Experimental Study On Outdoor Photochemical Smoke Box Simulation Of Light Oxidation Process Of Low Molecular Hydrocarbon Compounds

Online monitoring of atmospheric volatile organic compounds in typical urban area of Beijing was carried out continuously by Airmo VOC monitor. The concentration, variation, sources analysis and chemical reactivity of the 84 species of VOCs were studied from 6th August to 6th September, 2014. Based on the results, the atmospheric photochemical reaction process of the propane and propylene was investigated, using the newly built large scale outdoor smog chamber in Chinese Research Academy of Environmental Science, which was the first large-scale outdoor chamber in China. The study focused on the effect of VOCs/NOx ratio, initial oxidizing condition and meteorological conditions to the reaction process and the formation of products.(1) The average volume concentration(one hour average concentration) of the total VOCs was 34.04 ppb during the observation. Among the VOCs species, the proportion of alkanes was 39.80%, halogenated hydrocarbon(23.40%), alkenes(17.89%), aromatics(12.41%), alkyne(6.51%). The ten highest concentration species were propane, chloroform, ethane, 1,2,4-Trichlorobenzene, ethylene, acetylene, n-butane, toluene, 1,1-dichloroethane, cyclopentane. Analysis of the chemical reactivity of the observed ambient VOCs was conducted using the the OH radical loss rate(LOH) and ozone formation potential(OFP) methods. It showed that isoprene, 1,2,4-trimethylbenzene, m/p-xylene, ethylene, toluene, cyclopentane and trans-butene were the key reactive components, indicating their significant contribution to the photochemical production of Surface Ozone in Beijing.(2) In the reaction system of C3H8+NO, the increasing of initial oxidation had an obvious improvement on the photooxidation process of propane, while no evident effect on the final reaction mechanism. The concentration of NO in reaction system was more sensitive to the ozone formation. Real-time FT-IR study showed that acetone was an important product of atmospheric photochemical reaction of C3H8, which coincided with the reference result.(3) During the dark stage of the C3H8+NO reaction, no formation of micro particles was observed. After exposure to the sunlight, the micro particles formed at the beginning and the concentration increased rapidly in afternoon. Increasing of original NO led to a larger particle’s size range and higher maximum particle’s concentration. Preinjected ozone, which gave a higher oxidant condition to the reaction system, making the particle’s size range became wider, and the maximum particle size larger, which suggested that the formation and growth of the particle happened simultaneously.(4)In the C3H6+NO reaction system, the consumption of NO and the formation rate of O3 were higher than the C3H8 + NO reaction system, which indicated that propylene has higher chemical activity than propane. The products of propene potooxidation indicated the acetaldehyde formation during the photochemical process, also agreed with the reference result.(5)The C3H6+NO reaction system showed a weak reaction process even in the dark stage. Under the sunlight the particle formed rapidly even at the beginning of the photoreaction, the size range was wider and the maximum particle size was larger than the C3H8 system, and an obvious growth of the particle size was observed during the reaction processing. The results indicated that propylene might have more seriously effects on PM pollution during the atmospheric photochemical process.