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

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 NO x levels of 5–30 ppb. The NO x turnaround value (i.e., the NO x concentration at which P (O 3 ) 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 NO x in this area are responsible for the high O 3 production rates and the NO x turnaround value. The simulated O 3 production efficiency is typically 3–8 O 3 molecules per NO x molecule oxidized during the midday hours. The simulation reveals a RO 2 peak up to 70 ppt at night, and the reactions of alkene‐NO 3 and alkene‐O 3 are responsible for more than 80% of the nighttime RO 2 in the residual layer, contributing to over 70% and about 10%, respectively. Isoprene accounts for about 40% of the nighttime RO 2 peak concentration. The nighttime RO 2 level is limited by the availability of alkenes. Hydrolysis of N 2 O 5 on sulfate aerosols leads to an increase of HNO 3 by as much as 30–60% but to a decrease of NO x by 20–50% during the night in the lower troposphere. Heterogeneous conversion of NO 2 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 NO x ‐VOC transition regime on the basis of the current emission inventory. Doubling AVOC emissions leads to the NO x sensitive chemistry. Biogenic VOCs contribute about 5% on the average to the total near‐surface O 3 in the Houston area.