Concentrations of CO, SO
2, NO, NO
2, and NO
Y were measured atop the University of Houston's Moody Tower supersite during the 2006 TexAQS-II Radical and Aerosol Measurement Project (TRAMP). The lowest concentrations of all primary and secondary species were observed in clean marine air in southerly flow. SO
2 concentrations were usually low, but increased dramatically in sporadic midday plumes advected from sources in the Houston Ship Channel (HSC), located NE of the site. Concentrations of CO and NO
x displayed large diurnal variations in keeping with their co-emission by mobile sources in the Houston Metropolitan Area (HMA). CO/NO
x emission ratios of 5.81 ± 0.94 were observed in the morning rush hour. Nighttime concentrations of NO
x (NO
x = NO + NO
2) and NO
Y (NO
Y = NO + NO
2 + NO
3 + HNO
3 + HONO + 21N
2O
5 + HO
2NO
2 + PANs + RONO
2 + p-NO
3? + …) were highest in winds from the NNW-NE due to emission from mobile sources. Median ratios of NO
x/NO
Y were approximately 0.9 overnight, reflecting the persistence and/or generation of NO
Z (NO
Z = NO
Y ? NO
x) species in the nighttime Houston boundary layer, and approached unity in the morning rush hour. Daytime concentrations of NO
x and NO
Y were highest in winds from the HSC. NO
x/NO
Y ratios reached their minimum values (median ca 0.63) from 1300 to 1500 CST, near local solar noon, and air masses often retained enough NO
x to sustain additional O
3 formation farther downwind. HNO
3 and PANs comprised the dominant NO
Z species in the HMA, and on a median basis represented 17–20% and 12–15% of NO
Y, respectively, at midday. Concentrations of HNO
3, PANs, and NO
Z, and fractional contributions of these species to NO
Y, were at a maximum in NE flow, reflecting the source strength and reactivity of precursor emissions in the HSC. As a result, daytime O
3 concentrations were highest in air masses with HSC influence. Overall, our findings confirm the impact of the HSC as a dominant source region within the HMA. A comparison of total NO
Y measurements with the sum of measured NO
Y species (NO
Yi = NO
x + HNO
3 + PANs + HONO + p-NO
3?) yielded excellent overall agreement during both day ([NO
Y](ppb) = ([NO
Yi](ppb)11.03 ± 0.16) ? 0.42;
r2 = 0.9933) and night ([NO
Y](ppb) = ([NO
Yi](ppb)11.01 ± 0.16) + 0.18;
r2 = 0.9975). A similar comparison between NO
Y–NO
x concentrations and the sum of NO
Zi (NO
Zi = HNO
3 + PANs + HONO + p-NO
3?) yielded good overall agreement during the day ([NO
Z](ppb) = ([NO
Zi](ppb)11.01 ± 0.30) + 0.044 ppb;
r2 = 0.8527) and at night ([NO
Z](ppb) = ([NO
Zi](ppb)11.12 ± 0.69) + 0.16 ppb;
r2 = 0.6899). Median ratios of NO
Z/NO
Zi were near unity during daylight hours but increased to approximately 1.2 overnight, a difference of 0.15–0.50 ppb. Differences between NO
Z and NO
Zi rarely exceeded combined measurement uncertainties, and variations in NO
Z/NO
Zi ratios may have resulted solely from errors in conversion efficiencies of NO
Y species and changes in NO
Y composition. However, nighttime NO
Z/NO
Zi ratios and the magnitude of NO
Z ? NO
Zi differences were generally consistent with recent observations of ClNO
2 in the nocturnal Houston boundary layer.
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