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The atmospheric history of molecular hydrogen (H 2 ) from 1852 to 2003 was reconstructed from measurements of firn air collected at Megadunes, Antarctica. The reconstruction shows that H 2 levels in the southern hemisphere were roughly constant near 330 parts per billion (ppb; nmol H 2 mol −1 air) during the mid to late 1800s. Over the twentieth century, H 2 levels rose by about 70% to 550 ppb. The reconstruction shows good agreement with the H 2 atmospheric history based on firn air measurements from the South Pole. The broad trends in atmospheric H 2 over the twentieth century can be explained by increased methane oxidation and anthropogenic emissions. The H 2 rise shows no evidence of deceleration during the last quarter of the twentieth century despite an expected reduction in automotive emissions following more stringent regulations. During the late twentieth century, atmospheric CO levels decreased due to a reduction in automotive emissions. It is surprising that atmospheric H 2 did not respond similarly as automotive exhaust is thought to be the dominant source of anthropogenic H 2. The monotonic late twentieth century rise in H 2 levels is consistent with late twentieth-century flask air measurements from high southern latitudes. An additional unknown source of H 2 is needed to explain twentieth-century trends in atmospheric H 2 and to resolve the discrepancy between bottom-up and top-down estimates of the anthropogenic source term. The firn air–based atmospheric history of H 2 provides a baseline from which to assess human impact on the H 2 cycle over the last 150 y and validate models that will be used to project future trends in atmospheric composition as H 2 becomes a more common energy source. 

The Southern Ocean plays an important role in determining atmospheric carbon dioxide (CO 2 ), yet estimates of air-sea CO 2 flux for the region diverge widely. In this study, we constrained Southern Ocean air-sea CO 2 exchange by relating fluxes to horizontal and vertical CO 2 gradients in atmospheric transport models and applying atmospheric observations of these gradients to estimate fluxes. Aircraft-based measurements of the vertical atmospheric CO 2 gradient provide robust flux constraints. We found an annual mean flux of –0.53 ± 0.23 petagrams of carbon per year (net uptake) south of 45°S during the period 2009–2018. This is consistent with the mean of atmospheric inversion estimates and surface-ocean partial pressure of CO 2 ( P co 2 )–based products, but our data indicate stronger annual mean uptake than suggested by recent interpretations of profiling float observations. 

Dichloromethane (CH₂Cl₂) and perchloroethylene (C₂Cl₄) are chlorinated very short lived substances (Cl‐VSLS) with anthropogenic sources. Recent studies highlight the increasing influence of such compounds, particularly CH₂Cl₂, on the stratospheric chlorine budget and therefore on ozone depletion. Here, a multiyear global‐scale synthesis inversion was performed to optimize CH₂Cl₂(2006–2017) and C₂Cl₄(2007–2017) emissions. The approach combines long‐term surface observations from global monitoring networks, output from a three‐dimensional chemical transport model (TOMCAT), and novel bottom‐up information on prior industry emissions. Our posterior results show an increase in global CH₂Cl₂emissions from 637 ± 36 Gg yr⁻¹in 2006 to 1,171 ± 45 Gg yr⁻¹in 2017, with Asian emissions accounting for 68% and 89% of these totals, respectively. In absolute terms, Asian CH₂Cl₂emissions increased annually by 51 Gg yr⁻¹over the study period, while European and North American emissions declined, indicating a continental‐scale shift in emission distribution since the mid‐2000s. For C₂Cl₄, we estimate a decrease in global emissions from 141 ± 14 Gg yr⁻¹in 2007 to 106 ± 12 Gg yr⁻¹in 2017. The time‐varying posterior emissions offer significant improvements over the prior. Utilizing the posterior emissions leads to modeled tropospheric CH₂Cl₂and C₂Cl₄abundances and trends in good agreement to those observed (including independent observations to the inversion). A shorter C₂Cl₄lifetime, from including an uncertain Cl sink, leads to larger global C₂Cl₄emissions by a factor of ~1.5, which in some places improves model‐measurement agreement. The sensitivity of our findings to assumptions in the inversion procedure, including CH₂Cl₂oceanic emissions, is discussed.