Interhemispheric asymmetry in OH abundance inferred from measurements of atmospheric 14CO

Radiocarbon monoxide indicates increasing atmospheric oxidizing capacity

Hydroxyl (OH) is the atmosphere's main oxidant removing most pollutants including methane. Its short lifetime prevents large-scale direct observational quantification. Abundances inferred using anthropogenic trace gas measurements and models yield conflicting trend estimates. By contrast, radiocarbon monoxide (14CO), produced naturally by cosmic rays and almost exclusively removed by OH, is a tracer with a well-understood source. Here we show that Southern-Hemisphere 14CO measurements indicate increasing OH. New Zealand 14CO data exhibit an annual-mean decrease of 12 ± 2% since 1997, whereas Antarctic measurements show a December-January decrease of 43 ± 24%. Both imply similar OH increases, corroborating our own and other model results suggesting that OH has been globally increasing during recent decades. Model sensitivity simulations illustrate the roles of methane, nitrogen oxides, stratospheric ozone depletion, and global warming driving these trends. They have substantial implications for the budgets of pollutants removed by OH, and especially imply larger than documented methane emission increases.

Photo-enhanced oxidation of arsenite by biochar: The effect of pH, kinetics and mechanisms

The persistent and photo-induced free radicals of biochar play significant roles in the transformation or degradation of inorganic and organic pollutants. However, the redox capacity of biochar for arsenite (As(III)) photochemistry under different pH conditions remains unclear. In this study, we discovered that solar radiation primarily expedited the oxidation of As(III) by biochar by augmenting the production of reactive oxygen species (ROS). Biochar demonstrated a strong pH reliance on the photooxidation of As(III). Under acidic and neutral conditions, solar radiation amplified the generation of •OH (hydroxyl radicals) by BC-P (phenolic -OH of biochar) and semiquinone-type BC-PFRs (persistent free radicals of biochar) by 4.9 and 2.0 times, respectively, resulting in enhanced As(III) oxidation. Under alkaline conditions, BC-P and BC-Q (quinoid CO of biochar) facilitated the production of H2O2 (hydrogen peroxide) by 2.1 times through the spontaneous formation of semiquinone-type BC-PFRs via an anti-disproportionation reaction, promoting approximately 88.2% of As(III) photooxidation. Furthermore, solar radiation elevated around 11.8% As(III) oxidation driven by BC-Q and semiquinone-type BC-PFRs. This study provides a crucial theoretical foundation for using biochar to treat arsenic pollution in aquatic systems and understanding the migration and transformation of arsenic in different environments.

Observational evidence for interhemispheric hydroxyl-radical parity

The hydroxyl radical (OH) is a key oxidant involved in the removal of air pollutants and greenhouse gases from the atmosphere. The ratio of Northern Hemispheric to Southern Hemispheric (NH/SH) OH concentration is important for our understanding of emission estimates of atmospheric species such as nitrogen oxides and methane. It remains poorly constrained, however, with a range of estimates from 0.85 to 1.4 (refs 4, 7-10). Here we determine the NH/SH ratio of OH with the help of methyl chloroform data (a proxy for OH concentrations) and an atmospheric transport model that accurately describes interhemispheric transport and modelled emissions. We find that for the years 2004-2011 the model predicts an annual mean NH-SH gradient of methyl chloroform that is a tight linear function of the modelled NH/SH ratio in annual mean OH. We estimate a NH/SH OH ratio of 0.97 ± 0.12 during this time period by optimizing global total emissions and mean OH abundance to fit methyl chloroform data from two surface-measurement networks and aircraft campaigns. Our findings suggest that top-down emission estimates of reactive species such as nitrogen oxides in key emitting countries in the NH that are based on a NH/SH OH ratio larger than 1 may be overestimated.

Short-term variations in the oxidizing power of the atmosphere

The hydroxyl radical is the predominant atmospheric oxidant, responsible for removing a wide range of trace gases, including greenhouse gases, from the atmosphere. Determination of trends and variability in hydroxyl radical concentrations is critical to understanding whether the 'cleansing' properties of the atmosphere are changing. The variability in hydroxyl radical concentrations on annual to monthly timescales, however, is difficult to quantify. Here we show records of carbon monoxide containing radiocarbon (14CO), which is oxidized by hydroxyl radicals, from clean-air sites at Baring Head, New Zealand, and Scott Base, Antarctica, spanning 13 years. Using a model study, we correct for known variations in production of 14CO (refs 6, 7), allowing us to exploit this species as a diagnostic for short term changes in hydroxyl radical concentrations. We find no significant long-term trend in hydroxyl radical concentrations but provide evidence for recurring short-term variations of around ten per cent persisting for a few months. We also find decreases in hydroxyl radical concentrations of up to 20 per cent, apparently triggered by the eruption of Mt Pinatubo in 1991 and by the occurrence of extensive fires in Indonesia in 1997.