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

Abstract

Biogenic volatile organic compounds (BVOCs), primarily emitted by terrestrial vegetation, are highly reactive and have large effects on the oxidizing potential of the troposphere, air quality and climate. In terms of global emissions, isoprene is the most important BVOC. Droughts bring about changes in the surface emission of biogenic hydrocarbons mainly because plants suffer water stress. Past studies report that the current parameterization in the state-of-the-art Model of Emissions of Gases and Aerosols from Nature (MEGAN) v2.1, which is a function of the soil water content and the permanent wilting point, fails at representing the strong reduction in isoprene emissions observed in field measurements conducted during a severe drought. Since the current algorithm was originally developed based on potted plants, in this study, we update the parameterization in the light of recent ecosystem-scale measurements of isoprene conducted during natural droughts in the central U.S. at the Missouri Ozarks AmeriFlux (MOFLUX) site. The updated parameterization results in stronger reductions in isoprene emissions. Evaluation using satellite formaldehyde (HCHO), a proxy for BVOC emissions, and a chemical-transport model, shows that the adjusted parameterization provides a better agreement between the modelled and observed HCHO temporal variability at local and regional scales in 2011–2012, even if it worsens the model agreement in a global, long-term evaluation. We discuss the limitations of the current parameterization, a function of highly uncertain soil properties such as porosity.

Optimizing the Isoprene Emission Model MEGAN With Satellite and Ground‐Based Observational Constraints

“…Including 𝜸 SM in our optimization experiments therefore risks misattributing uncertainties in 𝜸 SM to other model parameters including E 0 and 𝜸 T . A recent study has found that the parameterization of the MEGAN 𝜸 SM can be improved using a combination of ecosystem‐scale isoprene flux measurements and satellite‐derived SM (Opacka et al., 2022). Future work in this direction, along with updates to the 𝜸 SM algorithm to account for the impact of moderate drought stress on emissions, should improve the utility of the MEGAN drought stress response and allow parameter optimization experiments to be reliably performed under drought conditions.…”

“…We used gridded 𝜸 SM values at 0.5° × 0.5° from 2005–2015 as obtained with Community Land Model (CLM) 4.5 and the MEGAN3 SM algorithm (X. Jiang et al., 2018). There are large uncertainties associated with the drought stress response 𝜸 SM (X. Jiang et al., 2018; Potosnak et al., 2014; Seco et al., 2015; Opacka et al., 2022). To quantify the sensitivity of our analysis to this model component, we run our E 0 optimization experiments both with and without 𝜸 SM enabled.…”

“…There are large uncertainties associated with the drought stress response 𝜸 SM (X. Jiang et al, 2018;Potosnak et al, 2014;Seco et al, 2015;Opacka et al, 2022).…”

Modeling Isoprene Emission Response to Drought and Heatwaves Within MEGAN Using Evapotranspiration Data and by Coupling With the Community Land Model

“…If the input is changed to other SM data sets for different depths, such as the surface SM data sets from satellite or root zone SM data sets, the current threshold values would not be appropriate (Opacka et al, 2022).…”

“…For instance, the wilting point, the SM at which a plant cannot extract water from soil, is used to define the severity of drought in MEGAN v2.1. However, some previous studies (Huang et al, 2015;Opacka et al, 2022;Potosnak et al, 2014;Seco et al, 2015) have shown that the wilting point estimates are a major source of uncertainty for isoprene emission estimation during the drought. Second, the SM driven drought algorithm is sensitive to the accuracy of the SM inputs, and the systematic errors of the SM data sets from land surface models or satellites will directly affect the estimation of isoprene emission (Emmerson et al, 2019;Opacka et al, 2022).…”