Researches on HO(x) radical chemistry would provide theoretical support for understanding the global climate change and regional air pollution control. At present stage, comprehensive field campaign including HO(x) radial measurements is one of the critical approaches to advance it. However, due to the very short lifetime and extremely low concentration of HO(x) in the atmosphere, direct measurement of HO(x) radical is one of most challenged works in atmospheric chemistry. This paper reviews the direct measurement techniques of the HO(x) radical, summarizes the observed dynamics range of its concentrations, introduces the current schematic diagram of the HO(x) radical chemistry and the important contributions from previous field studies, and discusses the main scientific questions that need further researches. Besides, the progress of the HO(x) radical chemistry in China is reviewed, and several potentially important research directions are pointed out.

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 (NO(x)) 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) + NO(2)) 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 NO(x)- 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 H(2)O(2), HNO(3), total OH reactivity, and HO(x) 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.