2.3.5 Trends in the Hydroxyl Free Radical

The hydroxyl free radical (OH) is the major oxidizing chemical in the atmosphere, destroying about 3.7 Gt of trace gases, including CH₄ and all HFCs and HCFCs, each year (Ehhalt, 1999). It therefore has a very significant role in limiting the LLGHG RF. IPCC/TEAP (2005) concluded that the OH concentration might change in the 21st century by –18 to +5% depending on the emission scenario. The large-scale concentrations and long-term trends in OH can be inferred indirectly using global measurements of trace gases for which emissions are well known and the primary sink is OH. The best trace gas used to date for this purpose is methyl chloroform; long-term measurements of this gas are reviewed in Section 2.3.4. Other gases that are useful OH indicators include ¹⁴CO, which is produced primarily by cosmic rays (Lowe and Allan, 2002). While the accuracy of the ¹⁴CO cosmic ray and other ¹⁴CO source estimates and the frequency and spatial coverage of its measurements do not match those for methyl chloroform, the ¹⁴CO lifetime (2 months) is much shorter than that of methyl chloroform (4.9 years). As a result, ¹⁴CO provides estimates of average concentrations of OH that are more regional, and is capable of resolving shorter time scales than those estimated from methyl chloroform. The ¹⁴CO source variability is better defined than its absolute magnitude so it is better for inferring relative rather than absolute trends. Another useful gas is the industrial chemical HCFC-22. It yields OH concentrations similar to those derived from methyl chloroform, but with less accuracy due to greater uncertainties in emissions and less extensive measurements (Miller et al., 1998). The industrial gases HFC-134a, HCFC-141b and HCFC-142b are potentially useful OH estimators, but the accuracy of their emission estimates needs improvement (Huang and Prinn, 2002; O’Doherty et al., 2004).

Indirect measurements of OH using methyl chloroform have established that the globally weighted average OH concentration in the troposphere is roughly 10⁶ radicals per cubic centimetre (Prinn et al., 2001; Krol and Lelieveld, 2003). A similar average concentration is derived using ¹⁴CO (Quay et al., 2000), although the spatial weighting here is different. Note that methods to infer global or hemispheric average OH concentrations may be insensitive to compensating regional OH changes such as OH increases over continents and decreases over oceans (Lelieveld et al., 2002). In addition, the quoted absolute OH concentrations (but not their relative trends) depend on the choice of weighting (e.g., Lawrence et al., 2001). While the global average OH concentration appears fairly well defined by these indirect methods, the temporal trends in OH are more difficult to discern since they require long-term measurements, optimal inverse methods and very accurate calibrations, model transports and methyl chloroform emissions data. From AGAGE methyl chloroform measurements, Prinn et al. (2001) inferred that global OH levels grew between 1979 and 1989, but then declined between 1989 and 2000, and also exhibited significant interannual variations. They concluded that these decadal global variations were driven principally by NH OH, with SH OH decreasing from 1979 to 1989 and staying essentially constant after that. Using the same AGAGE data and identical methyl chloroform emissions, a three-dimensional model analysis (Krol and Lelieveld, 2003) supported qualitatively (but not quantitatively) the earlier result (Prinn et al., 2001) that OH concentrations increased in the 1980s and declined in the 1990s. Prinn et al. (2001) also estimated the emissions required to provide a zero trend in OH. These required methyl chloroform emissions differed substantially from industry estimates by McCulloch and Midgley (2001) particularly for 1996 to 2000. However, Krol and Lelieveld (2003) argued that the combination of possible underestimated recent emissions, especially the >20 Gg European emissions deduced by Krol et al. (2003), and the recent decreasing effectiveness of the stratosphere as a sink for tropospheric methyl chloroform, may be sufficient to yield a zero deduced OH trend. As discussed in Section 2.3.4, estimates of European emissions by Reimann et al. (2005) are an order of magnitude less than those of Krol et al. (2003). In addition, Prinn et al. (2005a) extended the OH estimates through 2004 and showed that the Prinn et al. (2001) decadal and interannual OH estimates remain valid even after accounting for the additional recent methyl chloroform emissions discussed in Section 2.3.4. They also reconfirmed the OH maximum around 1989 and a larger OH minimum around 1998, with OH concentrations then recovering so that in 2003 they were comparable to those in 1979. They noted that the 1997 to 1999 OH minimum coincides with, and is likely caused by, major global wildfires and an intense El Niño at that time. The 1997 Indonesian fires alone have been estimated to have lowered global late-1997 OH levels by 6% due to carbon monoxide (CO) enhancements (Duncan et al., 2003).

Methyl chloroform is also destroyed in the stratosphere. Because its stratospheric loss frequency is less than that in the troposphere, the stratosphere becomes a less effective sink for tropospheric methyl chloroform over time (Krol and Lelieveld, 2003), and even becomes a small source to the troposphere beginning in 1999 in the reference case in the Prinn et al. (2001, 2005a) model. Loss to the ocean has usually been considered irreversible, and its rates and uncertainties have been obtained from observations (Yvon-Lewis and Butler, 2002). However, Wennberg et al. (2004) recently proposed that the polar oceans may have effectively stored methyl chloroform during the pre-1992 years when its atmospheric levels were rising, but began re-emitting it in subsequent years, thus reducing the overall oceanic sink. Prinn et al. (2005a) tried both approaches and found that their inferred interannual and decadal OH variations were present using either formulation, but inferred OH was lower in the pre-1992 years and higher after that using the Wennberg et al. (2004) formulation.

More recently, Bousquet et al. (2005) used an inverse method with a three-dimensional model and methyl chloroform measurements and concluded that substantial year-to-year variations occurred in global average OH concentrations between 1980 and 2000. This conclusion was previously reached by Prinn et al. (2001), but subsequently challenged by Krol and Lelieveld (2003) who argued that these variations were caused by model shortcomings and that models need, in particular, to include observationally-based, interannually varying meteorology to provide accurate annual OH estimates. Neither the two-dimensional Prinn et al. (2001) nor the three-dimensional Krol et al. (2003) inversion models used interannually varying circulation. However, the Bousquet et al. (2005) analysis, which uses observationally based meteorology and estimates OH on monthly time scales, yields interannual OH variations that agree very well with the Prinn et al. (2001) and equivalent Krol and Lelieveld (2003) estimates (see Figure 2.8). However, when Bousquet et al. (2005) estimated both OH concentrations and methyl chloroform emissions (constrained by their uncertainties as reported by McCulloch and Midgley, 2001), the OH variations were reduced by 65% (dashed line in Figure 2.8). The error bars on the Prinn et al. (2001, 2005a) OH estimates, which account for these emission uncertainties using Monte Carlo ensembles of inversions, also easily allow such a reduction in OH variability (thin vertical bars in Figure 2.8). This implies that these interannual OH variations are real, but only their phasing and not their amplitude, is well defined. Bousquet et al. (2005) also deduced that OH in the SH shows a zero to small negative trend, in qualitative agreement with Prinn et al. (2001). Short-term variations in OH were also recently deduced by Manning et al. (2005) using 13 years of ¹⁴CO measurements in New Zealand and Antarctica. They found no significant long-term trend between 1989 and 2003 in SH OH but provided evidence for recurring multi-month OH variations of around 10%. They also deduced even larger (20%) OH decreases in 1991 and 1997, perhaps triggered by the 1991 Mt. Pinatubo eruption and the 1997 Indonesian fires. The similarity of many of these results to those from methyl chloroform discussed above is very important, given the independence of the two approaches.

Figure 2.8. Estimates used to evaluate trends in weighted global average OH concentrations. (A) and (B): comparison of 1980 to 1999 OH anomalies (relative to their long-term means) inferred by Bousquet et al. (2005), Prinn et al. (2001) and Krol et al. (2003) from AGAGE methyl chloroform observations, and by Bousquet et al. (2005) when methyl chloroform emissions as well as OH are inferred; error bars for Bousquet et al. (2005) refer to 1 standard deviation inversion errors while yellow areas refer to the envelope of their 18 OH inversions. (C) OH concentrations for 1979 to 2003 inferred by Prinn et al. (2005a) (utilising industry emissions corrected using recent methyl chloroform observations), showing the recovery of 2003 OH levels to 1979 levels; also shown are results assuming uncorrected emissions and estimates of recent oceanic re-emissions. Error bars in Prinn et al. (2001, 2005a) are 1 standard deviation and include inversion, model, emission and calibration errors from large Monte Carlo ensembles (see Section 2.3.5 for details and references).

RF calculations of the LLGHGs are calculated from observed trends in the LLGHG concentrations and therefore OH concentrations do not directly affect them. Nevertheless OH trends are needed to quantify LLGHG budgets (Section 7.4) and for understanding future trends in the LLGHGs and tropospheric ozone.