Differences in ozone photochemical characteristics between the megacity Nanjing and its suburban surroundings, Yangtze River Delta, China.

Abstract

Rapid economic growth has led to a significant increase in ozone (O3) precursor emissions in many regions of China. Improved understanding of O3 formation in response to different precursor emissions is imperative to address the highly nonlinear O3 problem and to provide a solid scientific basis for efficient O3 abatement in these regions. A comparative study was conducted in summer using a set of observational data at urban and suburban sites in Nanjing. The results showed that high O3 concentrations were frequently encountered at both sites. The probability distributions of O3 in both sites show a fair resemblance to each other, suggesting strong regional mixing over the polluted Nanjing. A distinction between the characteristics of O3 precursors has been found at different sites. During the observation period, O3 concentrations varied monthly, reaching a minimum in June and peaking in August. The daily maximum O3 concentration was found to exceed 80 ppb for 27 days at residential area (RA), whereas it only exceeded 80 ppb for 22 days at industrial area (IA), 16 days at traffic area (TA), and 14 days at commercial area (CA). This pattern suggests a higher continuous ozone exposure risk at RA. The daily maximum O3 concentrations at different sites were 135.1 ppb (IA), 134.1 ppb (RA), 129.2 ppb (TA), and 110.6 ppb (CA), respectively. The daily maximum O3 concentration occurred at 16:00 in IA, at 17:00 in TA and CA, and at 18:00 in RA. Nitrogen dioxide (NO2) and carbon monoxide (CO) showed similar double-peak diurnal cycles. NO2 showed maximum values in June and minimum values in July. CO showed a similar diurnal variation to NO2. This effect may be explained by their common sources and the similar chemical losses. During the day, O3 tended to rapidly increase during the morning, reaching a maximum value of 9-11 ppb h(-1). The differences in O3 and NO2 between workdays and weekends were small. The CO levels were higher on weekdays than on weekends in urban areas and were higher on weekends than on weekdays in suburban areas. A sensitivity study performed with an observation-based model (OBM) showed alkenes to be the largest contributor to O3 production. The production of O3 in the Nanjing area is generally limited by volatile organic compounds (VOCs), whereas high nitric oxide (NO) concentrations suppress O3 concentrations.