Global average concentration and trend for hydroxyl radicals deduced from ALE/GAGE trichloroethane (methyl chloroform) data for 1978–1990

Abstract

Atmospheric measurements at several surface stations made between 1978 and 1990 of the anthropogenic chemical compound 1,1,1‐trichloroethane (methyl chloroform, CH 3 CCl 3) show it increasing at a global average rate of 4.4 ± 0.2% per year (1σ) over this time period. The measured trends combined with industrial emission estimates are used in an optimal estimation inversion scheme to deduce a globally averaged CH 3 CCl 3 tropospheric (and total atmospheric) lifetime of 5.7 (+0.7, −0.6) years (1σ) and a weighted global average tropospheric hydroxyl radical (OH) concentration of (8.7 ± 1.0) × 10 5 radical cm−3 (1σ). Inclusion of a small loss rate to the ocean for CH 3 CCl 3 of 1/85 year−1 does not affect the stated lifetime but lowers the stated OH concentration to (8.1 ± 0.9) × 10 5 radical cm−3 (1σ). The rate of change of the weighted global average OH concentration over this time period is determined to be 1.0 ± 0.8% per year (1σ) which has major implications for the oxidation capacity of the atmosphere and more specifically for methane (CH 4), which like CH 3 CCl 3 is destroyed primarily by OH radicals. Because the weighting strongly favors the tropical lower troposphere, this deduced positive OH trend is qualitatively consistent with hypothesized changes in tropical tropospheric OH and ozone concentrations driven by tropical urbanization, biomass burning, land use changes, and long‐term warming. We caution, however, that our deduced rate of change in OH assumes that current industry estimates of anthropogenic emissions and our absolute calibration of CH 3 CCl 3 are accurate. The CH 3 CCl 3 measurements at our tropical South Pacific station (Samoa) show remarkable sensitivity to the El Nino‐Southern Oscillation (ENSO), which we attribute to modulation of cross‐equatorial transport during the northern hemisphere winter by the interannually varying upper tropospheric zonal winds in the equatorial Pacific. Thus measurements of this chemical compound have led to the discovery of a previously unappreciated aspect of tropical atmospheric tracer transport.