The hydroxyl free radical (OH) is the major oxidizing chemical in the atmosphere, destroying about 3.7 petagrams (Pg) of trace gases each year, including many gases involved in ozone depletion, the greenhouse effect and urban air pollution. Measurements of 1,1,1-trichloroethane (methyl chloroform, CH₃CCl₃), which reacts with OH, provide the most accurate currently utilized method for determining the global behavior of OH. We report that CH₃CCl₃ levels rose steadily from 1978 to reach a maximum in 1992 and have since decreased rapidly to levels in 2004 less than 30% of the levels when measurements began in 1979. Analysis of these observations shows that global average OH levels had a small maximum around 1989 and a larger minimum around 1998, with OH concentrations in 2003 being comparable to those in 1979 (that is no significant long-term trend). This post-1998 recovery of OH reported here contrasts with the situation 4 years ago when reported OH was decreasing. The 1997-1999 OH minimum coincides with, and is likely caused by, major global wildfires and an intense El Niño event at this time.

For reprints and further information, please contact Ronald Prinn: rprinn@mit.edu

Article: Evidence for variability of atmospheric hydroxyl radicals over the past quarter century. Prinn, R.G., J. Huang, R.F. Weiss, D.M. Cunnold, P.J. Fraser, P.G. Simmonds, A. McCulloch, C. Harth, S. Reimann, P. Salameh, S. O'Doherty, R.H.J. Wang, L. W. Porter, B.R. Miller and P. B. Krummel, (2005), Geophysical Research Letters 32: L07809 (doi:10.1029/2004GL022228).