Measurements of primary trace gases and NO Y composition in Houston, Texas

Abstract

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 + 2∗N 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)∗1.03 ± 0.16) − 0.42; r 2 = 0.9933) and night ([NO Y](ppb) = ([NO Yi](ppb)∗1.01 ± 0.16) + 0.18; r 2 = 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)∗1.01 ± 0.30) + 0.044 ppb; r 2 = 0.8527) and at night ([NO Z](ppb) = ([NO Zi](ppb)∗1.12 ± 0.69) + 0.16 ppb; r 2 = 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.