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Concentrations of CO, SO2, NO, NO2, and NOY 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. SO2 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 NOx displayed large diurnal variations in keeping with their co-emission by mobile sources in the Houston Metropolitan Area (HMA). CO/NOx emission ratios of 5.81 ± 0.94 were observed in the morning rush hour. Nighttime concentrations of NOx (NOx = NO + NO2) and NOY (NOY = NO + NO2 + NO3 + HNO3 + HONO + 2∗N2O5 + HO2NO2 + PANs + RONO2 + p-NO3− + …) were highest in winds from the NNW-NE due to emission from mobile sources. Median ratios of NOx/NOY were approximately 0.9 overnight, reflecting the persistence and/or generation of NOZ (NOZ = NOY − NOx) species in the nighttime Houston boundary layer, and approached unity in the morning rush hour. Daytime concentrations of NOx and NOY were highest in winds from the HSC. NOx/NOY ratios reached their minimum values (median ca 0.63) from 1300 to 1500 CST, near local solar noon, and air masses often retained enough NOx to sustain additional O3 formation farther downwind. HNO3 and PANs comprised the dominant NOZ species in the HMA, and on a median basis represented 17–20% and 12–15% of NOY, respectively, at midday. Concentrations of HNO3, PANs, and NOZ, and fractional contributions of these species to NOY, were at a maximum in NE flow, reflecting the source strength and reactivity of precursor emissions in the HSC. As a result, daytime O3 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 NOY measurements with the sum of measured NOY species (NOYi = NOx + HNO3 + PANs + HONO + p-NO3−) yielded excellent overall agreement during both day ([NOY](ppb) = ([NOYi](ppb)∗1.03 ± 0.16) − 0.42; r2 = 0.9933) and night ([NOY](ppb) = ([NOYi](ppb)∗1.01 ± 0.16) + 0.18; r2 = 0.9975). A similar comparison between NOY–NOx concentrations and the sum of NOZi (NOZi = HNO3 + PANs + HONO + p-NO3−) yielded good overall agreement during the day ([NOZ](ppb) = ([NOZi](ppb)∗1.01 ± 0.30) + 0.044 ppb; r2 = 0.8527) and at night ([NOZ](ppb) = ([NOZi](ppb)∗1.12 ± 0.69) + 0.16 ppb; r2 = 0.6899). Median ratios of NOZ/NOZi were near unity during daylight hours but increased to approximately 1.2 overnight, a difference of 0.15–0.50 ppb. Differences between NOZ and NOZi rarely exceeded combined measurement uncertainties, and variations in NOZ/NOZi ratios may have resulted solely from errors in conversion efficiencies of NOY species and changes in NOY composition. However, nighttime NOZ/NOZi ratios and the magnitude of NOZ − NOZi differences were generally consistent with recent observations of ClNO2 in the nocturnal Houston boundary layer. |
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