A three-dimensional-model inversion of methyl chloroform to constrain the atmospheric oxidative capacity

Variations in the atmospheric oxidative capacity, largely determined by variations in the hydroxyl radical (OH), form a key uncertainty in many greenhouse and other pollutant budgets, such as that of methane (CH4). Methyl chloroform (MCF) is an often-adopted tracer to indirectly put observational constraints on large-scale variations in OH. We investigated the budget of MCF in a 4DVAR inversion using the atmospheric transport model TM5, for the period 1998–2018, with the objective to derive information on large-scale, interannual variations in atmospheric OH concentrations. While our main inversion did not fully converge, we did derive interannual variations in the global oxidation of MCF that bring simulated mole fractions of MCF within 1 %–2 % of the assimilated observations from the NOAA-GMD surface network at most sites. Additionally, the posterior simulations better reproduce aircraft observations used for independent validation compared to the prior simulations. The derived OH variations showed robustness with respect to the prior MCF emissions and the prior OH distribution over the 1998 to 2008 period. Although we find a rapid 8 % increase in global mean OH concentrations between 2010 and 2012 that quickly declines afterwards, the derived interannual variations were typically small (< 3 %/yr), with no significant long-term trend in global mean OH concentrations. The inverse system found strong adjustments to the latitudinal distribution of OH, relative to widely used prior distributions, with systematic increases in tropical and decreases in extra-tropical OH concentrations (both up to 30 %). These spatial adjustments were driven by intrahemispheric biases in simulated MCF mole fractions, which have not been identified in previous studies. Given the large amplitude of these adjustments, which exceeds spread between literature estimates, and a residual bias in the MCF intrahemispheric gradients, we suggest a reversal in the extratropical ocean sink of MCF in response to declining atmospheric MCF abundance (as hypothesized in Wennberg et al., 2004). This ocean source provides a more realistic explanation for the biases, possibly complementary to adjustments in the OH distribution. We identified significant added value in the use of a 3D transport model, since it implicitly accounts for variable transport and optimizes the observed spatial gradients of MCF, which is not possible in simpler models. However, we also found a trade-off in computational expense and convergence problems. Despite these convergence problems, the derived OH variations do result in an improved match with MCF observations relative to an interannually repeating prior for OH. Therefore, we consider that variations in OH derived from MCF inversions with 3D models can add value to budget studies of long-lived gases like CH4.

Trends in global tropospheric hydroxyl radical and methane lifetime since 1850 from AerChemMIP

We analyse historical (1850–2014) atmospheric hydroxyl (OH) and methane lifetime data from Coupled Model Intercomparison Project Phase 6 (CMIP6)/Aerosols and Chemistry Model Intercomparison Project (AerChemMIP) simulations. Tropospheric OH changed little from 1850 up to around 1980, then increased by around 9 % up to 2014, with an associated reduction in methane lifetime. The model-derived OH trends from 1980 to 2005 are broadly consistent with trends estimated by several studies that infer OH from inversions of methyl chloroform and associated measurements; most inversion studies indicate decreases in OH since 2005. However, the model results fall within observational uncertainty ranges. The upward trend in modelled OH since 1980 was mainly driven by changes in anthropogenic near-term climate forcer emissions (increases in anthropogenic nitrogen oxides and decreases in CO). Increases in halocarbon emissions since 1950 have made a small contribution to the increase in OH, whilst increases in aerosol-related emissions have slightly reduced OH. Halocarbon emissions have dramatically reduced the stratospheric methane lifetime by about 15 %–40 %; most previous studies assumed a fixed stratospheric lifetime. Whilst the main driver of atmospheric methane increases since 1850 is emissions of methane itself, increased ozone precursor emissions have significantly modulated (in general reduced) methane trends. Halocarbon and aerosol emissions are found to have relatively small contributions to methane trends. These experiments do not isolate the effects of climate change on OH and methane evolution; however, we calculate residual terms that are due to the combined effects of climate change and non-linear interactions between drivers. These residual terms indicate that non-linear interactions are important and differ between the two methodologies we use for quantifying OH and methane drivers. All these factors need to be considered in order to fully explain OH and methane trends since 1850; these factors will also be important for future trends.

A 3D-model inversion of methyl chloroform to constrain the atmospheric oxidative capacity

Variations in the atmospheric oxidative capacity, largely determined by variations in the hydroxyl radical (OH), form a key uncertainty in many greenhouse and other pollutant budgets, such as that of methane (CH4). Methyl chloroform (MCF) is an often-adopted tracer to indirectly put observational constraints on variations in OH. We investigated the budget of MCF in a 4DVAR inversion using the atmospheric transport model TM5, for the period 1998–2018, with the objective to derive information on interannual variations in OH and in its spatial distribution. We derived interannual variations in the global oxidation of MCF that bring simulated mole fractions of MCF within 1–2 % of the assimilated observations from the NOAA-GMD surface network at most sites. Additionally, the posterior simulations better reproduce aircraft observations used for independent validation. The derived OH variations showed robustness with respect to the prior MCF emissions and the prior OH distribution. The interannual variations were typically small (

Constraints on OH in a global 3D inversion of methyl chloroform

&lt;p&gt;The hydroxyl radical (OH) is the primary atmospheric oxidant. In this role, OH is involved in the removal of a wide variety of atmospheric pollutants and greenhouse gases. Despite the central role of OH in atmospheric chemistry, important metrics such as interannual variability and trends in OH on large spatial scales remain poorly constrained. This is mainly due to its low abundance and short lifetime of seconds.&lt;/p&gt;&lt;p&gt;Over the past decades, the anthropogenically emitted methyl chloroform (MCF) has been uniquely qualified as a tracer to indirectly constrain OH on large spatio-temporal scales. However, recent box model studies have shown that OH, as estimated from MCF observations, is still very uncertain &lt;sup&gt;1,2&lt;/sup&gt;&lt;sup&gt;,3&lt;/sup&gt;. This translates for example to large uncertainties in global methane (CH&lt;sub&gt;4&lt;/sub&gt;) emissions, even if changes in the global CH&lt;sub&gt;4&lt;/sub&gt; burden are well-defined. Box model studies however, do not fully capitalize on the MCF measurement network and the gradients therein. Moreover, they may introduce biases due to incorrect or incomplete representation of atmospheric transport.&lt;/p&gt;&lt;p&gt;Here, we present results from a 4DVAR inversion of MCF over the 1998-2018 period, performed in the 3D chemistry-transport model TM5. Starting from typical OH priors, we find adjustments in the OH spatio-temporal distribution that bring the simulated MCF mole fractions closer to observations. Large uncertainties in this improved estimate remain, but we find that no large interannual variability (&gt;2%) and no significant trend in global mean OH are needed to match MCF observations. We do find significant adjustments in the latitudinal gradients of OH (e.g. an increase in tropical OH).&lt;/p&gt;&lt;p&gt;&lt;sup&gt;1&amp;#160;&lt;/sup&gt;Rigby, M., et al. PNAS (2017), 114.21: 5373-5377&lt;/p&gt;&lt;p&gt;&lt;sup&gt;2 &lt;/sup&gt;Turner, A.J., et al. PNAS (2017), 114.21: 5367-5372&lt;/p&gt;&lt;p&gt;3 Naus, S et al. ACP (2019), 19.1: 407-424&lt;/p&gt;

Trends in global tropospheric hydroxyl radical and methane lifetime since 1850 from AerChemMIP

We analyse historical (1850–2014) atmospheric hydroxyl (OH) and methane lifetime data from CMIP6/AerChemMIP simulations. Global OH changed little from 1850 up to around 1980, then increased by around 10 %, with an associated reduction in methane lifetime. The model-derived OH trend since 1980 differs from trends found in several studies that infer OH from inversions of methyl chloroform measurements; however, these inversions are poorly constrained and contain large uncertainties that do not rule out the possibility of recent positive OH trends. The recent increases in OH that we find are consistent with one previous study that assimilated global satellite-derived carbon monoxide (CO) over the period 2002–2013. The upward trend in modelled OH since 1980 was mainly driven by changes in anthropogenic Near-Term Climate Forcer emissions (increases in anthropogenic nitrogen oxides and decreases in CO). Increases in halocarbon emissions since 1950 have made a small contribution to the increase in OH, whilst increases in aerosol-related emissions have slightly reduced OH. Halocarbon emissions have dramatically reduced the stratospheric methane lifetime, by about 15–40 %, which has been assumed to not change in most previous studies. We find that whilst the main driver of atmospheric methane increases since 1850 is emissions of methane itself, increased ozone precursor emissions have significantly modulated (in general reduced) methane trends. Halocarbon and aerosol emissions are found to have relatively small contributions to methane trends. All these factors, together with changes and variations of climate and climate-driven natural emissions, need to be included in order to fully explain OH and methane trends since 1850; these factors will also be important for future trends.

Atmospheric observations and emission estimates of ozone-depleting chlorocarbons from India

While the Montreal Protocol has been successful in reducing emissions of many long-lived ozone-depleting substances, growth in the global emission rates of unregulated very short-lived substances (VSLS) poses a potential threat to the recovery of the ozone layer. The sources of these VSLS are not well-constrained, with major source regions poorly monitored by existing measurement networks. Given India's rapidly growing economy, its emissions of both regulated chlorocarbons and unregulated VSLS chlorocarbons are suspected to have global significance. Furthermore, VSLS from the Asian monsoon regions have a greater impact on ozone-depletion than those emitted elsewhere due to the ability of monsoon systems to rapidly transport pollutants to the lower stratosphere. Previous atmospheric measurements of chlorocarbons from the Indian sub-continent are scarce. Here we present a new set of observations, derived from flask samples collected during a 2-month aircraft campaign in India and use these measurements to infer India's chlorocarbon emissions. We show that emissions of carbon tetrachloride from northern and central India (2.3 (1.5–3.4) Gg yr−1), are likely due to inadvertent production and release during the manufacture of chloromethanes (specifically dichloromethane and chloroform) and account for approximately 7 % of the global total. Emissions of methyl chloroform from the same region were estimated to be 0.07 (0.04–0.10) Gg yr−1 which account for less than 5 % of remaining global emissions. We used a population scaling to estimate India's emissions of the very short-lived chlorocarbons dichloromethane, perchloroethene and chloroform, which were estimated to be 69.2 (55.8–82.9) Gg yr−1, 2.9 (2.5–3.3) Gg yr−1 and 25.7 (21.6–29.9) Gg yr−1 respectively. In the case of dichloromethane, our estimate is consistent with a 3-fold increase in emissions since the last estimate derived from atmospheric data in 2008.