Satellite-derived Constraints on the Effect of Drought Stress on 1 Biogenic Isoprene Emissions in the Southeast US 2

While substantial progress has been made to improve our understanding of biogenic isoprene emissions 15 under unstressed conditions, there remain large uncertainties in isoprene emissions under stressed conditions. Here 16 we use the US Drought Monitor (USDM) as a weekly drought severity index and tropospheric columns of 17 formaldehyde (HCHO), the key product of isoprene oxidation, retrieved from the Ozone Monitoring Instrument (OMI) 18 to derive top-down constraints on the response of summertime isoprene emissions to drought stress in the Southeast 19 U.S. (SE US), a region of high isoprene emissions and prone to drought. OMI HCHO column density is found to be 20 5.3% (mild drought) - 19.8% (severe drought) higher than that in no-drought conditions. A global chemical transport 21 model, GEOS-Chem, with the MEGAN2.1 emission algorithm can simulate this direction of change, but the simulated 22 increases at the corresponding drought levels are 1.4-2.0 times of OMI HCHO, suggesting the need for a drought- 23 stress algorithm in the model. By minimizing the model-to-OMI differences in HCHO to temperature sensitivity under 24 different drought levels, we derived a top-down drought stress factor ( γ d_OMI ) in GEOS-Chem that parameterizes using 25 water stress and temperature. The algorithm led to an 8.6% (mild drought) - 20.7% (severe drought) reduction in 26 isoprene emissions in the SE US relative to the simulation without it. With γ d_OMI the model predicts a non-uniform 27 trend of increase in isoprene emissions with drought severity that is consistent with OMI HCHO and a single site’s 28 isoprene flux measurements. Compared with a previous drought stress algorithm derived from the latter, the satellite- 29 based drought stress factor performs better in capturing the regional scale drought-isoprene responses as indicated by 30 the close-to-zero mean bias between OMI and simulated HCHO columns under different drought conditions. The 31 drought stress algorithm also reduces the model’s high bias in organic aerosols (OA) simulations by 6.60% (mild 32 drought) to 11.71% (severe drought) over the SE US compared to the no-stress simulation. The simulated ozone 33 response to the drought stress factor displays a spatial disparity due to the isoprene suppressing effect on oxidants, 34 with an <1 ppb increase in O 3 in high-isoprene regions and a 1-3 ppbv decrease in O 3 in low-isoprene regions. This 35 study demonstrates the unique value of exploiting long-term satellite observations to develop empirical stress 36 algorithms on biogenic emissions where in situ flux measurements are limited. simulation has a mean high bias of 0.15×10 16 mole cm -2 - 0.42×10 16 mole cm -2 during D0-D4. The 378 γ d_OMI algorithm reduces the high bias to 0.02×10 16 mole cm -2 - 0.20×10 16 mole cm -2 . By contrast, the γ d_MOFLUX 379 algorithm reduces the HCHO simulations too much over the SE US and causes an overall underestimation of 0.12×10 16 380 mole cm -2 - 0.38×10 16 mole cm -2 . The γ d_MOFLUX algorithm also narrows the statistical distribution of HCHO as 381 indicated by the smaller boxes and shorter whiskers compared to OMI. This suggests that the γ d_MOFLUX algorithm 382 based on the single-site observations is incapable of representing the drought stress over the SE US. Thus, the γ d_OMI 383