16 Jun 2022
16 Jun 2022

Satellite-derived Constraints on the Effect of Drought Stress on Biogenic Isoprene Emissions in the Southeast US

Yuxuan Wang1, Nan Lin1, Wei Li1, Alex Guenther2, Joey C. Y. Lam3, Amos P. K. Tai3,4, Mark J. Potosnak5, and Roger Seco6 Yuxuan Wang et al.
  • 1Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas, USA
  • 2Earth System Science, University of California, Irvine, Irvine, California, USA
  • 3Earth System Science Programme and Graduate Division of Earth and Atmospheric Sciences, Faculty of Science, The Chinese University of Hong Kong, Hong Kong SAR, China
  • 4State Key Laboratory of Agrobiotechnology and Institute of Environment, Energy and Sustainability, The Chinese University of Hong Kong, Hong Kong SAR, China
  • 5Environmental Science and Studies, DePaul University, Chicago, IL, USA
  • 6Institute of Environmental Assessment and Water Research (IDAEA-CSIC), Carrer Jordi Girona 18-26, 08034 Barcelona, Spain

Abstract. While substantial progress has been made to improve our understanding of biogenic isoprene emissions under unstressed conditions, there remain large uncertainties in isoprene emissions under stressed conditions. Here we use the US Drought Monitor (USDM) as a weekly drought severity index and tropospheric columns of formaldehyde (HCHO), the key product of isoprene oxidation, retrieved from the Ozone Monitoring Instrument (OMI) to derive top-down constraints on the response of summertime isoprene emissions to drought stress in the Southeast U.S. (SE US), a region of high isoprene emissions and prone to drought. OMI HCHO column density is found to be 5.3 % (mild drought) – 19.8 % (severe drought) higher than that in no-drought conditions. A global chemical transport model, GEOS-Chem, with the MEGAN2.1 emission algorithm can simulate this direction of change, but the simulated increases at the corresponding drought levels are 1.4–2.0 times of OMI HCHO, suggesting the need for a drought-stress algorithm in the model. By minimizing the model-to-OMI differences in HCHO to temperature sensitivity under different drought levels, we derived a top-down drought stress factor (γd_OMI) in GEOS-Chem that parameterizes using water stress and temperature. The algorithm led to an 8.6 % (mild drought) – 20.7 % (severe drought) reduction in isoprene emissions in the SE US relative to the simulation without it. With γd_OMI the model predicts a non-uniform trend of increase in isoprene emissions with drought severity that is consistent with OMI HCHO and a single site’s isoprene flux measurements. Compared with a previous drought stress algorithm derived from the latter, the satellite-based drought stress factor performs better in capturing the regional scale drought-isoprene responses as indicated by the close-to-zero mean bias between OMI and simulated HCHO columns under different drought conditions. The drought stress algorithm also reduces the model’s high bias in organic aerosols (OA) simulations by 6.60 % (mild drought) to 11.71 % (severe drought) over the SE US compared to the no-stress simulation. The simulated ozone response to the drought stress factor displays a spatial disparity due to the isoprene suppressing effect on oxidants, with an <1 ppb increase in O3 in high-isoprene regions and a 1–3 ppbv decrease in O3 in low-isoprene regions. This study demonstrates the unique value of exploiting long-term satellite observations to develop empirical stress algorithms on biogenic emissions where in situ flux measurements are limited.

Yuxuan Wang et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-436', Anonymous Referee #1, 09 Jul 2022
  • RC2: 'Comment on egusphere-2022-436', Anonymous Referee #2, 11 Jul 2022
  • CC1: 'Comment on egusphere-2022-436', Valerio Ferracci, 19 Jul 2022

Yuxuan Wang et al.


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Short summary
Drought can impose large changes on biogenic isoprene emissions. In-situ field observations of isoprene emissions under droughts are confined by spatial coverage, thus providing limited constraints. We derived a drought stress factor based on satellite formaldehyde data for MEGAN2.1 in GEOS-Chem model using water stress and temperature. This factor reduces the overestimation of isoprene emissions under severe droughts and improves the simulated ozone and organic aerosol responses to droughts.