Impacts of atmospheric degradation mechanisms of biogenic volatile organic compounds on aerosol formation potential and property of cloud condensation nuclei

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Secondary organic aerosol (SOA) formation from volatile organic compounds (VOCs) was investigated using a photochemical reaction chamber with an inner volume of 6-m^3. Isoprene is the most abundant biogenic hydrocarbon and its dominant sink in the atmosphere is the reaction with OH radicals in the daytime. Due to its large emission rate, it is important to know if isoprene oxidation leads to SOA production and what is a dominant factor to control the SOA formation yield. SOA formation in the NO_x-photooxidation of isoprene was always observed after photochemical ozone was formed. The concentration of isoprene lost before SOA was formed became lower with decreasing the initial Nox concentration, whereas the total aerosol yield increased with increasing NOT. Experimental results could be explained in terms that RO_2-radical reactions with HO_2 radical contribute the formation of condensable species.SOA formation in the isoprene ozonolysis was also observed. The aerosol formation yield wa … More s markedly depressed by adding excess amount of OH-radical scavengers, while the size of depression of aerosol formation was insensitive to the species of scavenger. Aerosol mass spectrum observed in the presence of OH-radical scavenger, CO, was different from that observed in the absence of the scavenger. These results suggest that OH-initiated reactions are essential to the production of SOAandthe direct ozone reaction with isoprene less produces SOA than OH-oxidation reaction.The SOA mass yields from the photooxidation of aromatic hydrocarbons (AHCs), such as toluene, p-ethyltoluene, p-xylene and 1, 3, 5-trimethylbenzene, were also measured: The SOA yield from each AHC depended on the initial ARC concentration. A previously proposed reaction scheme, in which condensable products are assumed to be formed by secondary reactions of primary oxidation products with ozone, was examined for the interpretations of the present experimental results. As for each AHC, the SOA concentrations measured as a function of the concentration of reacted AHC were successfully explained under various conditions by the previously proposed two-step reaction scheme. Less