Observational evidence for interhemispheric hydroxyl-radical parity

Journal name:
Nature
Volume:
513,
Pages:
219–223
Date published:
DOI:
doi:10.1038/nature13721
Received
Accepted
Published online

The hydroxyl radical (OH) is a key oxidant involved in the removal of air pollutants and greenhouse gases from the atmosphere1, 2, 3. The ratio of Northern Hemispheric to Southern Hemispheric (NH/SH) OH concentration is important for our understanding of emission estimates of atmospheric species such as nitrogen oxides and methane4, 5, 6. It remains poorly constrained, however, with a range of estimates from 0.85 to 1.4 (refs 4, 7,8,9,10). Here we determine the NH/SH ratio of OH with the help of methyl chloroform data (a proxy for OH concentrations) and an atmospheric transport model that accurately describes interhemispheric transport and modelled emissions. We find that for the years 2004–2011 the model predicts an annual mean NH–SH gradient of methyl chloroform that is a tight linear function of the modelled NH/SH ratio in annual mean OH. We estimate a NH/SH OH ratio of 0.97 ± 0.12 during this time period by optimizing global total emissions and mean OH abundance to fit methyl chloroform data from two surface-measurement networks and aircraft campaigns11, 12, 13. Our findings suggest that top-down emission estimates of reactive species such as nitrogen oxides in key emitting countries in the NH that are based on a NH/SH OH ratio larger than 1 may be overestimated.

At a glance

Figures

  1. Temporal evolution of measured (symbols) and simulated (lines) CH3CCl3 in the atmosphere.
    Figure 1: Temporal evolution of measured (symbols) and simulated (lines) CH3CCl3 in the atmosphere.

    a, Monthly mean concentrations at MHD (blue), RPB (green), SMO (red) and CGO (black). Observations (symbols) are taken at four AGAGE sites using GC–MD, and the ACTM simulations (lines) correspond to the ‘Control’ case of total emissions and annual mean OH. b, MHD–CGO concentration differences are shown in comparison with ACTM_0.99 (red) and ACTM_1.26 (blue) simulations. Note that because of the coarse horizontal resolution (T42 spectral truncations; ~2.8° × 2.8°) site representation errors are large for Mace Head when intense emissions occurred over Western Europe, for example until 2000 for CH3CCl3. ACTM_0.99 simulation at a horizontal resolution of T106 spectral truncations (~1.1° × 1.1°; ACTM_T106 in inset to b; green) for the period 2002–2011 shows no significant difference for CH3CCl3 from the T42 resolution run, indicating that site representation error does not affect our results. The inset to b also shows a model simulation (ACTM_UNEP; purple) using a different emission distribution, based on countries reporting to the United Nations Environment Programme (UNEP), but with identical global emission totals and OH distribution as for ACTM_0.99. The model lines are broken because of a missing observation in August 2010 at CGO. Representative CH3CCl3 emission distributions for the ‘Control’ and ‘UNEP’ cases are shown in Extended Data Fig. 2. Similar CH3CCl3 concentration gradients, based on a greater number of NOAA flask sampling sites, are shown in Extended Data Fig. 3.

  2. Role of global total emissions and chemical loss on inter-site CH3CCl3 differences.
    Figure 2: Role of global total emissions and chemical loss on inter-site CH3CCl3 differences.

    a, Model–measurement mismatch (derived as J = √{[(CN − CS)model − (CN − CS)measured]2/N}; CN and CS are CH3CCl3 concentrations in the NH and SH, respectively, and N is the number of data points) as a function of total emissions (E) and chemical loss (CL) due to global mean OH abundance varied together in a manner consistent with the observed global decline in CH3CCl3 concentrations. The mismatch is shown in terms of standard deviations of simulated ALT–PSA (black, ACTM_0.99; red, ACTM_1.26) and MHD–CGO (green, ACTM_0.99; blue, ACTM_1.26) CH3CCl3 concentration differences with respect to measurements as monthly averages over the period 2004–2011. b, c, Annual means of inter-site difference at monthly intervals (b) and peak-to-trough seasonal cycle amplitude in the inter-site difference (c), to decompose the contribution of E and CL to the model–measurement mismatch. The observed values are shown by horizontal purple lines for ALT-PSA and light blue lines for MHD-CGO. These results are independent of sampling network (MHD–CGO from AGAGE GC–MD and ALT–PSA from NOAA). All the sensitivity simulations were for 2001–2011. Simulations for 2001–2003 have been considered as spin-up, and are excluded for calculating statistics.

  3. Meridional gradients of CH3CCl3 during five HIPPO campaigns suggest that the NH/SH OH ratio is close to 1.
    Figure 3: Meridional gradients of CH3CCl3 during five HIPPO campaigns suggest that the NH/SH OH ratio is close to 1.

    Latitudinal distributions of CH3CCl3 are shown as measured from the Advanced Whole Air Sampling (AWAS) flask air (black), and as simulated by ACTM_0.99 (red) and ACTM_1.26 (blue) with ‘Control’ global emissions and global mean OH concentrations. a, HIPPO 1, 12–23 January 2009; b, HIPPO 3, 26 March to 15 April 2010; c, HIPPO 4, 16 June to 10 July 2011; d, HIPPO 5, 19 August to 8 September 2011; e, HIPPO 2, 2–21 November 2009. The panels are arranged in seasonal order. The median concentrations are shown at 5° latitude intervals for a 1–4-km altitude range for the meridional gradients. The y-axis range is maintained at 1.5 p.p.t., however, the absolute values differ, reflecting time differences between the campaigns. Both ACTM results are adjusted to the mean observed values corresponding to >25° S and the altitude range 1–4 km for each of the HIPPO campaigns separately (+0.07, +0.05, −0.24, +0.05 and −0.05 for ACTM_0.99, and −0.90, −0.85, −1.09, −0.75 and −0.80 for ACTM_1.26 for HIPPO 1–5, respectively), to allow for uncertainties in decadal emissions and lifetimes of CH3CCl3 (and bias in concentration gradients for ACTM_1.26), but this systematic shift with the SH reference does not affect the meridional gradient northward of 25° S.

  4. Estimation of the NH/SH OH concentration ratio from CH3CCl3 interhemispheric gradients.
    Figure 4: Estimation of the NH/SH OH concentration ratio from CH3CCl3 interhemispheric gradients.

    NH–SH CH3CCl3 concentration differences for different measurement data sets (black, AGAGE GC-MD, MHD–CGO; red, AGAGE Medusa, MHD–CGO; blue, NOAA flask, ALT–PSA; green, HIPPO, between 30° N and 30° S) based on the ACTM sensitivity simulations for various NH/SH OH ratios during the period 2004–2011 considering the case of ‘Control’ global total emissions and global mean OH concentrations. ACTM simulation results (open symbols) using different OH distributions (ACTM_0.99; ACTM_0.99 ± sine functions; ACTM_0.99 and ACTM_1.26 mixtures; and ACTM_1.26; Extended Data Table 2b) are sampled for the AGAGE GC–MD, AGAGE Medusa, NOAA flasks and HIPPO sampling locations. The observed NH–SH CH3CCl3 concentration gradients (closed symbols) are calculated using MHD and CGO for AGAGE, ALT and PSA for NOAA flasks, and averages of data in the latitudes polewards of 30° in the altitude range 1–4 km for HIPPO, which are then used for calculating the NH/SH OH ratio with the fitted lines (GC–MD, y = 1.556 − 1.191x; Medusa, y = 1.589 − 1.188x; flask, y = 1.589 − 1.188x; HIPPO, y = 0.899 − 0.481x). Model outputs for the Medusa and GC–MD measurements differ slightly because gaps in the records from the two instruments during the 2004–2011 time period are not coincident.

  5. Latitude-height distributions of zonal-mean OH and CH3CCl3 lifetime in the troposphere.
    Extended Data Fig. 1: Latitude–height distributions of zonal-mean OH and CH3CCl3 lifetime in the troposphere.

    Results are shown for two months in distinct seasons, January (left column) and July (middle column), and annual mean (right column) for ACTM_0.99 OH (a), ACTM_1.26 OH (b) and ACTM_9.99 CH3CCl3 (c) lifetime. The vertical model coordinate is defined by sigma-pressure = (P − Ptop)/P0, where P, Ptop and P0 are pressure at a given model level, model top level and model surface layer, respectively. Although there is overall agreement for the seasonal variations and spatial gradients, the annual mean NH/SH OH ratio is ~26% higher for ACTM_1.26 than for ACTM_0.99. The higher OH in the NH for ACTM_1.26 is caused mainly by greater OH amounts near the Earth’s surface over the regions of active air pollution chemistry, such as industrialized Asia, Europe and North America. The difference in annual mean NH/SH OH ratio between ACTM_1.26 and ACTM_0.99 diminishes at a sigma-pressure height of 0.5 (mid-troposphere) and above. Note that the CH3CCl3 lifetime in the lower troposphere over the tropical latitudes of the summer hemisphere can be shorter than 2 years, which is of the same order of magnitude as the interhemispheric exchange time of 1.3 years in ACTM6, 22. Thus both chemistry and transport in the troposphere are expected to influence the meridional distributions of CH3CCl3. The monthly (at 0.5° S in January and 7.3° N in July) and annual (4.2° N) locations of the ITCZ determined from the dynamical and chemical equators are marked approximately by a vertical line in a and b.

  6. Longitude-latitude distributions of CH3CCl3 emissions, trends in global total emissions and CH3CCl3 global lifetimes in ACTM_0.99, and sensitivity of the MHD-CGO CH3CCl3 difference to the NH/SH emission ratio.
    Extended Data Fig. 2: Longitude–latitude distributions of CH3CCl3 emissions, trends in global total emissions and CH3CCl3 global lifetimes in ACTM_0.99, and sensitivity of the MHD–CGO CH3CCl3 difference to the NH/SH emission ratio.

    a, b, The ‘Control’ CH3CCl3 emission case uses interannually varying spatial distributions until 1999, and the 1999 spatial distribution for all later years (a, top row), and the UNEP emission distribution depends on country reports for each year (b). There is an order of magnitude difference in colour scales for 1995 and 2005/2010. Although the UNEP-based maps show no emissions over Europe, the atmospheric observations suggest continued emissions of CH3CCl3 up to and including 2011 (ref. 48). Thus we continued to use the 1990s emission map for 2000 and later years in the Control case. The surface observation site numbers are shown in b. c, Global total CH3CCl3 emissions are shown in comparison with ref. 26 (Extended Data Table 1), and agree within 0.8 Gg per year or 13% on average during 2000–2009. Lifetimes of CH3CCl3 are estimated by using two different methods (black line, τSS = burden/loss; red line, d(burden)/dt = emission − burden/τtotal). The total lifetimes are adjusted for CH3CCl3 loss on the oceanic surface for this comparison plot. d, The global total emissions of SF6, scaled to ref. 34, and HFC-134a (EDGAR4.2) as used in the ACTM simulations (Methods). The SF6 and HFC-134a emissions distributions are from the EDGAR4.2 emission database25. e, The observed MHD–CGO difference is shown as horizontal lines at 0.39 p.p.t. for GC–MD and 0.44 p.p.t. for Medusa instruments, which suggests that generally a solution exists for simulating MHD–CGO differences for ACTM_0.99 at a NH/SH emission ratio of >10 (the ‘Control’ case is shown by the vertical line at ~16.6). We have used monthly mean model output for this plot; therefore no distinction between Medusa and GC–MD sampling times can be made for model results (unlike in Fig. 4). The ACTM simulated symbols at the right end of each line correspond to all emissions in the NH (NH/SH ratio = ∞) and are not scaled on the x axis. No solution can be achieved for ACTM_1.26 for the Control global CH3CCl3 emissions. Because the NH/SH emission ratios are in the range 17–40 for UNEP-based emissions for the 2000s, we find the ACTM_0.99 MHD–CGO concentration differences to be in good agreement with those observed (Fig. 1b, inset).

  7. Longitude-latitude distributions of simulated CH3CCl3, and comparisons of simulated and measured CH3CCl3 variations at NOAA HATS sites.
    Extended Data Fig. 3: Longitude–latitude distributions of simulated CH3CCl3, and comparisons of simulated and measured CH3CCl3 variations at NOAA HATS sites.

    a, b, The right column is for annual mean concentration and the left two columns are for two distinct months: January and July. Results are presented for the lowest model level for 2010, considering ‘Control’ global emissions and annual mean OH concentrations. Variable colour scales are used to account for the decrease in CH3CCl3 concentrations. Offsets (indicated at the bottom of each panel in b) are subtracted from the CH3CCl3 ACTM_1.26 run to match colour shading over Antarctica for ACTM_0.99 and ACTM_1.26 runs. The distributions of SF6 with decreasing concentrations from NH to SH (not shown) are controlled by emission distributions and atmospheric transport, whereas those for CH3CCl3 are governed by the loss due to chemical reaction with tropospheric OH, transport and emissions. c, d, Monthly mean concentrations at four representative sites (left column) and inter-site differences with respect to PSA for ALT, KUM, SMO and SPO (middle column), and for BRW, THD, MHD and NWR (right column). ACTM_0.99 (c) and ACTM_1.26 (d) simulation results for the ‘Control’ global emissions. All measurements are monthly means derived from the NOAA flask network.

  8. Relationship between lifetime and emission change for simulating the observed decay in CH3CCl3 concentration and the NH-SH CH3CCl3 gradient.
    Extended Data Fig. 4: Relationship between lifetime and emission change for simulating the observed decay in CH3CCl3 concentration and the NH–SH CH3CCl3 gradient.

    a, Implied emissions calculated for different lifetimes of CH3CCl3 (by decreasing or increasing the loss rates by 10%, 20% or 30% with respect to a ‘Control’ loss case corresponding to a lifetime of 4.9 years). Because both the emissions and burden change with time, no general conclusion can be drawn, apart from the linearity between lifetime (primarily governed by the OH abundance) and implied or required global total emissions for simulating the observed concentration decay rates. b, As a, but all values scaled with respect to the control value. This allows us to conclude that there is a range of global emission and global OH values that can successfully simulate the observed global decline in CH3CCl3 mixing ratio over time that are a constant relative adjustment to the ‘control’ global emissions and global mean OH concentrations. The ACTM results for a +30% to −30% change in chemical loss (CL) and simultaneous +117% to −117% change in global total emissions (E), respectively, are shown for ACTM_0.99 (c) and for ACTM_1.26 (d) in comparison with the measurements (2004–2011). The 2004–2011 average of MHD–CGO and ALT–PSA differences and peak-to-trough seasonal cycle amplitudes are summarized in Fig. 2.

  9. State of weather during the five HIPPO campaigns, and representativeness of HIPPO measurements over the central Pacific Ocean.
    Extended Data Fig. 5: State of weather during the five HIPPO campaigns, and representativeness of HIPPO measurements over the central Pacific Ocean.

    a, Locations of HIPPO profiles, with flight tracks marked by research flight (RF) numbers during each of the five HIPPO campaigns, plotted with rainfall rates33. The onward transects, from the Arctic to the Antarctic, over the Central Pacific Ocean are used in here. Although data from selected flights over the central Pacific Ocean are used here, each of the HIPPO campaigns consisted of a series of 10–14 flights in the Pacific region spanning 67° S–87° N (from north of Alaska to south of New Zealand). Measurement periods for the HIPPO campaigns are 8–30 January 2009, 31 October–22 November 2009, 24 March–16 April 2010, 14 June–11 July 2011 and 9 August–9 September 2011. The pentad-mean CMAP rainfall rates are provided by NOAA/OAR/ESRL PSD, Boulder, Colorado, USA (http://www.esrl.noaa.gov/psd). b, c, As a check for representativeness of HIPPO over the central Pacific Ocean, we show comparisons of ACTM_0.99 simulated zonal mean (shaded) seasonal cycles of CH3CCl3 (b) and SF6 (c) at the surface with those simulated for three different longitudes (contour lines) (top row, central Pacific Ocean; middle row, central Atlantic Ocean; bottom row, central Indian Ocean). The zonal mean values for both CH3CCl3 and SF6 agree to within 0.1 p.p.t. with those at 180° E, suggesting that HIPPO measurements of these gases over the central Pacific represent zonal averages. The differences between the zonal mean values and those at different longitude regions decrease with increasing altitude. The zonal differences between different sectors are governed primarily by the emissions; for example, larger differences between the zonal mean are observed for the Indian Ocean sector at ~30° N for SF6 (d, bottom row) owing to Indian emissions. The zonal differences for CH3CCl3 are apparent but are less distinct because the surface emissions are small over the selected longitudes (see Extended Data Fig. 2).

  10. Comparisons of simulated and measured SF6 during HIPPO.
    Extended Data Fig. 6: Comparisons of simulated and measured SF6 during HIPPO.

    ah, Measurements from the PANTHER GC-ECD (left column) and ACTM simulations (right column) for January (HIPPO 1), June–July (HIPPO 4), August–September (HIPPO 5) and November (HIPPO 2) over the central Pacific (research flight no. 2-8). All the data are binned and averaged at intervals of 2.5° latitude and 1 km altitude. The white areas indicate no flights at those latitudes and altitudes (no PANTHER measurements were conducted during HIPPO 3). Although data from selected flights are shown here, each of the HIPPO campaigns consisted of a series of 10–14 flights in the Pacific region spanning 67° S–87° N from north of Alaska to south of New Zealand (Extended Data Fig. 5a). ir, Latitudinal (im; 1–3 km average) and vertical (nr; 1–3 km average to 5–7 km average) SF6 gradients simulated by ACTM using emissions from EDGAR4.2 (extended for 2009–2011) and measured during the five HIPPO campaigns. The y-axis range of 0.8 p.p.t. is fixed for all the panels in the left column to show the meridional gradients, but the absolute values differ to account for the increase in concentration from January 2009 to September 2011. A 0.1 p.p.t. offset is added to the simulated SF6 concentrations for better comparison with the observations. Because SF6 is an inert tracer in the troposphere, an arbitrary offset does not affect our interpretation of model interhemispheric transport. The altitude range of 1–3 km is chosen here, as opposed to 1–4 km in Fig. 2, for obtaining representative vertical gradients because the number of observations decreases significantly above 7 km.

  11. Estimation of of NH/SH OH ratios from the relationships of the NH-SH CH3CCl3 gradient with NH/SH OH ratio.
    Extended Data Fig. 7: Estimation of of NH/SH OH ratios from the relationships of the NH–SH CH3CCl3 gradient with NH/SH OH ratio.

    ad, Comparisons of Mace Head to Cape Grim gradients in CH3CCl3 as measured by AGAGE and simulated by nine cases of ACTM with varying NH/SH ratios of OH, but for only the ‘Control’ global emissions and OH concentration scenario. The results for ACTM_0.99, with OH modified using a sine (latitude) function (Extended Data Table 2b), are shown in the top row, and those for mixing the ACTM_0.99 and ACTM_1.26 OH fields are shown in the bottom row. Time series at monthly mean intervals are shown in the left column, and annual means in the right column. Most of the observed differences between MHD and CGO (symbols) lie above the ACTM_0.99 simulated line, and towards simulations using NH/SH OH ratios of less than 1. The first 3 years of simulations are considered as model spin-up and are not used to calculate statistics. e, f, Similar to Fig. 4, but for AGAGE GC–MD observations for different years between 2004 and 2011 (e) and using HIPPO observations below 4 km during individual campaigns (f) for the ‘Control’ global emissions and global OH concentrations (right). This figure shows the changes in MHD (NH)–CGO (SH) CH3CCl3 gradients because of the decrease in emissions with time. We show only the averaged (2004–2011) results in Fig. 4 of the main text by sampling the model results at the time of measurements to avoid any bias from the changing NH–SH CH3CCl3 gradients. The HIPPO NH–SH CH3CCl3 gradients are for the hemispheres separated at the Equator, whereas the results in Fig. 4 separate the hemispheres using data in the latitudes polewards of 30°, to avoid the tropical region so as to estimate a NH/SH OH ratio that is more comparable with those estimated from the surface sites chosen for comparison (for example MHD and CGO). The cross and plus symbols mark the location of NH–SH CH3CCl3 concentration difference for deriving the NH/SH OH ratio. The calculated NH/SH (separated at the Equator) OH ratio is 1.01 ± 0.16 averaged over five HIPPO campaigns (1.07, 1.05, 0.85, 1.24 and 0.87 for HIPPO 1–5, respectively, during January 2009, October–November 2009, March–April 2010, June–July 2011 and August–September 2011). The large variability (±0.16) between the HIPPO campaigns is caused by the seasonal cycle in OH and transport as well as uncertainties in emissions.

Tables

  1. List of annual total emissions and emission/burden (E/B)
    Extended Data Table 1: List of annual total emissions and emission/burden (E/B)
  2. Details of the surface measurement sites and OH fields used in ACTM simulations
    Extended Data Table 2: Details of the surface measurement sites and OH fields used in ACTM simulations
  3. Evaluation of ACTM simulations with the use of HIPPO aircraft measurements
    Extended Data Table 3: Evaluation of ACTM simulations with the use of HIPPO aircraft measurements

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Author information

Affiliations

  1. Department of Environmental Geochemical Cycle Research, JAMSTEC, Yokohama 236 0001, Japan

    • P. K. Patra,
    • A. Ghosh,
    • K. Ishijima,
    • K. Miyazaki &
    • M. Takigawa
  2. CAOS, Graduate School of Studies, Tohoku University, Sendai 980 8578, Japan

    • P. K. Patra
  3. Wageningen University, Droevendaalsesteeg 3a, 6708 PB, The Netherlands

    • M. C. Krol
  4. National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory, Boulder, Colorado 80305, USA

    • S. A. Montzka,
    • J. W. Elkins,
    • E. J. Hintsa,
    • D. F. Hurst,
    • B. R. Miller &
    • F. L. Moore
  5. Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093, USA

    • T. Arnold,
    • J. Mühle &
    • R. F. Weiss
  6. The Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149, USA

    • E. L. Atlas
  7. Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08901, USA

    • B. R. Lintner
  8. National Center for Atmospheric Research (NCAR), Boulder, Colorado 80301, USA

    • B. B. Stephens
  9. School of Engineering and Applied Science, Harvard University, Cambridge, Massachusetts 02138, USA

    • B. Xiang &
    • S. C. Wofsy
  10. Centre for Australian Weather and Climate Research, Commonwealth Scientific and Industrial Research Organisation (CSIRO) Oceans and Atmosphere Flagship, Aspendale, Victoria 3195, Australia

    • P. J. Fraser,
    • P. B. Krummel &
    • L. P. Steele
  11. National Institute of Polar Research, 10-3, Midoricho, Tachikawa, Tokyo 190-8518, Japan

    • A. Ghosh
  12. CIRES, University of Colorado, Boulder, Colorado 80309, USA

    • E. J. Hintsa,
    • D. F. Hurst,
    • B. R. Miller &
    • F. L. Moore
  13. School of Chemistry, University of Bristol, Cantock’s Close, BS8 1TS, UK

    • S. O’Doherty &
    • D. Young
  14. Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

    • R. G. Prinn
  15. School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332, USA

    • H. J. Wang

Contributions

P.K.P., M.K., S.A.M., B.X., B.B.S., B.R.L., T.A. and A.G. designed the model experiments and performed data analysis. T.A., E.L.A., S.A.M., B.B.S., J.W.E., P.J.F., E.J.H., D.F.H., P.B.K., B.R.M., F.L.M., J.M., S.O.D., R.G.P., L.P.S., H.J.W., R.F.W., S.C.W. and D.Y. conducted measurements. All co-authors participated in writing the manuscript and contributed through discussions.

Competing financial interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to:

Author details

Extended data figures and tables

Extended Data Figures

  1. Extended Data Figure 1: Latitude–height distributions of zonal-mean OH and CH3CCl3 lifetime in the troposphere. (674 KB)

    Results are shown for two months in distinct seasons, January (left column) and July (middle column), and annual mean (right column) for ACTM_0.99 OH (a), ACTM_1.26 OH (b) and ACTM_9.99 CH3CCl3 (c) lifetime. The vertical model coordinate is defined by sigma-pressure = (P − Ptop)/P0, where P, Ptop and P0 are pressure at a given model level, model top level and model surface layer, respectively. Although there is overall agreement for the seasonal variations and spatial gradients, the annual mean NH/SH OH ratio is ~26% higher for ACTM_1.26 than for ACTM_0.99. The higher OH in the NH for ACTM_1.26 is caused mainly by greater OH amounts near the Earth’s surface over the regions of active air pollution chemistry, such as industrialized Asia, Europe and North America. The difference in annual mean NH/SH OH ratio between ACTM_1.26 and ACTM_0.99 diminishes at a sigma-pressure height of 0.5 (mid-troposphere) and above. Note that the CH3CCl3 lifetime in the lower troposphere over the tropical latitudes of the summer hemisphere can be shorter than 2 years, which is of the same order of magnitude as the interhemispheric exchange time of 1.3 years in ACTM6, 22. Thus both chemistry and transport in the troposphere are expected to influence the meridional distributions of CH3CCl3. The monthly (at 0.5° S in January and 7.3° N in July) and annual (4.2° N) locations of the ITCZ determined from the dynamical and chemical equators are marked approximately by a vertical line in a and b.

  2. Extended Data Figure 2: Longitude–latitude distributions of CH3CCl3 emissions, trends in global total emissions and CH3CCl3 global lifetimes in ACTM_0.99, and sensitivity of the MHD–CGO CH3CCl3 difference to the NH/SH emission ratio. (542 KB)

    a, b, The ‘Control’ CH3CCl3 emission case uses interannually varying spatial distributions until 1999, and the 1999 spatial distribution for all later years (a, top row), and the UNEP emission distribution depends on country reports for each year (b). There is an order of magnitude difference in colour scales for 1995 and 2005/2010. Although the UNEP-based maps show no emissions over Europe, the atmospheric observations suggest continued emissions of CH3CCl3 up to and including 2011 (ref. 48). Thus we continued to use the 1990s emission map for 2000 and later years in the Control case. The surface observation site numbers are shown in b. c, Global total CH3CCl3 emissions are shown in comparison with ref. 26 (Extended Data Table 1), and agree within 0.8 Gg per year or 13% on average during 2000–2009. Lifetimes of CH3CCl3 are estimated by using two different methods (black line, τSS = burden/loss; red line, d(burden)/dt = emission − burden/τtotal). The total lifetimes are adjusted for CH3CCl3 loss on the oceanic surface for this comparison plot. d, The global total emissions of SF6, scaled to ref. 34, and HFC-134a (EDGAR4.2) as used in the ACTM simulations (Methods). The SF6 and HFC-134a emissions distributions are from the EDGAR4.2 emission database25. e, The observed MHD–CGO difference is shown as horizontal lines at 0.39 p.p.t. for GC–MD and 0.44 p.p.t. for Medusa instruments, which suggests that generally a solution exists for simulating MHD–CGO differences for ACTM_0.99 at a NH/SH emission ratio of >10 (the ‘Control’ case is shown by the vertical line at ~16.6). We have used monthly mean model output for this plot; therefore no distinction between Medusa and GC–MD sampling times can be made for model results (unlike in Fig. 4). The ACTM simulated symbols at the right end of each line correspond to all emissions in the NH (NH/SH ratio = ∞) and are not scaled on the x axis. No solution can be achieved for ACTM_1.26 for the Control global CH3CCl3 emissions. Because the NH/SH emission ratios are in the range 17–40 for UNEP-based emissions for the 2000s, we find the ACTM_0.99 MHD–CGO concentration differences to be in good agreement with those observed (Fig. 1b, inset).

  3. Extended Data Figure 3: Longitude–latitude distributions of simulated CH3CCl3, and comparisons of simulated and measured CH3CCl3 variations at NOAA HATS sites. (810 KB)

    a, b, The right column is for annual mean concentration and the left two columns are for two distinct months: January and July. Results are presented for the lowest model level for 2010, considering ‘Control’ global emissions and annual mean OH concentrations. Variable colour scales are used to account for the decrease in CH3CCl3 concentrations. Offsets (indicated at the bottom of each panel in b) are subtracted from the CH3CCl3 ACTM_1.26 run to match colour shading over Antarctica for ACTM_0.99 and ACTM_1.26 runs. The distributions of SF6 with decreasing concentrations from NH to SH (not shown) are controlled by emission distributions and atmospheric transport, whereas those for CH3CCl3 are governed by the loss due to chemical reaction with tropospheric OH, transport and emissions. c, d, Monthly mean concentrations at four representative sites (left column) and inter-site differences with respect to PSA for ALT, KUM, SMO and SPO (middle column), and for BRW, THD, MHD and NWR (right column). ACTM_0.99 (c) and ACTM_1.26 (d) simulation results for the ‘Control’ global emissions. All measurements are monthly means derived from the NOAA flask network.

  4. Extended Data Figure 4: Relationship between lifetime and emission change for simulating the observed decay in CH3CCl3 concentration and the NH–SH CH3CCl3 gradient. (344 KB)

    a, Implied emissions calculated for different lifetimes of CH3CCl3 (by decreasing or increasing the loss rates by 10%, 20% or 30% with respect to a ‘Control’ loss case corresponding to a lifetime of 4.9 years). Because both the emissions and burden change with time, no general conclusion can be drawn, apart from the linearity between lifetime (primarily governed by the OH abundance) and implied or required global total emissions for simulating the observed concentration decay rates. b, As a, but all values scaled with respect to the control value. This allows us to conclude that there is a range of global emission and global OH values that can successfully simulate the observed global decline in CH3CCl3 mixing ratio over time that are a constant relative adjustment to the ‘control’ global emissions and global mean OH concentrations. The ACTM results for a +30% to −30% change in chemical loss (CL) and simultaneous +117% to −117% change in global total emissions (E), respectively, are shown for ACTM_0.99 (c) and for ACTM_1.26 (d) in comparison with the measurements (2004–2011). The 2004–2011 average of MHD–CGO and ALT–PSA differences and peak-to-trough seasonal cycle amplitudes are summarized in Fig. 2.

  5. Extended Data Figure 5: State of weather during the five HIPPO campaigns, and representativeness of HIPPO measurements over the central Pacific Ocean. (730 KB)

    a, Locations of HIPPO profiles, with flight tracks marked by research flight (RF) numbers during each of the five HIPPO campaigns, plotted with rainfall rates33. The onward transects, from the Arctic to the Antarctic, over the Central Pacific Ocean are used in here. Although data from selected flights over the central Pacific Ocean are used here, each of the HIPPO campaigns consisted of a series of 10–14 flights in the Pacific region spanning 67° S–87° N (from north of Alaska to south of New Zealand). Measurement periods for the HIPPO campaigns are 8–30 January 2009, 31 October–22 November 2009, 24 March–16 April 2010, 14 June–11 July 2011 and 9 August–9 September 2011. The pentad-mean CMAP rainfall rates are provided by NOAA/OAR/ESRL PSD, Boulder, Colorado, USA (http://www.esrl.noaa.gov/psd). b, c, As a check for representativeness of HIPPO over the central Pacific Ocean, we show comparisons of ACTM_0.99 simulated zonal mean (shaded) seasonal cycles of CH3CCl3 (b) and SF6 (c) at the surface with those simulated for three different longitudes (contour lines) (top row, central Pacific Ocean; middle row, central Atlantic Ocean; bottom row, central Indian Ocean). The zonal mean values for both CH3CCl3 and SF6 agree to within 0.1 p.p.t. with those at 180° E, suggesting that HIPPO measurements of these gases over the central Pacific represent zonal averages. The differences between the zonal mean values and those at different longitude regions decrease with increasing altitude. The zonal differences between different sectors are governed primarily by the emissions; for example, larger differences between the zonal mean are observed for the Indian Ocean sector at ~30° N for SF6 (d, bottom row) owing to Indian emissions. The zonal differences for CH3CCl3 are apparent but are less distinct because the surface emissions are small over the selected longitudes (see Extended Data Fig. 2).

  6. Extended Data Figure 6: Comparisons of simulated and measured SF6 during HIPPO. (661 KB)

    ah, Measurements from the PANTHER GC-ECD (left column) and ACTM simulations (right column) for January (HIPPO 1), June–July (HIPPO 4), August–September (HIPPO 5) and November (HIPPO 2) over the central Pacific (research flight no. 2-8). All the data are binned and averaged at intervals of 2.5° latitude and 1 km altitude. The white areas indicate no flights at those latitudes and altitudes (no PANTHER measurements were conducted during HIPPO 3). Although data from selected flights are shown here, each of the HIPPO campaigns consisted of a series of 10–14 flights in the Pacific region spanning 67° S–87° N from north of Alaska to south of New Zealand (Extended Data Fig. 5a). ir, Latitudinal (im; 1–3 km average) and vertical (nr; 1–3 km average to 5–7 km average) SF6 gradients simulated by ACTM using emissions from EDGAR4.2 (extended for 2009–2011) and measured during the five HIPPO campaigns. The y-axis range of 0.8 p.p.t. is fixed for all the panels in the left column to show the meridional gradients, but the absolute values differ to account for the increase in concentration from January 2009 to September 2011. A 0.1 p.p.t. offset is added to the simulated SF6 concentrations for better comparison with the observations. Because SF6 is an inert tracer in the troposphere, an arbitrary offset does not affect our interpretation of model interhemispheric transport. The altitude range of 1–3 km is chosen here, as opposed to 1–4 km in Fig. 2, for obtaining representative vertical gradients because the number of observations decreases significantly above 7 km.

  7. Extended Data Figure 7: Estimation of of NH/SH OH ratios from the relationships of the NH–SH CH3CCl3 gradient with NH/SH OH ratio. (372 KB)

    ad, Comparisons of Mace Head to Cape Grim gradients in CH3CCl3 as measured by AGAGE and simulated by nine cases of ACTM with varying NH/SH ratios of OH, but for only the ‘Control’ global emissions and OH concentration scenario. The results for ACTM_0.99, with OH modified using a sine (latitude) function (Extended Data Table 2b), are shown in the top row, and those for mixing the ACTM_0.99 and ACTM_1.26 OH fields are shown in the bottom row. Time series at monthly mean intervals are shown in the left column, and annual means in the right column. Most of the observed differences between MHD and CGO (symbols) lie above the ACTM_0.99 simulated line, and towards simulations using NH/SH OH ratios of less than 1. The first 3 years of simulations are considered as model spin-up and are not used to calculate statistics. e, f, Similar to Fig. 4, but for AGAGE GC–MD observations for different years between 2004 and 2011 (e) and using HIPPO observations below 4 km during individual campaigns (f) for the ‘Control’ global emissions and global OH concentrations (right). This figure shows the changes in MHD (NH)–CGO (SH) CH3CCl3 gradients because of the decrease in emissions with time. We show only the averaged (2004–2011) results in Fig. 4 of the main text by sampling the model results at the time of measurements to avoid any bias from the changing NH–SH CH3CCl3 gradients. The HIPPO NH–SH CH3CCl3 gradients are for the hemispheres separated at the Equator, whereas the results in Fig. 4 separate the hemispheres using data in the latitudes polewards of 30°, to avoid the tropical region so as to estimate a NH/SH OH ratio that is more comparable with those estimated from the surface sites chosen for comparison (for example MHD and CGO). The cross and plus symbols mark the location of NH–SH CH3CCl3 concentration difference for deriving the NH/SH OH ratio. The calculated NH/SH (separated at the Equator) OH ratio is 1.01 ± 0.16 averaged over five HIPPO campaigns (1.07, 1.05, 0.85, 1.24 and 0.87 for HIPPO 1–5, respectively, during January 2009, October–November 2009, March–April 2010, June–July 2011 and August–September 2011). The large variability (±0.16) between the HIPPO campaigns is caused by the seasonal cycle in OH and transport as well as uncertainties in emissions.

Extended Data Tables

  1. Extended Data Table 1: List of annual total emissions and emission/burden (E/B) (197 KB)
  2. Extended Data Table 2: Details of the surface measurement sites and OH fields used in ACTM simulations (381 KB)
  3. Extended Data Table 3: Evaluation of ACTM simulations with the use of HIPPO aircraft measurements (158 KB)

Additional data