Evaluation of ozone-nitrogen oxides-volatile organic compound sensitivity of Cincinnati, Ohio

摘要

Ambient concentrations of ozone (O-3), nitrogen oxides (NOx), total reactive nitrogen (NOy), nitric acid (HNO3), and hydrogen peroxide (H2O2) were measured during September 2003 at an urban site of Cincinnati, OH. The aim of this study was two-fold: to investigate whether O-3 formation in this population exposure-type site is NOx sensitive or volatile organic compound (VOC) sensitive and to test the practicality of using two combined observational-based methods to identify the sensitivity of O-3 formation in midlevel polluted locations. The evaluation of the indicator species: NOy, O-3/NOy, O-3/HNO3, H2O2/HNO3, and O-3/(estimated NO, reaction products), as well as the combined hypothesis testing analysis of the weekend/weekday (WE/WD) differences of 1-hr and 8-hr average maximum O-3 and of the 6:00 a.m.-9:00 a.m. average nitric oxide and NO, concentrations, show evidence that Cincinnati is likely VOC sensitive. Average WE 1-hr and 8-hr maximum O-3, as well as duration of WE O-3 accumulation, were not lower than the corresponding WD levels in spite of the observed significant reduction in NOx emissions on WE, a typical situation in VOC-sensitive locations. The possibility that the seasonal transition from summer to autumn could have influenced the results was also investigated through an exploratory analysis of the afternoon O-3maximum/NOx measured and of the WE/WD differences of peak O-3 and morning average NO and NOx concentrations observed at this site from June through September 2003. The results suggest that a VOC-sensitive chemistry regime dominated along the summer season. The findings of this study suggest that additional reductions in regional NOx emissions in Cincinnati, a potential nonattainment area under the 8-hr O-3 standard, may cause an increase in local O-3. Future strategies to reduce O-3 in Southwest Ohio should be further evaluated carefully. The combination of observational-based methods might provide a consistent complementary approach in the identification of the NOx-VOC sensitive characteristics of mid-to-moderate polluted urban areas.