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 O₃ and its precursors together with meteorological parameters at both sites are presented. Gross ozone production rate P(O₃) and sensitivity to nitric oxides (NO_x) and volatile organic compounds (VOCs) were investigated using an observation-based photochemical box model (OBM). P(O₃) 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 (O₃ + NO₂) concentrations. The O ₃ episodes typically appeared under southerly wind conditions with high P(O₃), especially at the urban site. Sensitivity studies with and without measured nitrous acid (HONO) as a model constraint suggested that the estimated P(O₃) at both sites was strongly enhanced by radical production from HONO photolysis. Both NO_x- and VOC-sensitive chemistries existed over time scales from hours to days at the two sites. The variation in O₃-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 HO_x radicals. The OBM was generally capable of correctly simulating the levels of P(O₃), although it might tend to overpredict the VOC-sensitive chemistry.