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Abstract. Future trajectories of the stratospheric trace gas background will alter the rates of bromine- and chlorine-mediated catalytic ozone destruction via changes in the partitioning of inorganic halogen reservoirs and the underlying temperature structure of the stratosphere. The current formulation of the bromine alpha factor, the ozone-destroying power of stratospheric bromine atoms relative to stratospheric chlorine atoms, is invariant with the climate state. Here, we refactor the bromine alpha factor, introducing normalization to a benchmark chemistry–climate state, and formulate Equivalent Effective Stratospheric Benchmark-normalized Chlorine (EESBnC) to reflect changes in the rates of both bromine- and chlorine-mediated ozone loss catalysis with time. We show that the ozone-processing power of the extrapolar stratosphere is significantly perturbed by future climate assumptions. Furthermore, we show that our EESBnC-based estimate of the extrapolar ozone recovery date is in closer agreement with extrapolar ozone recovery dates predicted using more sophisticated 3-D chemistry–climate models than predictions made using equivalent effective stratospheric chlorine (EESC). 

In Spring 2016, 27 whole air samples were collected from an aircraft ∼500–∼3,500 m over Hebei Province, China and analyzed for 16 halocarbons, including chlorofluorocarbons (CFCs). Mixing ratios (median, 25th–75th percentiles) of CFC‐11 (281, 255–318 ppt), CFC‐12 (546, 473–591 ppt), CFC‐113 (79, 73–85 ppt), CFC‐114 (22, 19–25 ppt), HCFC‐22 (345, 308–432 ppt), and CCl₄(88, 75–104 ppt) were often observed to be higher than their global tropospheric background levels. The significantly elevated mixing ratios of ozone depleting substances (ODSs) combined with strong correlations with anthropogenic tracers known to have substantial use and emission in this region (HCFC‐22 and CH₂Cl₂) suggest continuing emissions of multiple Montreal Protocol‐controlled gases at the time of measurement. We use HYSPLIT trajectory clusters and potential source contribution function methods to identify principal transport pathways of CFCs. We find the highest mixing ratios of ODSs in air originating from Inner Mongolia, Hebei, and Shandong. The strong correlations between CFC‐11 and CFC‐12 with the feedstock CCl₄suggest new production is prevalent in all three regions. We find no evidence for new production of CFC‐113, but the strong correlation of CFC‐114 with the feedstock C₂Cl₄suggests new production of CFC‐114 from the southeast of China. The findings of this study confirm high mixing ratios of ODSs over Hebei in Spring 2016 and suggest new production and use (rather than release from banks), which is in conflict with the Montreal Protocol agreement that bans the production of CFCs.

 

Many Chemistry‐Climate Models (CCMs) include a simplified treatment of brominated very short‐lived (VSL^Br) species by assuming CH₃Br as a surrogate for VSL^Br. However, neglecting a comprehensive treatment of VSL^Brin CCMs may yield an unrealistic representation of the associated impacts. Here, we use the Community Atmospheric Model with Chemistry (CAM‐Chem) CCM to quantify the tropospheric and stratospheric changes between various VSL^Brchemical approaches with increasing degrees of complexity (i.e., surrogate, explicit, and full). Our CAM‐Chem results highlight the improved accuracy achieved by considering a detailed treatment of VSL^Brphotochemistry, including sea‐salt aerosol dehalogenation and heterogeneous recycling on ice‐crystals. Differences between the full and surrogate schemes maximize in the lowermost stratosphere and midlatitude free troposphere, resulting in a latitudinally dependent reduction of ∼1–7 DU in total ozone column and a ∼5%–15% decrease of the OH/HO₂ratio. We encourage all CCMs to include a complete chemical treatment of VSL^Brin the troposphere and stratosphere.

 

Natural gas production in the United States has increased rapidly over the past decade, along with concerns about methane (CH₄) fugitive emissions and its climate impacts. Quantification of CH₄emissions from oil and natural gas (O&NG) operations is important for establishing scientifically sound policies for mitigating greenhouse gases. We use the aircraft mass balance approach for three flight experiments in August and September 2015 to estimate CH₄emissions from O&NG operations over the southwestern Marcellus Shale. We estimate a mean CH₄emission rate as 21.2 kg/s with 28% coming from O&NG operations. The mean CH₄emission rate from O&NG operations was estimated to be 1.1% of total NG production. The individual best‐estimate emission rates from the three flight experiments ranged from 0.78 to 1.5%, with overall limits of 0% and 3.5%. These emission rates are at the low end of other top‐down studies, but consistent with the few observational studies in the Marcellus Shale region as well as the U.S. Environmental Protection Agency CH₄inventory. A substantial source of CH₄(~70% of observed CH₄emissions) was found to contain little ethane, possibly due to coalbed CH₄emitted either directly from coal mines or from wells drilled through coalbed layers in O&NG operations. Recent regulations requiring capture of gas from the completion‐venting step of hydraulic fracturing appear to have reduced the atmospheric release of CH₄. Our study suggests that for a 20‐year time scale, energy derived from the combustion of natural gas extracted from this region likely exerts a net climate benefit compared to coal.