Deriving Global OH Abundance and Atmospheric Lifetimes for Long‐Lived Gases: A Search for CH₃CCl₃ Alternatives

Abstract

An accurate estimate of global hydroxyl radical (OH) abundance is important for projections of air quality, climate, and stratospheric ozone recovery. As the atmospheric mixing ratios of methyl chloroform (CH3CCl3) (MCF), the commonly used OH reference gas, approaches zero, it is important to find alternative approaches to infer atmospheric OH abundance and variability. The lack of global bottom‐up emission inventories is the primary obstacle in choosing a MCF alternative. We illustrate that global emissions of long‐lived trace gases can be inferred from their observed mixing ratio differences between the Northern Hemisphere (NH) and Southern Hemisphere (SH), given realistic estimates of their NH‐SH exchange time, the emission partitioning between the two hemispheres, and the NH versus SH OH abundance ratio. Using the observed long‐term trend and emissions derived from the measured hemispheric gradient, the combination of HFC‐32 (CH2F2), HFC‐134a (CH2FCF3, HFC‐152a (CH3CHF2), and HCFC‐22 (CHClF2), instead of a single gas, will be useful as a MCF alternative to infer global and hemispheric OH abundance and trace gas lifetimes. The primary assumption on which this multispecies approach relies is that the OH lifetimes can be estimated by scaling the thermal reaction rates of a reference gas at 272 K on global and hemispheric scales. Thus, the derived hemispheric and global OH estimates are forced to reconcile the observed trends and gradient for all four compounds simultaneously. However, currently, observations of these gases from the surface networks do not provide more accurate OH abundance estimate than that from MCF.

Spatial and temporal variability of interhemispheric transport times

“…However, understanding the temporal variability of the transport is important for understanding and interpreting the observed temporal variations in tracer concentrations and determining the relative role of changes in transport, emissions, sinks, and chemistry for different species. For example, observations of methyl chloroform, or other species with reaction with OH as their primary sink, can be used to infer the abundance of OH (e.g., Krol and Lelieveld, 2003;Prinn et al, 2005;Montzka et al, 2011;Liang et al, 2017), and knowledge of the seasonal and interannual variability of the transport is required to isolate similar variability in the OH abundance. Similarly, knowledge of the interannual variability of transport from the NH is required for estimates of the variability in emissions or sinks (ocean uptake) of CO 2 from measurements of CO 2 in the SH (e.g., Francey and Frederiksen, 2016).…”

Monitoring Global Tropospheric OH Concentrations using Satellite Observations of Atmospheric Methane

“…Interpretation of these trends has generally focused on changing emissions (Rice et al, 2016;Hausmann et al, 2016;Nisbet et al, 2016;Schaefer et al, 2016), but recent studies have suggested that the growth over the past decade could be contributed by a decline in global OH concentration Rigby et al, 2017). On the other hand, the trend in atmospheric CO over the past decade suggests an increase in global OH concentrations (Gaubert et al, 2017).…”

“…Inferring OH trends from methyl chloroform will become more difficult in the future as concentrations approach the detection limit (Liang et al, 2017) and evasion from the ocean complicates interpretation (Wennberg et al, 2004). Finding an alternative proxy for tropospheric OH is viewed as a pressing problem in the atmospheric chemistry community (Lelieveld et al, 2006).…”

“…Huang and Prinn (2002) pointed out that the major limitation to hydrochlorofluorocarbons and hydrofluorocarbons as the alternative proxy is the lack of accurate global emission inventory. To alleviate this difficulty, 15 Liang et al (2017) proposed to use the hemispheric gradient of a suite of these compounds to jointly retrieve global emissions and tropospheric OH, but their approach may be limited by the sparsity of the surface observation network.…”

An advanced method of contributing emissions to short-lived chemical species (OH and HO<sub>2</sub>): the TAGGING 1.1 submodel based on the Modular Earth Submodel System (MESSy 2.53)

“…As emissions of CH 3 CCl 3 steadily decline, Liang et al (2017) suggest an alternative method: they combine several trace gases such as CH 2 F 2 , CH 2 FCF 3 , CH 3 CHF 2 and CHClF 2 in a gradient-trend-based two-box model approach to derive a global OH concentration of 11.2 × 10 5 molec cm −3 . Overall, global chemistry climate models estimate a tropospheric OH concentration of around 11 × 10 5 molec cm −3 , which compares well with the observation-based results from Prinn et al (2005) and Liang et al (2017).…”