Evidence for Substantial Variations of Atmospheric Hydroxyl Radicals in the Past Two Decades (original) (raw)
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Atmospheric Trends and Lifetime of CH3CCI3 and Global OH Concentrations
Science, 1995
Determination of the atmospheric concentrations and lifetime of trichloroethane (CH3CC13) is very important in the context of global change. This halocarbon is involved in depletion of ozone, and the hydroxyl radical (OH) concentrations determined from its lifetime provide estimates of the lifetimes of most other hydrogen-containing gases involved in the ozone layer and climate. Global measurements of trichloroethane indicate rising concentrations before and declining concentrations after late 1991. The lifetime of CH3CC13 in the total atmosphere is 4.8 i 0.3 years, which is substantially lower than previously estimated. The deduced hydroxyl radical concentration, which measures the atmosphere's oxidizing capability, shows little change from 1978 to 1994.
Journal of Geophysical Research, 1992
Atmospheric measurements at several surface stations made between 1978 and 1990 of the anthropogenic chemical compound l,l,l-trichloroethane (methyl chloroform, CH3CC13) show it increasing at a global average rate of 4.4-+ 0.2% per year (lcr) 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 CH3CC13 tropospheric (and total atmospheric) lifetime of 5.7 (+0.7,-0.6) years (1 or) and a weighted global average tropospheric hydroxyl radical (OH) concentration of (8.7-+ 1.0) x 105 radical cm-3 (lcr). Inclusion of a small loss rate to the ocean for CH3CC13 of 1/85 year-• does not affect the stated lifetime but lowers the stated OH concentration to (8.1-+ 0.9) x 105 radical cm-3 (1 or). 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 (lcr) which has major implications for the oxidation capacity of the atmosphere and more specifically for methane (CH4), which like CH3CC13 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 CH3CC13 are accurate. The CH3CC13 measurements at our tropical South Pacific station (Samoa) show remarkable sensitivity to the E1 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. 1. oxy radical (HO2) by NO, 03, HO2, and organoperoxy radicals [e.g., Donahue and Prinn, 1990]. It is destroyed on a time scale of about 1 s by oxidizing CO, SO2, NO2, and a wide range of hydrocarbons, including methane (CH4). Direct measurement of tropospheric OH is very difficult with long-baseline spectroscopy possessing a sensitivity level not much lower than ambient OH levels [Callies et al., 1989]. An alternative, indirect approach to OH measurement is to determine the rate of loss of a chemical whose sources are known and whose major sink is reaction with OH. This approach can be used with a globally dispersed chemical like 1,1,1-trichloroethane (CH3CC13) to yield a weighted global average OH concentration [Singh,
Global OH trend inferred from methylchloroform measurements
Journal of Geophysical Research, 1998
Methylchloroform (MCF) measurements taken at the Atmospheric Lifetime Experiment / Global Atmospheric Gases Experiment (ALE/GAGE) measurement stations are used to deduce the tropospheric OH concentration and its linear trend between 1978 and 1993. Global three-dimensional fields of OH are calculated with a transport model that includes background photochemistry. Despite the large uncertainties in these OH fields, the simulated MCF concentrations at the five ALE/GAGE stations compare reasonably well to the measurements. As a next step, the OH fields are adjusted to fit the measurements optimally. An ensemble (Monte Carlo) technique is used to optimize the OH scaling factor and to derive the linear trend in OH. The optimized OH fields and trend imply a MCF lifetime in the troposphere of 4.7 years in 1978 and of 4.5 years in 1993. For CH4 these lifetimes (due to OH destruction only) are 9.2 and 8.6 years in 1978 and 1993, respectively. Uncertainties in these estimates are discussed using box-model calculations. The optimized OH concentration is sensitive to the strength of other MCF sinks in the model and is constrained to 1 an+o.09 0 6 ... o.•s x 1 molecules cm -a in 1978 and to 1 07+0.09 ... o. •7 x 106 molecules cm -a in 1993. The deduced OH trend is sensitive to the trend in the MCF emissions and is confined to the interval between -0.1 and +1.1% yr -• with a most likely value of 0.46% yr -•. Possible causes of a global increase in OH are discussed. A positive OH
On the role of hydroxyl radicals in the self-cleansing capacity of the troposphere
Atmospheric Chemistry and Physics, 2004
Thousands of megatons natural and anthropogenic gases are released and subsequently removed from the troposphere each year. Photochemical reactions, initiated by hydroxyl (OH) radicals, oxidise most gases to products which are more easily removed by precipitation and dry deposition at the earth's surface. Since human-induced pollution emissions strongly affect OH formation and loss, large global changes in OH concentrations are possible. Global models and observations of trace gas distributions from global networks have been used to study geographical and temporal changes in tropospheric OH. Here we present a synopsis of recent studies, indicating that global mean OH has changed remarkably little in the past century, even though regional changes have probably been substantial. Globally, depletion of OH by reactive carbon gases has been compensated by increased OH formation by nitrogen oxides, an act of "inadvertent geo-engineering". However, OH analyses for the past 1-2 decades, partly based on methyl chloroform measurements, are inconclusive. Some work, assuming that methyl chloroform emissions have largely ceased, suggests a very strong downward global OH trend in the 1990s, inconsistent with modelling studies. The discrepancy could be much reduced by assuming continued small emissions of methyl chloroform. We recommend the continuation of high precision monitoring of this compound and improved analyses based on detailed meteorological-chemical models.
Atmospheric Chemistry and Physics, 2013
We have analysed results from 17 global models, participating in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), to explore trends in hydroxyl radical concentration (OH) and methane (CH 4 ) lifetime since preindustrial times (1850) and gain a better understanding of their key drivers. For the present day 20 to a 4.3 ± 1.9 % decrease in the methane lifetime. Analysing sensitivity simulations performed by 10 models, we find that preindustrial to present day climate change decreased the methane lifetime by about 4 months, representing a negative feedback on the climate system. Further, using a subset of the models, we find that global mean OH increased by 46.4 ± 12.2 % in response to preindustrial to present day anthropogenic 25 NO x emission increases, and decreased by 17.3 ± 2.3 %, 7.6 ± 1.5 %, and 3.1 ± 3.0 % due to methane burden, and anthropogenic CO, and NMVOC emissions increases, respectively.
Journal of Geophysical Research: Atmospheres
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 (CH 3 CCl 3) (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 (CH 2 F 2), HFC-134a (CH 2 FCF 3 , HFC-152a (CH 3 CHF 2), and HCFC-22 (CHClF 2), 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.