Simulated HONO concentrations were compared with values measured in-situ by a mist-chamber/ion chromatograph (MC/IC) system at the top of the Moody Tower (60 m a.g.l.) on the University of Houston (UH) campus (Stutz et al, 2010) and are shown in Fig. 1a for simulation cases G, GEH, and GEHP
When data points with wrong NO2 prediction were ignored and only NO2 values simulated within 70 % of measured value were considered the correlation coefficient for HONO increased to 0.82
Summary
The importance of nitrous acid (HONO) in the chemistry of the atmosphere stems from its photo-dissociation that serves as a significant source of hydroxyl radical (OH) (Lammel and Cape, 1996; Alicke et al, 2002, 2003; Kleffmann et al, 2005). Sarwar et al (2008) performed CMAQ modeling of HONO for the north-eastern area of the US in which, in addition to the gas phase chemistry, HONO emissions, heterogeneous HONO formation involving NO2 and H2O and the photolysis of HNO3 adsorbed on surfaces were accounted for. This approach improved surface HONO predictions and resulted in an average ozone increase of 1.4 ppbv. The simulated concentrations of HONO are compared with in-situ as well as with long-path measurements at three different altitudes obtained in Houston, Texas
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