“A method to estimate the contribution of unidentified VOCs to OH reactivity”

GC-FID analysis of polluted urban air samples detects a considerable number of unidentified peaks of volatile organic compounds (VOCs). In this study, a new method is proposed to estimate the contribution of the OH reactivity of such unidentified VOCs using a relative rate method. An air sample was analyzed by two methods: the conventional GC-FID method and a method where the sample was first exposed to the OH radicals generated in the photolysis of water vapor using a pen ray mercury lamp and then analyzed by the same conventional method. The difference in the peak areas of each species with and without OH exposure ideally depends on the OH reactivity of the species of interest. This difference in a standard gas sample that included 56 VOCs was observed through test measurements, and the validity of the system using the proposed technique was confirmed. An ambient air sample obtained in suburban Tokyo was then examined. From the peak area change of the identified species, the contribution of the unidentified VOCs to the OH reactivity was found to be about 22% of that of the identified VOCs. In a separate experiment, simultaneous measurements of the total OH reactivity, various trace gases, and the proposed technique to estimate the contribution of unidentified VOCs were conducted at a Tokyo suburb. The contribution of the unidentified VOCs to the total OH reactivity in suburban air was up to about 6% of the total OH reactivity, and the difference between the observed and estimated total OH reactivity was up to 20%.