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.

中文翻译：

---

卫星衍生的对干旱胁迫对美国东南部生物异戊二烯排放影响的约束

摘要。虽然在提高我们对无压力条件下生物异戊二烯排放的理解方面取得了实质性进展，但在压力条件下异戊二烯排放仍然存在很大的不确定性。在这里，我们使用美国干旱监测仪 (USDM) 作为每周干旱严重程度指数和甲醛 (HCHO) 的对流层柱（异戊二烯氧化的关键产物），从臭氧监测仪 (OMI) 检索得到自上而下的响应约束美国东南部 (SE US) 夏季异戊二烯排放量对干旱压力的影响，该地区异戊二烯排放量高且容易发生干旱。发现 OMI HCHO 柱密度比非干旱条件下高 5.3 %（轻度干旱）– 19.8 %（严重干旱）。具有 MEGAN2 的全球化学输运模型 GEOS-Chem。1排放算法可以模拟这种变化方向，但在相应干旱水平下模拟的增加量是OMI HCHO的1.4-2.0倍，表明模型中需要干旱胁迫算法。通过最小化不同干旱水平下 HCHO 对温度敏感性的模型与 OMI 差异，我们推导出了一个自上而下的干旱胁迫因子（_{γd_OMI} ) 在 GEOS-Chem 中使用水应力和温度进行参数化。相对于没有它的模拟，该算法导致美国东南部异戊二烯排放减少 8.6%（轻度干旱）至 20.7%（严重干旱）。用_{γd_OMI}该模型预测异戊二烯排放量增加的不均匀趋势与干旱严重程度一致，这与 OMI HCHO 和单个站点的异戊二烯通量测量值一致。与从后者派生的先前干旱胁迫算法相比，基于卫星的干旱胁迫因子在捕获区域尺度干旱-异戊二烯响应方面表现更好，如在不同干旱条件下 OMI 和模拟 HCHO 柱之间的接近于零的平均偏差所示条件。与无胁迫模拟相比，干旱胁迫算法还将模型在美国东南部有机气溶胶 (OA) 模拟中的高偏差降低了 6.60%（轻度干旱）至 11.71%（严重干旱）。由于异戊二烯对氧化剂的抑制作用，模拟的臭氧对干旱胁迫因子的响应显示出空间差异，<₃在高异戊二烯区域，O ₃在低异戊二烯区域减少 1-3 ppbv。这项研究证明了利用长期卫星观测来开发生物排放经验应力算法的独特价值，其中原位通量测量是有限的。