Tropospheric Oh And Stratospheric Oh And Cl Concentrations Determined From Ch4, Ch3cl, And Sf6 Measurements

Abstract

The hydroxyl (OH) radical is the key oxidant in the global atmosphere as it controls the concentrations of toxic gases like carbon monoxide and climate relevant gases like methane. In some regions, oxidation by chlorine (CI) radical is also important, and in the stratosphere both OH and Cl radicals impact ozone. An empirical method is presented to determine effective OH concentrations in the troposphere and lower stratosphere, based on CH4, CH3Cl, and SF6 data from aircraft measurements (IAGOS-CARIBIC) and a ground-based station (NOAA). Tropospheric OH average values of 10.9 × 105 (σ = 9.6 × 105) molecules cm¯³ and stratospheric OH average values of 1.1 × 105 (σ = 0.8 × 105) molecules cm³ were derived over mean ages derived from SF6. Using CH4 led to higher OH estimates due to the temperature dependence of the CH4 + OH reaction in the troposphere and due to the presence of Cl in the stratosphere. Exploiting the difference in effective OH calculated from CH3Cl and CH4 we determine the main altitude for tropospheric CH4 oxidation to be 4.5 ~ 10.5 km and the average Cl radical concentration in the lower stratosphere to be 1.1 × 104 (σ = 0.6 × 104) molecules cm³ (with a 35% measurement uncertainty). Furthermore, the data are used to examine the temporal trend in annual average stratospheric OH and Cl radical concentrations between 2010 and 2015. The year 2013 showed highest stratospheric OH and lowest Cl but no clear temporal trend was observed in the data in this period. These data serve as a baseline for future studies of stratospheric circulation changes.

Generated Summary

This research presents a novel empirical method for monitoring the effective OH concentration in the global troposphere and lower stratosphere over extended periods. The approach leverages long-term measurements of SF6, CH3Cl, and CH4 from aircraft measurements (IAGOS-CARIBIC) and a ground-based station (NOAA). The study aims to determine the “effective OH concentration” that has acted in the troposphere and lower stratosphere over longer timescales. SF6 measurements are used to derive the mean air age of the airborne samples, which allows for the determination of the initial surface mixing ratios of CH3Cl and CH4. By exploiting the reaction rate differences between CH3Cl and CH4, the study attempts to determine the height in the troposphere where CH4 oxidation by OH is largest and estimate Cl radical concentration in the lowermost stratosphere. The research employs a data-based approach to analyze the atmospheric oxidation capacity and markers for future circulation changes. The study examines the temporal trends in annual average stratospheric OH and Cl radical concentrations between 2010 and 2015, seeking to establish a baseline for future research.

Key Findings & Statistics

• The hydroxyl (OH) radical is the key oxidant in the global atmosphere.
• Tropospheric OH average values of 10.9 × 10⁵ (σ = 9.6 × 10⁵) molecules cm^-3.
• Stratospheric OH average values of 1.1 × 10⁵ (σ = 0.8 × 10⁵) molecules cm^-3.
• Median tropospheric OHeff were 9.93 × 10⁵ and 2.63 × 10⁶ molecules cm^-3 for CH3Cl and CH4, respectively.
• Median stratospheric values from 200-1100 days were 1.69 × 10⁵ and 3.35 x 10⁵ molecules cm^-3.
• Tropospheric OHeff values exhibit a much larger variability than stratospheric values.
• Median tropospheric OHeff derived from CH3Cl for years 2008–2015 is 9.9 x 10⁵ molecules cm^-3.
• The median tropospheric OHeff derived from CH4, 2.6 × 10⁶ molecules cm^-3, is between two and three times larger.
• The average Cl concentration over all periods is 1.1 (±0.6) × 10⁴ molecules cm^-3.
• The overall stratospheric OH, of 1.1 (±0.8) × 10⁵ molecules cm^-3 for the years 2010-2015 is derived.
• The tropospheric median OHeff derived from CH3Cl for years 2008–2015 is 9.9 × 105 molecules cm–3, which matches the tropospheric mean OH concentration of 1.1 × 105 molecules cm–3 reported from a recent global modeling study.
• The median tropospheric OHeff derived from CH4, 2.6 × 106 molecules cm–3, is between two and three times larger.
• The tropospheric median OHeff derived from CH3Cl for years 2008–2015 is 9.9 x 10⁵ molecules cm^-3, which matches the tropospheric mean OH concentration of 1.1 x 10⁵ molecules cm^-3 reported from a recent global modeling study.
• The median tropospheric OHeff derived from CH4, 2.6 × 10⁶ molecules cm^-3, is between two and three times larger.

Other Important Findings

• Exploiting the difference in effective OH calculated from CH3Cl and CH4, the study determined the main altitude for tropospheric CH4 oxidation to be 4.5 ~ 10.5 km.
• The average Cl radical concentration in the lower stratosphere was found to be 1.1 × 104 (σ = 0.6 × 104) molecules cm-3.
• The study examined the temporal trend in annual average stratospheric OH and Cl radical concentrations between 2010 and 2015.
• The year 2013 showed the highest stratospheric OH and lowest Cl concentrations.
• A schematic representation of the major atmospheric transport pathways associated with CARIBIC samples between 30°-60°N. The blue arrows indicate fast transport from ground to the tropical tropopause, the red arrows indicate downward transport from the stratosphere into the lowermost stratosphere.
• The study determined a representative annual “effective OH concentration” for the troposphere and the lower stratosphere.
• The study found that the rates of reaction with OH and Cl are much more dependent on temperature in the case of CH4.
• The study calculates that most tropospheric oxidation of CH4 occurs at circa 7 km (4.5~ 10.5 km, 220 ~ 260 K).
• The annual Cl radical concentration derived from air samples with age larger than 200 days in the stratosphere over the time period 2010-2015.

Limitations Noted in the Document

• The method relies on the assumption that both CH3Cl and CH4 are predominantly oxidized by OH, which may not fully hold in the upper stratosphere due to minor photolysis and reaction with O’D radicals.
• The assumption that time segregation accurately delineates troposphere, mixed troposphere-stratosphere, and stratosphere could be subject to some uncertainty.
• The study acknowledges that the CH3Cl estimate might be more closely aligned with recent global model studies.
• The study acknowledges potential overestimation of stratospheric Cl concentration due to the photolysis of chlorine containing compounds.
• Uncertainties in the method stem from instrumental errors, variations in source regions, and temperature-dependent reaction rates, influencing the calculated OHeff.
• The choice of source region could influence the calculation, and the SF6 is not perfectly mixed globally.

Conclusion

The study introduces a new empirical method for monitoring the effective OH concentration in the global troposphere and lower stratosphere, offering a valuable tool for understanding atmospheric oxidation capacity and potential changes due to events such as volcanic eruptions or alterations in stratospheric circulation. The approach is based on long-term measurements of SF6, CH3Cl, and CH4, enabling the determination of the mean age of air samples and the initial mixing ratios of CH3Cl and CH4. By comparing the reaction rates of CH3Cl and CH4 with OH, the research identifies the main altitude for CH4 oxidation and estimates Cl radical concentrations. The research also addresses limitations, such as the potential for overestimation in the stratosphere due to the reactions with the radicals. The analysis of trends from 2010 to 2015 reveals insights into the variations in stratospheric OH and Cl concentrations. The study determines a representative annual “effective OH concentration” for the troposphere and the lower stratosphere. The study shows the rates of reaction with OH and Cl are much more dependent on temperature in the case of CH4. The authors calculate that most tropospheric oxidation of CH4 occurs at circa 7 km (4.5~ 10.5 km, 220 ~ 260 K). The derived values are compared with those from other studies, adding to the understanding of OH and Cl in the atmosphere. These findings provide an important basis for comparison with modeling approaches. This data is valuable for comparison with models and for deriving lifetime estimates for other atmospheric species. The study concludes with the importance of ongoing measurements, suggesting that future research should focus on capturing how the OH and Cl levels are affected by major volcanic events and/or by stratospheric circulation changes.

DOI

10.1038/s41612-018-0041-9