Impact of meteorological conditions on BVOC emission rate from Eastern Mediterranean vegetation under drought

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 focused on two main objectives: i) test our hypothesis 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; ii) test the impact of a plant under drought stress receiving a small amount of precipitation on BVOC emission rate, and the manner in which the emission rate is influenced by meteorological parameters. 

To address these objectives we employed: i) proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) 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 ; and ii) branch-enclosure sampling measurements in Ramat Hanadiv Eastern Mediterranean Nature Park, both under natural drought and after irrigation, for six selected branches of Phillyrea latifolia, the highest BVOC emitter in this park, during September–October 2020.

Notably, both independent analyses revealed that instantaneous changes in meteorological conditions, especially in relative humidity (RH), can serve as a better proxy for drought-related changes in BVOC emission rate than the absolute values of meteorological parameters. However, after irrigation (equivalent to 5.5–7 mm precipitation), the correlation of the detected BVOC emission rate with the instantaneous changes in RH became significantly more moderate, or even reversed. Our findings highlight that under drought, the instantaneous changes in RH, and to a lesser extent in temperature (T), are the best proxy for the emission rate of monoterpenes (MTs) and sesquiterpenes (SQTs), whereas under moderate drought conditions, T or RH serves as the best proxy for MT and SQT emission rate, respectively. In addition, the detected emission rates of MTs and SQTs increased by 150% and 545%, respectively, after the small amount of irrigation. 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.