Keywords:biogenic VOCs, nitrogen oxides, standard addition method
Biogenic volatile organic compounds (BVOCs) have been focused on as precursors of tropospheric ozone (O3) and secondary organic aerosols. Various species of BVOCs have C=C double bonds and can react with O3. To capture BVOCs comprehensively, a total ozone reactivity (RO3, the sum of ki[VOCi]) analyzer has been developed [1-4]. RO3 of sample BVOCs can be determined when decrease of O3 due to BVOCs+O3 is precisely monitored. In our previous studies, the detection limit of the analyzer reached 6.3x10-5 s-1 (S/N=3, 60-s average, 50-s reaction) . To apply the analyzer to field observations where the samples consist of multiple compounds, characteristics of the analyzer should be explored further. For example, nitric oxide, NO, which exists significantly everywhere in the troposphere, can react with ozone rapidly. When BVOCs are captured as RO3, NO can be detected simultaneously. A model study reported that, even when RO3 in the forest atmosphere is focused, contribution of NO to RO3 can be critical . Generally, NO concentration can be captured easily utilizing an NO analyzer. Thus, if dependence of the output of RO3 analyzer on NO concentration is understood, contribution of NO to observed RO3 can be separated and RO3 due to BVOCs can be accurately quantified. In this study, mixtures of isoprene and NO standard gases were prepared and response of RO3 analyzer to the mixtures was experimentally captured in order to explore the possibilities of the standard addition method of NO to BVOCs samples. As a result, among the isoprene standard sample (A) and NO-added isoprene samples (B, C, D), a strong correlation was observed between the concentration of NO and the measured RO3 as shown in Fig.1. The intercept of the regression line agreed well with the observed RO3 of isoprene standard sample (A). It was experimentally confirmed that, when NO addition was conducted, the contribution of BVOCs (isoprene in the figure) to RO3 could be determined as the intercept of the regression line between NO concentration and measured RO3. Consequently, the method of standard addition to separate the contribution of NO from observed RO3 was established. Evaluation of BVOCs as RO3 is possible even when NO coexists in the sample. As a next step, application of this method to RO3 observations for studying BVOCs emission from real plants and/or ambient samples is promising.
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