Modelling the effect of the 2018 summer heatwave and drought on isoprene emissions in a UK woodland

Abstract

Projected future climatic extremes such as heatwaves and droughts are expected to have major impacts on emissions and concentrations of biogenic volatile organic compounds (bVOCs) with potential implications for air quality, climate and human health. While the effects of changing temperature and photosynthetically active radiation (PAR) on the synthesis and emission of isoprene, the most abundant of these bVOCs, are well known, the role of other environmental factors such as soil moisture stress are not fully understood and are therefore poorly represented in land surface models. As part of the Wytham Isoprene iDirac Oak Tree Measurements campaign, continuous measurements of isoprene mixing ratio were made throughout the summer of 2018 in Wytham Woods, a mixed deciduous woodland in southern England. During this time, the United Kingdom experienced a prolonged heatwave and drought, and isoprene mixing ratios were observed to increase by more than 400% at Wytham Woods under these conditions. We applied the state‐of‐the‐art FORest Canopy‐Atmosphere Transfer canopy exchange model to investigate the processes leading to these elevated concentrations. We found that although current isoprene emissions algorithms reproduced observed mixing ratios in the canopy before and after the heatwave, the model underestimated observations by ~40% during the heatwave–drought period implying that models may substantially underestimate the release of isoprene to the atmosphere in future cases of mild or moderate drought. Stress‐induced emissions of isoprene based on leaf temperature and soil water content (SWC) were incorporated into current emissions algorithms leading to significant improvements in model output. A combination of SWC, leaf temperature and rewetting emission bursts provided the best model‐measurement fit with a 50% improvement compared to the baseline model. Our results highlight the need for more long‐term ecosystem‐scale observations to enable improved model representation of atmosphere–biosphere interactions in a changing global climate.

Amazonian biogenic volatile organic compounds under global change

“…The availability of water within ecosystems affects BVOC production, whether as humidity, foliar uptake or content in soil. Drought reduces stomatal conductance, decreases intracellular CO 2 mixing ratios and as a result, increases isoprene emissions (Pegoraro, Rey, Bobich, et al, 2004), although this is dependent on the severity of drought (Niinemets, 2010; Otu‐Larbi et al, 2020; Pegoraro, Rey, Greenberg, et al, 2004; Potosnak et al., 2014). GLV emission can be stimulated at high temperatures and under drought conditions (Jardine, Chambers, et al, 2015; Pfannerstill et al., 2018).…”

Current and future impacts of drought and ozone stress on Northern Hemisphere forests

“…FORCAsT is a 1D model of biosphere-atmosphere chemical exchange which has previously been used to study canopy structure and mixing (Bryan et al, 2012(Bryan et al, , 2015, stomatal regulation and atmospheric chemistry within and above forest canopies (Ashworth et al, 2015(Ashworth et al, , 2016 and the impact of drought stress on biogenic volatile organic compound emissions and forest gas exchange (Otu-Larbi, Bolas, et al, 2020). A full description of the FORCAsT model can be found in Ashworth et al (2015).…”

iDirac: a field-portable instrument for long-term autonomous measurements of isoprene and selected VOCs

“…For high concentrations (e.g. strong leaf emissions) a smaller volume should be used to avoid nonideal behaviour of the adsorbent, as described by Peters and Bakkeren (1994). The instrument has an effective upper volume limit of 250 mL (see Sect.…”

“…The breakthrough volume for the adsorbent traps used in the iDirac was determined. This is a test that evaluates the volume of gas that causes isoprene to pass through the trap in a single sample run, and is typically independent of the analyte concentration (Peters and Bakkeren, 1994). This test is performed by placing an additional adsorbent trap in the instrument upstream of the main trap, at the exit of valves 1-4 from the valve box.…”

“…Both iDirac instruments were run off-grid, powered only by solar-powered batteries. The experiment and results are described in detail by Ferracci et al (2020a) and Otu-Larbi et al (2019). Data were collected from May to October 2018; here, the performance of the instruments is assessed for more than 5 months of continuous use in a forest environment.…”