Supplementary material to "Enhancing Long-Term Trend Simulation of Global Tropospheric OH and Its Drivers from 2005–2019: A Synergistic Integration of Model Simulations and Satellite Observations"

Tropospheric ozone trend analysis Data:To gain insight into the MERRA2-GMI tropospheric ozone columns trends, we employ a data product created from OMI and Microwave Limb Sounder (MLS) data that spans a latitude range of 60 o S-60 o N. Ziemke et al. (2006) employed a technique wherein tropospheric ozone columns were derived by subtracting MLS stratospheric columns from OMI total columns for each grid point daily.The precision of this dataset is estimated to be 1.3 DU.A comparison of these observations with ozonesondes for the period 2004-2005 yielded an RMSE value near 5 DU (Ziemke et al. 2006).It is worth noting that Ziemke et al. ( 2019) have observed a moderate drift (-1.0 DU/decade) caused by the row anomaly, which is currently being corrected for.This product does not follow an optimal estimation method to provide its dependency on the prior profiles (i.e., averaging kernels); therefore, we do not use this product to constrain our model. Analysis:In this study, despite the absence of observational constraints on tropospheric ozone concentrations from satellites in our study, it is deemed essential to provide insights into the long-term trends of MERRA2-GMI and their comparison with OMI/MLS observations.Here, we use daily-averaged MERRA2-GMI data without enforcing any masks used for OMI/MLS observations.The statistical significance of the linear trend maps of tropospheric ozone columns observed by OMI/MLS from 2005 to 2019 and simulated by MERRA2-GMI are depicted in Figure S3.OMI/MLS demonstrates a widespread increase in tropospheric ozone concentrations (+1-3 DU/decade) across the globe.The analysis shows that the increases in tropospheric ozone concentrations are more prominent in the northern hemisphere than in the southern hemisphere.The upward ozone trends are mainly concentrated in Asia due to a rise in anthropogenic emissions.The impact of Asian emissions on ozone pollution is not limited to its own region as it can transport hemispherically to the western United States, thereby increasing the background concentration levels (Cooper et al., 2010; Lin et al., 2017).The OMI/MLS trend map over the Pacific Ocean illustrates this observation clearly.The observations by OMI/MLS also indicate a hotspot of increased ozone concentrations over the subtropical Atlantic Ocean, which is potentially due to African biomass burning activities (Cuevas et al., 2013).The comparison of the OMI/MLS and MERRA2-GMI datasets indicates a strong degree of agreement in the northern hemisphere.Gaudel et al. (2018) have compiled long-term mountaintop nighttime ozone measurements in the northern hemisphere and found that most of them exhibit upward trends, representative of the free-tropospheric trends.The trends observed in the southern hemisphere by OMI/MLS do not align with those simulated by MERRA2-GMI (Ziemke et al., 2019).Lu et al. (2019) indirectly supports the upward trends detected by OMI/MLS by compiling long-term records of several surface observations and ozonesonde measurements at high latitudes in the southern hemisphere.Using a global model, they hypothesized that the upward trends may resulted from the expansion of Hadley circulation, particularly in austral autumn (March until May), leading to greater stratospheric contributions to the surface and a more effective mixing of ozone precursors from heavily polluted tropical regions to the free troposphere.However, it remains unverified whether this expansion occurs throughout the year, resulting in widespread upward trends observed by OMI/MLS.A more recent study, Thompson et al. (2021), utilized Southern Hemisphere Additional Ozonesondes (SHADOZ) data and suggested that the free-tropospheric ozone trends in 1998-2019 were fainter than those detected by satellite observations and varied greatly from season to season due to atmospheric dynamics (i.e., expansion or shrink in the tropopause height).An important caveat to consider is that satellite-based tropospheric ozone concentration can be largely uncertain due to limited sensitivity of the observed radiance to the optical path of ozone in the lower tropospheric region.Gaudel et