Abstract. Despite decades of effort, the drivers of global long-term trends in tropospheric ozone are not well understood, impacting estimates of ozone radiative forcing and the global ozone budget. We analyze tropospheric ozone trends since 1980 using ozonesondes and remote surface measurements around the globe and investigate the ability of two atmospheric chemical transport models, GEOS-Chem and MERRA2-GMI, to reproduce these trends. Global tropospheric ozone trends measured at 25 ozonesonde sites from 1990–2017 (9 sites since 1980s) show increasing trends averaging 2.1 ± 1.3 ppb decade^-1 across sites in the free troposphere (800–400 hPa). Relative trends in sondes are more pronounced closer to the surface (5.1 % decade^-1 above 700 hPa, 4.0 % decade^-1 below 700 hPa on average), suggesting the importance of emissions in observed changes. While most surface sites (148 of 238) in the United States and Europe exhibit decreases in high daytime ozone values due to regulatory efforts, 73 % of global sites outside those regions (24 of 33 sites) show increases from 1990–2014 that average 1.4 ± 0.9 ppb decade^-1. In all regions, increasing ozone trends both at the surface and aloft are at least partially attributable to increases in 5^th percentile ozone, which average 1.7 ± 1.0 ppb decade^-1 and reflect the global increase of background ozone. Observed ozone percentile distributions at the surface have shifted notably across the globe: all regions show increases in low tails (i.e., below 25^th percentile), North America and Europe show decreases in high tails (above 75^th percentile), and the Southern Hemisphere and Japan show increases across the entire distribution. Three model simulations comprising different emissions inventories, chemical schemes, and resolutions, sampled at the same locations and times of observations, are not able to replicate long-term ozone trends either at the surface or free troposphere, often underestimating trends. We find that ~60 % of the average ozone trend from 800–400 hPa across the 25 ozonesonde sites is captured by MERRA2-GMI and <15 % is captured by GEOS-Chem. MERRA2-GMI performs better than GEOS-Chem in the northern mid-latitude free troposphere, reproducing 71 % of increasing trends since 1990 and capturing stratosphere-troposphere exchange (STE) determined via a stratospheric ozone tracer. While all models tend to capture the direction of shifts in the ozone distribution and typically capture changes in high and low tails, they tend to underestimate the magnitude of the shift in medians. However, each model shows an 8–12 % (or 23–32 Tg) increase in total tropospheric ozone burden from 1980 to 2017. Sensitivity simulations using GEOS-Chem and the stratospheric ozone tracer in MERRA2-GMI suggest that in the northern mid- and high latitudes, dynamics such as STE are most important for reproducing ozone trends in models in the middle and upper troposphere, while emissions are more important closer to the surface. Our model evaluation for the last 4 decades reveals that the recent version of the GEOS-Chem model underpredicts free tropospheric ozone across this long time period, particularly in winter and spring over mid-to high latitudes. Such widespread model underestimation of tropospheric ozone highlights the need for better understanding of the processes that transport ozone and promote its production.

中文翻译：

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来自臭氧探空仪和地表观测的全球对流层臭氧的数十年增加：模型能否再现臭氧趋势？

摘要。尽管进行了数十年的努力，对流层臭氧的全球长期趋势的驱动因素仍未得到很好的理解，这影响了对臭氧辐射强迫和全球臭氧预算的估计。我们使用臭氧探空仪和全球远程地表测量分析了自 1980 年以来的对流层臭氧趋势，并研究了两种大气化学传输模型 GEOS-Chem 和 MERRA2-GMI 重现这些趋势的能力。1990 年至 2017 年期间在 25 个臭氧探空仪站点（自 1980 年代以来 9 个站点）测量的全球对流层臭氧趋势显示，自由对流层（800–400 hPa）各站点的平均增长趋势为 2.1 ± 1.3 ppb ^{-1 。}探空仪的相对趋势更接近地表（700 hPa 以上 5.1 % 十年 -1，4.0 %^十年-1平均低于 700 hPa），表明排放在观测到的变化中的重要性。尽管由于监管措施，美国和欧洲的大多数地表站点（238 个中的 148 个）显示白天高臭氧值下降，但这些地区以外的 73% 的全球站点（33 个站点中的 24 个）显示从 1990 年到 2014 年平均增加了 1.4 ± 0.9 ppb 十倍频^-1。在所有地区，地表和高空臭氧的增加趋势至少部分归因于 5 % 的臭氧增加，平均为 1.7 ± 1.0 ppb ^-1并反映了全球背景臭氧的增加。观测到的地表臭氧百分位分布在全球范围内发生了显着变化：所有区域均显示低尾增加（即低于 25 ^th百分位），北美和欧洲显示高尾减少（超过 75 ^th百分位），而南半球和日本在整个分布中都显示出增加。三个模型模拟包括不同的排放清单、化学方案和分辨率，在相同的观测地点和时间取样，无法复制地表或自由对流层的长期臭氧趋势，往往低估了趋势。我们发现，MERRA2-GMI 捕获了 25 个臭氧探空仪站点 800-400 hPa 的平均臭氧趋势的约 60%，而 GEOS-Chem 捕获了 <15%。MERRA2-GMI 在北部中纬度自由对流层中的表现优于 GEOS-Chem，再现了自 1990 年以来 71% 的增长趋势，并捕获了通过平流层臭氧示踪剂确定的平流层-对流层交换 (STE)。虽然所有模型都倾向于捕捉臭氧分布的变化方向，并且通常捕捉高尾和低尾的变化，但它们往往低估了中位数变化的幅度。然而，从 1980 年到 2017 年，每个模型都显示对流层臭氧总负荷增加了 8-12%（或 23-32 Tg）。使用 GEOS-Chem 和 MERRA2-GMI 中的平流层臭氧示踪剂进行的敏感性模拟表明，在北部中部在高纬度地区，STE 等动力学对于再现对流层中层和高层模型中的臭氧趋势最重要，而排放在靠近地表的地方更为重要。我们对过去 40 年的模型评估表明，最新版本的 GEOS-Chem 模型低估了在很长一段时间内的自由对流层臭氧，特别是在中高纬度地区的冬季和春季。这种对对流层臭氧普遍低估的模型凸显了更好地了解臭氧运输和促进其生产的过程的必要性。