Impact of Drought on Isoprene Fluxes Assessed Using Field Data, Satellite-based GLEAM Soil Moisture and HCHO Observations From OMI

Among the biogenic volatile organic compounds (BVOCs) emitted by plant foliage, isoprene is by far the most important in terms of both global emission and atmospheric impact. Droughts can substantially affect its emissions in ways that are largely unknown. In this study, we use the state-of-the-art MEGAN biogenic emission model (Guenther et al., 2006; 2012) coupled with the canopy model MOHYCAN (Müller et al., 2008) to estimate isoprene emissions and evaluate two different parameterizations of the stress factor accounting for the impact of drought on isoprene emissions: (a) the scheme used in MEGANv2.1, which consists of a simple dependence on soil water content and the permanent wilting point with inputs from the GLEAMv3 reanalysis (Martens et al., 2017), and (b) the MEGANv3 parameterization (Jiang et al., 2018), which considers the physiological effects of drought stress on plant photosynthesis as defined in the Community Land Model (CLM4.5), which embeds the MEGAN model. The effect of the stress factor on isoprene flux estimates is assessed against measurements of isoprene fluxes at the Missouri Ozarks AmeriFlux (MOFLUX) field site, located in a temperate forest in the central U.S., collected during the mild drought in 2011 (Potosnak et al., 2014) and the severe drought in 2012 (Seco et al., 2015). Based on the measured isoprene fluxes at the MOFLUX site, the MEGANv2.1 parameterization is adjusted through the optimization of a critical parameter, namely the threshold soil moisture content below which isoprene emission is suppressed due to drought. The adjusted parameterization is further evaluated over the U.S. using spaceborne formaldehyde (HCHO) columns observed by the Ozone Monitoring Instrument (OMI) sounder. To this end, we perform two-year simulations (2011-2012) of atmospheric composition over the U.S. with the MAGRITTEv1.1 regional chemistry-transport model (Müller et al., 2019) using isoprene emission datasets obtained using MEGANv2.1 either without or with the updated soil moisture stress parameterization. The resulting HCHO columns during the summertime over the MOFLUX site and over the U.S. point to an overall better agreement of the optimized parameterization with OMI. Two regions, Texas and southeast U.S., exhibit the largest changes in simulations and are examined in greater detail.