1. G – gas-phase HONO chemistry;
2. GEH – gas phase chemistry, HONO emissions, and HONO heterogeneous formation;
3. \\end{itemize} \\begin{figure*}[t] \\includegraphics[width=13cm]{acp-2012-40-f01.pdf} \\caption{Comparison of measured vs. simulated HONO time series at the UH Moody Tower for the time period 25 August-20 September 2006. Dots represent measured values, the solid lines represent CMAQ predicted concentration from G, GEH, and GEHP cases (explanation see text). Dashed vertical lines indicate midnight times. <strong>(a)</strong>: Comparison with data measured in-situ by a MC/IC system at the top of the Moody Tower, at 60 m a.g.l. <strong>(b-d)</strong>: Time series comparison of HONO measured from the Moody Tower by DOAS low light-path <strong>(b)</strong>, middle light path <strong>(c)</strong>, and upper path <strong>(d)</strong>.} \\end{figure*} \\section{Results and discussion} \\subsection{Evaluation of HONO modeling} 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. The highest HONO mixing ratios up to 2 ppbv were measured during nighttimes and in the early mornings while daytime concentrations are much lower, but still appreciable. HONO values simulated with only gas-phase chemistry (case G) persistently show significant under prediction of HONO concentrations. HONO mixing ratios from GEH and GEHP cases are much closer to the observed values (e.g. 31 August, 12 and 20 September). The advantage of including photochemical HONO sources can nicely be seen on 30 August, 7, 9, and 13 September (and others) when daytime HONO values from the GEHP case are higher and closer to measurements than HONO values from the GEH case. In some cases a mismatch between observed and simulated HONO values occurs (e.g. 1 and 6 September). This is mostly related to mismatch in NO<sub>2</sub>
4. concentrations as discussed further below. In order to evaluate HONO modeling for different altitudes in the urban boundary layer observational HONO data detected by Differential Optical Absorption Spectroscopy (DOAS) were utilized. These measurements were taken along different paths between the Moody Tower super site and Downtown Houston (Stutz et al., 2010). The low light-path detected mixing ratios between 20-70 m height which corresponds to the first and second CMAQ model layer, the middle light-path between 70-130 m corresponding to the second and third layer, and the upper light-path between 130-300 m, which falls into model layers three to five. Figures 1b-d shows comparisons of measured and simulated HONO values. While daytime measurements show only slight dependence on altitude, HONO mixing ratios at night and early morning decrease with altitude, with maximum values reaching about 2 ppbv at the low level and only about 0.5 ppbv at the upper level. Contrary to the measured values, HONO mixing ratios from the G case do not show variation with height. HONO values obtained from GEH and GEHP cases correctly capture the trend towards lower nighttime and early morning mixing ratios at higher altitudes. In addition, including photolytic HONO sources in the GEHP case resulted in average 100 ppt higher daytime HONO concentrations at the low DOAS level and an average daytime increase of 50 and 30 ppt at the middle and upper DOAS levels, respectively. Since most of the photolytic HONO production occurs by NO<sub>2</sub>
5. based on the same data set (25 August-20 September 2006) for all simulated cases as well as MC/IC observed values. This presentation summarizes clearly the general differences in HONO model simulations. It can be seen that higher daytime values were obtained from the GEHP case, which includes photolytic HONO formation, in comparison with the GEH case, in which heterogeneous HONO production dominates HONO sources. The model tends to overpredict NO<sub>2</sub>