Supplementary material to "Instantaneous intraday changes in key meteorological parameters as a proxy for the mixing ratio of BVOCs over vegetation under drought conditions"

Abstract. Biogenic volatile organic compounds (BVOCs) exert a significant influence on photochemical air pollution and climate change, with their emissions strongly affected by meteorological conditions. However, the effect of drought on BVOC emissions is not well-characterized, limiting the predictive power of this feedback on climate change and air quality. This study hypothesized that under severe drought conditions, BVOC emissions will be more sensitive to instantaneous intraday variations in meteorological parameters than to the absolute values of those parameters. To test this hypothesis, we employed proton transfer reaction time-of-flight mass spectrometry to quantify the mixing ratios of a suite of soluble and insoluble VOCs, including isoprene, monoterpenes, sesquiterpenes, acetone, acetaldehyde, methanol, ethanol, formaldehyde, formic acid, acetic acid, 1,3-butadiene, dimethyl sulfide (DMS), and H2S, under severe drought conditions in a natural Eastern Mediterranean forest in autumn 2016. Except for H2S, which was used as a control, and to a certain extent DMS, all measured VOCs exhibited a strong response to changes in relative humidity, with lower mixing ratios observed around noon, suggesting inhibition of BVOC emission under the relatively high temperature and low relative humidity of drought conditions. Notably, our analysis revealed that instantaneous changes in meteorological conditions, especially in relative humidity, can serve as a better proxy for drought-related changes in BVOC emission rate than the absolute values of meteorological parameters. These findings are supported by direct flux measurements conducted in a mixed Mediterranean forest under drought conditions, in the same region, and presented as a companion article. The findings further highlight the importance of analyzing the effect of meteorological conditions on BVOC emissions under drought conditions on a daily—or shorter—timescale, and support biogenic emission sources for 1,3-butadiene.