Chemical characterization of ozone formation in the Houston-Galveston area: A chemical transport model study

[1] An episodic simulation is conducted to characterize ozone (O-3) formation and to investigate the dependence of O-3 formation on precursors in the Houston-Galveston (HG) area using a regional chemical transport model (CTM). The simulated net photochemical O-3 production rates, P(O-3), in the Houston area are higher than those in most other U. S. urban cities, reaching 20 - 40 ppb hr(-1) for the daytime ground NOx levels of 5 - 30 ppb. The NOx turnaround value (i.e., the NOx concentration at which P( O3) reaches a maximum) is also larger than those observed in most other U. S. cities. The large abundance and high reactivity of anthropogenic volatile organic compounds (AVOCs) and the coexistence of abundant AVOCs and NOx in this area are responsible for the high O-3 production rates and the NOx turnaround value. The simulated O-3 production efficiency is typically 3 - 8 O-3 molecules per NOx molecule oxidized during the midday hours. The simulation reveals a RO2 peak up to 70 ppt at night, and the reactions of alkene-NO3 and alkene-O-3 are responsible for more than 80% of the nighttime RO2 in the residual layer, contributing to over 70% and about 10%, respectively. Isoprene accounts for about 40% of the nighttime RO2 peak concentration. The nighttime RO2 level is limited by the availability of alkenes. Hydrolysis of N2O5 on sulfate aerosols leads to an increase of HNO3 by as much as 30 - 60% but to a decrease of NOx by 20 - 50% during the night in the lower troposphere. Heterogeneous conversion of NO2 to HONO on the surfaces of soot aerosol accelerates the O-3 production by about 1 hour in the morning and leads to a noticeable increase of 7 ppb on average in the daytime O-3 level. The sensitivity study suggests that the near-surface chemistry over most of the Houston metropolitan area is in or close to the NOx-VOC transition regime on the basis of the current emission inventory. Doubling AVOC emissions leads to the NOx sensitive chemistry. Biogenic VOCs contribute about 5% on the average to the total near-surface O-3 in the Houston area.