Differences in ozone photochemical characteristics between the megacity Tianjin and its rural surroundings

Ground level ozone and its precursors were measured from July 10 to September 30, 2009 within Tianjin. The data were used to analyze differences in ozone photochemical oxidant production in urban and rural areas. Results showed more pronounced risk of O3 exposure at the rural site, Wuqing. During the observation period, ozone varied monthly, peaking in Jul. and reaching a minimum in Sep. The daily maximum ozone concentration was found to exceed 80 ppb for 28 days 100 ppb for 12 days, 120 ppb for 7 days at Wuqing, while it exceeded 80 ppb for 10 days, 100 ppb for 2 days, and 120 ppb for 1 day at the urban site, Tieta. The daily maximum ozone concentrations at Wuqing and Tieta were 193.7 ppb and 130.4 ppb. The daily maximum ozone concentration occurred at noon in Tieta and at 14:00 in Wuqing. NO and NOx peaked in September and reached minimum values in Jul., CO showed little variation at both sites. NOx and CO showed similar double-peak diurnal cycles resulted from a combination of diurnal variation of emission and the Planetary Boundary Layer During the VOCs (volatile organic compounds) sampling period, the average total VOCs concentration showed considerable day to day variation, which was 87.91 ppb with a range of 27.2 ppb-437.3 ppb at Tieta, and the average total VOCs was 197.95 ppb with a range of 63.48 ppb-473.97 ppb at Wuqing. A sensitivity study performed with the NCAR-MM model showed alkenes to be the most numerous contributors to O3 production, accounting for 53.3% of the total. Aromatics and alkanes accounted for 35.1% and 9.2%, respectively.