Oxidant (O3 + NO2) production processes and formation regimes in Beijing

For CareBeijing-2006, two sites were established in urban and suburban regions of Beijing in summer 2006. Observations of O-3 and its precursors together with meteorological parameters at both sites are presented. Gross ozone production rate P(O-3) and sensitivity to nitric oxides (NOx) and volatile organic compounds (VOCs) were investigated using an observation-based photochemical box model (OBM). P(O-3) varied from nearly zero to 120 and 50 ppb h(-1) for urban and suburban sites, respectively. These rates were greater than the accumulation rates of the observed oxidant (O-3 + NO2) concentrations. The O-3 episodes typically appeared under southerly wind conditions with high P(O-3), especially at the urban site. Sensitivity studies with and without measured nitrous acid (HONO) as a model constraint suggested that the estimated P(O-3) at both sites was strongly enhanced by radical production from HONO photolysis. Both NOx- and VOC-sensitive chemistries existed over time scales from hours to days at the two sites. The variation in O-3-sensitive chemistry was relatively well explained by the ratio of the average daytime total VOC reactivity (k(TVOC)) to NO, with the transition chemistry corresponding to a k(TVOC)/NO value of 2-4 s(-1) ppb(-1). Pronounced diurnal variations in the O-3 production regime were found. In the morning, conditions were always strongly VOC-limited, while in the afternoon, conditions were variable for different days and different sites. The model-calculated results were tested by measurements of H2O2, HNO3, total OH reactivity, and HOx radicals. The OBM was generally capable of correctly simulating the levels of P(O-3), although it might tend to overpredict the VOC-sensitive chemistry.