As the primary atmospheric oxidant, the OH radical has a key role in the removal or production of major air pollutants, greenhouse gases and many ozone-depleting substances^{1, 2, 3}. A better understanding of the NH/SH OH ratio will lead to significantly improved source estimates of reactive species and to an improved prediction of chemistry–climate interactions from changing human activities. Because of its very short lifetime (~1 s), OH manifests high spatiotemporal variability. Moreover, because OH concentrations are typically very low (~10⁶ molecules cm⁻³), in situ measurements are challenging, and large differences between observations have prevented a direct validation of model-simulated OH distributions or an evaluation of uncertainties in the chemical mechanisms responsible for OH recycling under different environmental conditions^{14, 15, 16, 17}. Rather, indirect estimates of the total abundance and interannual variations of global OH have been made with methyl chloroform (CH₃CCl₃) or ¹⁴CO measurements and simulations by chemistry-transport models (CTMs)^{8, 9, 10, 12, 18, 19, 20}.

Constraints on the meridional OH gradient are needed for estimating hemispheric source and sink magnitudes of gases and aerosols that are produced or destroyed through reactions with OH (see Methods). For example, large increases in NO_x (NO + NO₂) emissions from China compared with emission inventories are estimated by using a data assimilation system⁵ and the CHASER (Chemical AGCM for Studies of Atmospheric Environment and Radiative forcing²¹) CTM. An overestimate of NH OH could account for such discrepancies, because it affects the hemispheric budgets of NO_x and other reactive species such as carbon monoxide (CO) and methane (CH₄). CHASER-simulated OH is ~26% higher in the NH than in the SH, and this model predicts a smaller than observed NH–SH CH₄ gradient⁶. In the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), the multi-model average and 1σ spread of annual mean NH/SH OH ratios was 1.28 ± 0.10 (range 1.13–1.42). The simulated NH/SH OH ratio being greater than 1 is related primarily to OH production from modelled ozone, which is biased high in the NH and low in the SH, compared with observations⁴. Other evidence exists for significantly lower NH/SH OH ratios. On the basis of ¹⁴CO and CH₃CCl₃ observations, the NH/SH ratio of OH is suggested to be significantly lower than 1 (refs 7, 8). A NH/SH OH ratio of 0.98 using models has been shown⁹, but it was concluded that accurate estimation of the ratio would require more accurate CH₃CCl₃ emissions (also in ref. 8) and models that accurately describe interhemispheric exchange.

We constrain the NH/SH ratio of OH by using an atmospheric general circulation model (AGCM)-based CTM, the Japan Agency for Marine-Earth Sciences and Technology (JAMSTEC) Atmospheric Chemistry Transport Model (ACTM)^{6, 22}. CH₃CCl₃ simulations are performed with two spatially distinct tropospheric OH distributions, namely, ACTM_0.99 and ACTM_1.26, which have annual mean NH/SH OH ratios of 0.99 and 1.26, respectively^{21, 23} (Extended Data Fig. 1), with the NH and SH separated at the geographical Equator. The NH/SH OH ratios are 0.87 for ACTM_0.99 and 1.18 for ACTM_1.26, when the two hemispheres are divided in accordance with the monthly locations of the Intertropical Convergence Zone (ITCZ). The emissions of CH₃CCl₃ and sulphur hexafluoride (SF₆) are taken from the transport model intercomparison (TransCom)-CH₄ experiment^{6, 24, 25} and extrapolated for the years after 2008 (Extended Data Fig. 2 and Extended Data Table 1). We show below that the simulated NH–SH gradient of CH₃CCl₃ is equally sensitive to the NH/SH OH ratio in the model with regard to the emissions since 2000. Thus we can use the observed NH–SH CH₃CCl₃ gradient to constrain the NH/SH OH ratio in the model, provided that little uncertainties are contributed by emission estimates and transport parameterization.

Results from both the ACTM_0.99 and ACTM_1.26 simulations reproduce the temporal evolution of CH₃CCl₃ for the period 1994–2011 (Fig. 1), confirming that the global annual mean OH concentrations are in good balance with the ‘Control’ surface emissions used in the simulations. Concentrations of CH₃CCl₃ have decreased exponentially since the late 1990s as a result of the stringent implementation of the Montreal Protocol and its amendments to mitigate stratospheric ozone depletion^{3, 12} (Fig. 1a). The simulated CH₃CCl₃ concentration decay rates are not significantly different, namely 18.28 ± 0.14% per year (average ± 1σ as interannual variability) for ACTM_0.99 and 17.27 ± 0.13% per year of the annual mean concentrations for ACTM_1.26, compared with the observed 17.85 ± 0.29% per year during 2002–2011 at five Advanced Global Atmospheric Gases Experiment (AGAGE) sites and nine National Oceanic and Atmospheric Administration (NOAA) sites (Extended Data Table 2a). The average CH₃CCl₃ lifetimes are calculated to be 4.91 ± 0.03 years for ACTM_0.99 and 5.19 ± 0.03 years for ACTM_1.26. These lifetimes agree well with the observation-based lifetimes for given inventory emissions of 5.0 (range 4.87–5.23) years^{3, 26}.

Given specified source distribution and magnitude, the meridional CH₃CCl₃ concentration gradients are controlled mainly by loss due to reaction with OH and by meridional transport²³. This is because the local lifetimes of 1–3 years in the tropical troposphere are of similar magnitude as the interhemispheric transport time of 1.3 years in the ACTMs^{6, 22}. Figure 1b shows CH₃CCl₃ concentration gradients between Mace Head, Ireland (MHD), and Cape Grim, Australia (CGO). Results from ACTM_0.99 reveal a closer agreement with the observed MHD–CGO CH₃CCl₃ concentration gradients than with those for ACTM_1.26, given the set of ‘Control’ global emissions and global mean OH concentrations (also noted using NOAA data from multiple sites; see Extended Data Fig. 3). The differences between ACTM_1.26 and ACTM_0.99 are readily apparent for the 2000s, when the yearly emissions of CH₃CCl₃ are less than 3% of the atmospheric burden, compared with the early 1990s, when yearly emissions were as large as ~20% of the burden. Sensitivity simulations are conducted using ‘Control’ global emissions and mean OH at increased horizontal resolution (ACTM_T106) and with a different spatial distribution of CH₃CCl₃ emissions (ACTM_UNEP). ACTM_T106 and ACTM_UNEP show equally good agreement with observed concentration gradients between MHD and CGO (Fig. 1b, inset), suggesting model resolution and source distribution are, unlike the NH/SH OH ratio, not strong drivers for the NH–SH CH₃CCl₃ concentration gradient. Generally, the ratio of annual mean MHD–CGO CH₃CCl₃ gradients to annual mean concentrations is extremely stable at 2.87 ± 0.41% for AGAGE gas chromatograph–multi detector (GC–MD) data during the 2000s. The ACTM_0.99 simulation well predicted this ratio (2.88 ± 0.19%), whereas it is only 0.54 ± 0.22% for ACTM_1.26. The ratio of the MHD–CGO gradient to the mean CH₃CCl₃ concentration decreased rapidly in the 1990s, from ~34% in 1990 to ~2.9% in 1999 and the 2000s, in proportion to the decrease in global total emissions, mostly occurring in the NH.

Because no formal emission inventory of CH₃CCl₃ exists, uncertainties in the NH/SH emission ratio and in the global totals should be quantified. We find that the MHD–CGO CH₃CCl₃ differences are not particularly sensitive to NH/SH emission ratios of 10 or greater (~16.6 in the ‘Control’ case) in the period 2004–2011. To assess the uncertainties contributed by the global mean OH concentration and total CH₃CCl₃ emission jointly, we derive a linear relationship between the two (percentage lifetime change = −3.9 × percentage emission change) for simulating the observed CH₃CCl₃ growth rate (Extended Data Fig. 4). We find that ACTM_0.99 produces a minimum for the model-observation mismatch (J) for the magnitude of global emission and chemical loss given as the ‘Control’ case (Fig. 2). A slightly larger mismatch minimum is observed for ACTM_1.26 when we consider +20% chemical loss and +78% global CH₃CCl₃ emissions during the period 2004–2011 (Fig. 2a, b). However, a significant increase in emission and loss deteriorates the agreement between simulated and observed seasonal cycle amplitudes in the CH₃CCl₃ concentration difference between MHD–CGO or Alert, Canada (ALT), and Palmer Station, Antarctica (PSA) (Fig. 2c). Thus significantly larger global CH₃CCl₃ emissions than considered in the ‘Control’ case can be ruled out for the period 2004–2011, as opposed to the period of the late 1990s (ref. 8), and therefore the possibility of significantly more OH in the NH than in the SH is also deemed inconsistent with these observations and their simulation.

To exclude erroneous interhemispheric transport in the ACTM model, we briefly present results for SF₆, which is purely anthropogenic and chemically inert in the troposphere, and thus comprises an excellent tracer of atmospheric transport given its relatively high emission rate relative to the global burden and well-known meridional emission distribution^{22, 27, 28}. The meridional transport of SF₆ in ACTM has been validated extensively using surface sites^{6, 22}. Further, fine-grained measurements of various species during High-performance Instrumented Airborne Platform for Environmental Research (HIAPER) Pole-to-Pole Observation (HIPPO) campaigns¹³ represent zonal mean cross-sections of the troposphere (Extended Data Fig. 5). In general, ACTM-simulated SF₆ matches the observations well along all flight paths: the simulated SF₆ values frequently lie within the variability observed by the three instruments that recorded SF₆ on the HIPPO aircraft (Extended Data Fig. 6). The ACTM simulated NH–SH differences agree within ±0.02 parts per trillion (p.p.t.) or ±6% of the observed values for each HIPPO campaign, averaged over all latitudes and 1–3 km altitude for the three instruments (Extended Data Table 3).

The effect of the NH/SH OH ratio on NH–SH CH₃CCl₃ concentration gradients is well supported by the HIPPO measurements over the Pacific Ocean. The ACTM_0.99 simulated CH₃CCl₃ meridional gradients, given the ‘Control’ emissions (NH–SH = 0.19 p.p.t. averaged over all HIPPO campaigns), are in close agreement with the HIPPO observations (0.21 p.p.t.), with model-observation differences mostly <0.2 p.p.t. (or <2%) at all latitudes, during all seasons (Fig. 3). However, the results using ACTM_1.26 show systematically smaller NH–SH CH₃CCl₃ differences (0.05 p.p.t. averaged over all HIPPO campaigns), suggesting that the loss due to the CH₃CCl₃ + OH reaction is too high in the NH troposphere or too low in the SH. The poleward increase in CH₃CCl₃ from the SH subtropics is caused mainly by the greater abundance of OH in the SH tropics than in the subtropics in austral summer (HIPPO 1, 2 and 3).

For estimating the possible range of the annual mean NH/SH OH ratio, we performed nine sensitivity simulations using synthetic OH distributions (Extended Data Table 2b) for the period 2001–2011. The synthetic OH distributions are prepared by adding and subtracting sine functions with a phasing of 2 × latitude for ACTM_0.99 and mixing the two OH distributions. Figure 4 shows the dependence of average CH₃CCl₃ concentration gradients between NH and SH on the NH/SH OH ratio. Using the linear fits (shown as lines in Fig. 4) to the model simulations (open symbols), annual NH/SH OH ratios are calculated on the basis of the observed CH₃CCl₃ concentration gradients (filled symbols) averaged over the period 2004–2011 (2009–2011 for HIPPO). The average (±1σ of annual values) of NH/SH OH ratios are estimated to be 0.98 ± 0.12, 0.97 ± 0.11 and 0.96 ± 0.63 using the CH₃CCl₃ concentration gradients between MHD and CGO from AGAGE (average of GC–MD and Medusa instruments), ALT and PSA from NOAA flask samples, and NH and SH averages (1–4 km altitude, latitudes >30°) from HIPPO, respectively. Combining the results from the AGAGE and NOAA surface network, the decadal average NH/SH OH ratio is 0.97 ± 0.12. The decadal average NH/SH OH ratio estimated from the surface network is in excellent agreement with that estimated from five HIPPO campaigns covering greater geographical areas and vertical extents of both hemispheres but sampling over a briefer period. The uncertainty of about ±13% for the surface networks includes the measurement and model errors (<3%), emission uncertainties, and interannual and seasonal variations in OH within each of the hemispheres, although relative contributions of emission uncertainty and OH variations cannot be quantified.

The precise and well-calibrated measurements from different networks, combined with a transport model that accurately describes interhemispheric transport and modelled emissions, allow us to conclude that a global OH distribution with substantially more OH in the NH is inconsistent with CH₃CCl₃ observations. Our result of an NH/SH OH ratio of close to 1 is in strong contrast to higher modelled OH in the NH⁴. Our results may be explained in various ways. Either NH OH sources are overestimated, possibly owing to O₃ that is biased high in the NH⁴, or NH OH sinks are underestimated²⁹. Alternatively, SH OH sources may be underestimated³⁰ or SH OH sinks are overestimated (less likely). Further refinements of OH distributions and OH budgets are required for an accurate estimation of surface emissions of many important short-lived species that affect the Earth’s radiative budget and air pollution chemistry. For example, to match the observed interhemispheric gradient in atmospheric CH₄ (based on data from the TransCom experiment⁶), CH₄ emissions in the NH have to be increased from 398 to 430 Tg of CH₄ per year, and decreased from 151 to 119 Tg of CH₄ per year in the SH if ACTM_1.26 OH is used instead of ACTM_0.99. Our results also imply that top-down emission estimates of reactive species (for example, CO, NO_x and SO_x) in key emitting countries in the NH are probably overestimated if OH fields are used with an NH/SH OH ratio much larger than 1.

The JAMSTEC ACTM

The CCSR/NIES/FRCGC (Center for Climate System Research/National Institute for Environmental Studies/Frontier Research Center for Global Change) atmospheric general circulation model (AGCM)-based ACTM is developed for simulations of long-lived gases in the atmosphere^{6, 22}. Scenarios of surface emissions are used in ACTM to simulate atmospheric concentrations of methyl chloroform and SF₆ to derive constraints on the model transport and to determine the NH/SH ratio of OH abundance. The ACTM simulations analysed here have a horizontal resolution of T42 spectral truncation (~2.8° × 2.8°) with 67 sigma-pressure levels in the vertical and model top at ~90 km. The horizontal winds and temperature of ACTM are nudged with Japan Meteorological Agency Reanalysis (JRA) data products³¹.

We use predefined tropospheric OH concentrations²³, after scaling the global totals by 0.92 to simulate global CH₃CCl₃ decay rates as in the TransCom-CH₄ experiment⁶ (Extended Data Fig. 1a; hereafter called ACTM_0.99 because the NH/SH OH ratio is 0.99). The hemispheres are delineated at the Equator. A previous study²³ used a semi-empirical method to estimate seasonally varying OH fields by using observed distributions of O₃, H₂O, NO_t (NO₂ + NO + 2N₂O₅ + NO₃ + HNO₂ + HNO₄), CO, hydrocarbons, temperature and cloud optical depth. To test the sensitivity to different interhemispheric OH distributions, we also consider tropospheric OH simulated by CHASER with online chemistry²¹, scaled by 0.88 to simulate global CH₃CCl₃ decay rates more accurately⁶ (Extended Data Fig. 1b; hereafter called ACTM_1.26 because the NH/SH OH ratio is 1.26). For both cases, stratospheric OH distributions are adopted from the full stratospheric chemistry model version of the ACTM³². The NH/SH OH ratios for ACTM_0.99 and ACTM_1.26 are estimated to be 0.87 and 1.18, respectively, when the hemispheres are separated at the ITCZ (named observed ‘dynamical’ Equator) following the location of maximum tropical rainfall³³ or the maximum in meridional gradient of ACTM_0.99 simulated concentration of HFC-134a (CH₂FCF₃) (named modelled ‘chemical’ Equator). Annual mean locations of the ITCZ defined by CMAP and HFC-134a are at 4.2° N and 4.3° N, respectively, suggesting good performance of ACTM interhemispheric transport.

ACTM simulation setup for SF₆, CH₃CCl₃ and HFC-134a.

We took SF₆ and HFC-134a emission distributions from the Emissions Database for Global Atmospheric Research (EDGAR; version 4.2)²⁵ for 1980–2008 (Extended Data Table 1). Global total SF₆ emissions are scaled to those estimated previously³⁴. The 2008 emission distributions are repeated for 2009–2011, but global total emissions are scaled to maintain the 2007–2008 increase rate for HFC-134a, and for SF₆ in proportion with fossil fuel emissions trends (Carbon Dioxide Information Analysis Center, http://cdiac.ornl.gov/trends/emis/meth_reg.html). The spatial distributions in EDGAR4.2 emissions are based on proxy data sets, mainly urban population (G. Maenhout, personal communication, April 2013). The EDGAR4.2 CH₃CCl₃ emissions are from ref. 24 (1980–2000), and are assumed to decrease exponentially in subsequent years, following E(t) = E₀ exp(−t/τ), where emissions E₀ are at t = 0 (year 2000) and lifetime τ = 5 years. The global totals and spatial distributions of CH₃CCl₃ emissions are identical to the TransCom-CH₄ experiment for 1987–2008 (‘Control’ case) (prepared by M. Krol)⁶. We also prepared alternative CH₃CCl₃ emission distribution maps using yearly country consumption (United Nations Environment Programme, UNEP). Emissions within a country are gridded at 1° × 1° on the basis of population distributions. Contrasting CH₃CCl₃ emission distributions used in the ‘Control’ and UNEP cases are depicted in Extended Data Fig. 2a, b. The time evolution of global total emissions is shown in Extended Data Fig. 2c, d and Extended Data Table 1.

We have considered these equations to simulate OH chemistry, deposition and stratospheric photolysis and rate constants for CH₃CCl₃ and HFC-134a:

Photochemical losses are calculated online in ACTM in accordance with recommended temperature-dependent reaction and wavelength-dependent photolysis rates³⁵. We have not considered uncertainties in reaction rates, which can be as high as 50% for some of the species. CH₃CCl₃ deposition on the oceanic surface is parameterized⁶ to give a partial global lifetime of 197 years. The O(¹D) concentrations are calculated in ACTM, and the monthly mean Cl climatology is from ref. 32. Zonal mean gridded CH₃CCl₃ lifetimes due to equations (1) and (2) are shown in Extended Data Fig. 1c. The species lifetime at steady state (τ_SS)³⁶ can be defined as burden/loss (M × ΣN_gas/M × ΣL_gas), where N and L are the moles present and lost in a model grid box, respectively, and M is the molecular mass. We calculated τ_SS for CH₃CCl₃ (all loss by reaction with OH and photolysis) as 4.91 ± 0.03 (s.d. over multiple years) and for HFC-134a as 13.65 ± 0.11 years in ACTM_0.99 (average 2001–2010). Their lifetimes are slightly longer (5.19 ± 0.03 and 14.23 ± 0.11 years, respectively) in ACTM_1.26. Global CH₃CCl₃ lifetimes are also estimated with a different method: d(burden)/dt = emission − burden/τ_total, as compared in Extended Data Fig. 2c with τ_SS. Good agreements between the two lifetimes after 2000 indicate that although some CH₃CCl₃ emissions still occur, τ_total estimation depends less on emission trends during our analysis period (2004–2011). Small interannual lifetime variations after ~2000 are caused by the temperature dependence of equation (1). Both ACTM estimated CH₃CCl₃ lifetimes agree within 0.2 years with the 5.0-year observationally defined lifetime³ and with other independent estimates, for example, 4.87–5.23 years²⁶.

Measurements and data processing

Simulations span 1980–2011, after a 10-year spin-up to stabilize the stratospheric distribution. Three-dimensional model output is sampled hourly for comparison with observations. We use in situ data from five AGAGE observatories¹¹ (from two types of instrument: the GC–MD¹¹ and the Medusa gas chromatograph–mass spectrometer (GC–MS)³⁷) and flask measurements from the NOAA halocarbon and other trace species (HATS) network^{6, 8}. For surface measurement sites see Extended Data Fig. 2b and Extended Data Table 2a. We calculate concentration gradients for each network separately and therefore do not consider calibration scale differences.

We also use data from five HIPPO campaigns¹³ (http://hippo.ornl.gov; HIPPO_all_missions_merge_10s_20121129.tbl). Model results are interpolated in space and time along the aircraft trajectory. HIPPO SF₆ data are from three instruments (NOAA whole air sampler (NWAS) using GC–MS analysis, Unmanned Aircraft Systems (UAS) Chromatograph for Atmospheric Trace Species (UCATS), PAN and other Trace Hydrohalocarbon Experiment (PANTHER) GC-ECD). CH₃CCl₃ data are from the AWAS system using GC–MS analysis (University of Miami). Simulated and measured values are binned by 1 km vertically and 2.5° latitudinally.

The model was sampled at hourly intervals and we chose the nearest model grid (within ~1.45 degrees) for each site. Monthly or annual means are calculated by including model results at the time of measurements, unless stated otherwise.

Model–measurement comparison (surface sites)

Because the NH/SH SF₆ emission ratio is ~36 and SF₆ has no tropospheric loss, the observed and simulated SF₆ concentrations in the NH exceed SH concentrations. Given accurate emission estimates, SF₆ observations can therefore be used to verify the interhemispheric mixing times^{22, 34}. Meridional CH₃CCl₃ gradients are more complex than those of SF₆ as a result of reaction with seasonally varying OH. The higher NH CH₃CCl₃ concentrations than those of the SH for ACTM_0.99 are caused by small lingering NH emissions combined with equal OH removal in the SH and NH (see Extended Data Fig. 3). In contrast, higher NH emissions are compensated for by higher NH OH loss in ACTM_1.26.

Considering ‘Control’ global emissions and global OH concentrations, ACTM_0.99 CH₃CCl₃ simulations are in very good agreement with AGAGE and NOAA surface measurements (Fig. 1 and Extended Data Fig. 3c, d); however, ACTM_1.26 always underestimates measured meridional CH₃CCl₃ gradients. These comparisons show that the CH₃CCl₃ change rate is well simulated from the 1990s to the present, suggesting that assumed global total emissions comply well with assumed global OH totals in this ‘Control’ scenario (note that annual mean OH levels are assumed constant from year to year, with only seasonal variation). Spatial distributions of emissions and emission magnitudes are more difficult to validate (this is addressed later). Extended Data Fig. 3c, d show concentration differences for eight NOAA sites relative to Palmer Station (65° S), suggesting that spatial emission distributions are reasonably estimated for the past 15 years. CH₃CCl₃ at SMO is found to be consistently lower than at PSA (Extended Data Fig. 3c, d, middle column)⁸, as a result of higher CH₃CCl₃ loss in the SH tropics (SMO at 14° S) than in the SH mid and high latitudes (PSA at 65° S). Both ACTM_0.99 and ACTM_1.26 capture the intra-SH gradient, suggesting that intra-SH OH gradients and intra-hemispheric transport of CH₃CCl₃ are well represented in both ACTM versions.

CH₃CCl₃ interhemispheric gradients are sensitive to the NH/SH OH ratio and emission magnitudes and may be calculated differently depending on the model’s horizontal resolution. We have performed test simulations using, first, two different emission maps (‘Control’ and UNEP-based) with geographically differing emissions, for example India and China versus Europe and the USA (Extended Data Fig. 2a, b), and second, varied horizontal resolutions (T42 and T106 spectral truncations). MHD–CGO CH₃CCl₃ gradients are simulated consistently using ACTM_0.99 since the later 2000s for all four test simulation cases (Fig. 1).

Sensitivity of the NH/SH OH ratio to assumed CH₃CCl₃ emission magnitudes and NH/SH emission ratio (ERs)

We have simulated MHD–CGO CH₃CCl₃ gradients using ACTM_0.99 for NH/SH CH₃CCl₃ ERs from 0.23 (more SH emissions) to infinity (only NH emissions). Extended Data Fig. 2e suggests that MHD–CGO CH₃CCl₃ gradients are not very sensitive to NH/SH ER, when the ER is greater than that assumed for the ‘Control’ emission case. The NH/SH ER is maintained at 16.6 for our sensitivity simulations (2001–2011).

We have estimated how much change in global total emissions is needed for a given change in chemical loss rate, to fit the observed global mean CH₃CCl₃ decline. Since 2001, the atmosphere still contains a significant CH₃CCl₃ burden and one would expect the OH estimate to be relatively independent of emissions⁸. Simulations using ACTM_0.99 (Extended Data Fig. 4a, b) suggest that if emissions are wrong by 10% in 2001, the lifetime will be in error by 10/3.9 = 2.6%. A consistency in emission-lifetime relative errors (2001, 2006 and 2011) is also observed because the emission/burden ratios remained almost constant (Extended Data Table 1) due to the same e-folding time for emission and burden after 2001.

Both the observed global mean CH₃CCl₃ decline and MHD–CGO gradients are simulated well with ACTM_1.26 when global total emissions and global mean OH are adjusted by roughly +78% and +20%, respectively (Fig. 2). The seasonal cycle amplitude in NH–SH CH₃CCl₃ difference is an additional constraint on global emissions, which is due mainly to the seasonality in loss through reaction with OH (Extended Data Fig. 4c, d). Although this simulation (+78% emission; +20% OH) fits the average NH–SH concentration difference in CH₃CCl₃ only slightly worse than ACTM_0.99 (‘Control’) (see Fig. 2a, b), the simulation seasonal cycle amplitude (peak-to-trough height within each year) clearly deteriorates (Fig. 2c). Too high emissions and chemical loss strengths increase the seasonality of the concentration differences substantially more than observed for both site pairs. This is primarily because NH sites (MHD and ALT) experience larger increases in concentration due to emissions during the NH winter (lower OH) and larger decreases in concentration due to chemical loss during the NH summer (higher OH).

The best model–observation agreement is found for ACTM_0.99 with ‘Control’ cases of global total emissions and global mean OH for both the MHD–CGO and ALT–PSA CH₃CCl₃ concentration differences.

Apart from the poor simulation of the observed seasonal cycle, 78% higher global total emissions (compared with ‘Control’) are unlikely on the basis of CH₃CCl₃ production and consumption reported to UNEP (http://ozone.unep.org/new_site/en/ozone_data_tools_access.php). The ‘Control’ emissions derived here are typically 10–20% higher than those estimated from the UNEP database for individual years between 1990 and 2009, but not for 2010 and 2011, when UNEP reported a factor of 4.8 and 31.4, respectively, lower than the ‘Control’ global total emissions.

SF₆ and CH₃CCl₃ interhemispheric and vertical gradients (HIPPO campaigns)

Measurements and modelling of long-lived gases have been widely used to infer air mass transport and mixing^{38, 39, 40, 41}. HIPPO provides a unique opportunity to test model transport, emission and chemistry of multiple chemical species of widely varying lifetimes¹³. We have used flights only over the central Pacific Ocean for this analysis, to avoid greater site representation error in coarse-resolution transport models (for example, 2.8° × 2.8° and ~500 m height intervals in the troposphere for ACTM) near the source regions. The locations of all research flights (denoted by numbers) are plotted on top of precipitation maps (Extended Data Fig. 5a). Because the chosen measurement locations are away from major sources of anthropogenic species, the site representation errors in our coarse-resolution ACTM are minimal, and measurements over the middle of the Pacific Ocean are representative of zonal means (Extended Data Fig. 5b, c).

Comparison of the location of the sharp SF₆ decrease (Extended Data Fig. 6a–h) with observed precipitation (Extended Data Fig. 5a) shows it to be coincident with the Pacific ITCZ. Over the NH mid to high latitudes, SF₆ concentrations at 6 km are within 0.1 p.p.t. of surface values. With deeper penetrating convection and a higher tropopause in the tropics, values comparable to the surface occur up to 12 km. We observed a reversed vertical gradient in the SH low to mid latitudes, with SF₆ concentrations aloft exceeding those at the surface. This reversed vertical gradient is interpreted in terms of the bulk of interhemispheric transport occurring in the mid to upper troposphere^{22, 28, 42, 43, 44}.

Both observed meridional and vertical SF₆ gradients are well simulated by ACTM (Extended Data Fig. 6i–r), mostly within 0.1 p.p.t., confirming that large-scale transport processes are well represented. Previously, such conclusions were drawn on the basis of limited data sets, measured at only a few surface sites^{6, 22}. CH₃CCl₃ simulation results using ACTM_0.99 agree well with HIPPO AWAS aircraft data (Fig. 3) when the ‘Control’ global emissions and global mean OH concentrations are considered, whereas ACTM_1.26 always underestimates meridional CH₃CCl₃ gradients (Extended Data Table 3), as for surface measurements.

Estimation of the NH/SH OH ratio from CH₃CCl₃ interhemispheric gradients

Sensitivity simulations are also carried out using nine synthetic OH distributions cases: three scenarios derived by combining the OH distributions used in ACTM_0.99 and ACTM_1.26, and six scenarios derived by manipulating the OH distributions used in ACTM_0.99 with a sine function (Extended Data Table 2b).

Extended Data Fig. 7 shows the differences in monthly and annual mean CH₃CCl₃ concentrations between MHD and CGO measured by AGAGE compared with ACTM simulations using the synthetic NH–SH OH distributions. It is clear that simulations with a NH/SH OH ratio of ~1 show the closest agreement with the annual mean measurements for most years, except for 2007, when anomalously high (>0.7 p.p.t.) MHD–CGO CH₃CCl₃ differences were measured with the Medusa instrument. We performed the same analysis using ALT and PSA sites from the NOAA flask network, and differences in averaged HIPPO measurements in the NH (north of 30° N) and SH (south of 30° S) below 4 km. A summary of these results is shown in Fig. 4. The 2010 anomaly in AGAGE Medusa CH₃CCl₃ differences is not found for ALT–PSA (or MHD–CGO) using NOAA data (Extended Data Fig. 3c, d), underlining the advantages of two cooperating yet independent observational networks.

The ACTM sensitivity runs derived with ‘Control’ global emissions and global mean OH concentrations form tight linear relationships for each year, although NH–SH CH₃CCl₃ gradients vary significantly with time in proportion to total CH₃CCl₃ emissions (Extended Data Fig. 7e, f). Using the ‘Control’ CH₃CCl₃ emission scenario, we have estimated 2004–2011 average (±1σ of annual values) NH/SH OH ratios of 0.99 ± 0.10, 0.97 ± 0.13, 0.97 ± 0.11 and 0.96 ± 0.63 using CH₃CCl₃ concentrations from AGAGE GC–MD, AGAGE Medusa, NOAA flasks and the five HIPPO campaigns, respectively. The large variability (±0.63) between the HIPPO campaigns is caused by the seasonal OH cycle and transport as well as uncertainties in emissions. However, because five HIPPO campaigns cover the full seasonal cycle, the mean value can be considered as annually representative. The average NH/SH OH ratio estimated from surface observation networks (0.97) agrees very well (within in 2%) with the mean value estimated from the five HIPPO campaigns (0.96).

We use the standard deviation of individual annual mean values as an estimate of uncertainty in the long-term mean value. The uncertainty of about ±13% in the NH/SH OH ratio for the surface networks includes the measurement and model errors (<3%), emission uncertainties and interannual and seasonal variations in OH within both hemispheres, but the relative contributions of emission uncertainty and OH variations cannot be quantified. The model transport errors from SF₆ interhemispheric gradients are estimated to be 3% when averaged over five HIPPO campaigns, which is likely to decrease for longer time-averaging to ~1% (see ref. 6 for 2004–2007). An integrated estimate of measurement uncertainty has been produced²⁶, essentially using the standard deviation of the differences between global NOAA and AGAGE data. Their quoted 1% gives us an upper limit to the attainable measurement uncertainty, which is consistent across years. It includes the error in calibration propagation. However, because we are unable to quantify interannual variations in CH₃CCl₃ emissions accurately⁴⁵, the effects of interannual OH variations⁴⁶ within each hemisphere on the NH/SH ratio are not addressed here. For example, a recent study⁴⁷ shows that lightning mainly affects the air chemistry at ~300 mbar over the tropics, although the systematic difference in ACTM_0.99 and ACTM_1.26 mainly occur below 500 mbar in the extratropical to mid-latitude regions of the NH.

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We thank the HIPPO science team and the crew and support staff at the NCAR Research Aviation Facility, and all the laboratory staff working for AGAGE and NOAA measurement networks. This work is partly supported by the Japan Society for the Promotion of Science/Grants-in-Aid for Scientific Research (KAKENHI) Kiban-A (grant no. 22241008) and Ministry of Education, Culture, Sports, Science and Technology (MEXT) Arctic GRENE projects. NCAR is sponsored by the National Science Foundation (NSF). HIPPO was supported by NSF grants ATM-0628575, ATM-0628519, ATM-0628388 ATM-0628452 and ATM-1036399, by NASA award NNX11AF36G, and by NCAR. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of NSF, NOAA or NASA. M.C.K. is supported by EU FP7 project PEGASOS. AGAGE is supported principally by NASA grants to Massachusetts Institute of Technology (NNX11AF17G) and Scripps Institution of Oceanography (NNX11AF16G) and also by NOAA and the CSIRO. Mace Head is supported by the Department of Energy and Climate Change, award GA0201. We thank the CSIRO Oceans and Atmosphere Flagship and the Bureau of Meteorology for Cape Grim project funding. NOAA flask measurements are supported in part by NOAA’s Climate Program Office and its Atmospheric, Chemistry, Carbon Cycle and Climate Program.