Night-time NO3 and OH radical concentrations in the United Kingdom inferred from hydrocarbon measurements

The hydrocarbon decay method (Rivett et al., 2003) has been used to analyse hydrocarbon data from four contrasting sites in the United Kingdom to estimate night-time levels of NO3 and OH. Remarkably consistent results emerge using alkenes, revealing NO3 and OH levels in the range of 0.01-10 ppt and 1 x 10(4)-1 x 10(6) molecule cm(-3), respectively. Weak seasonal cycles are observed where NO3 levels peak in spring and OH in summer. Analysis using alkanes suggests that Cl atom levels of around 2 x 10(4) molecule cm(-3) may be present. How Cl may be formed at night in such high quantities is unknown and may not be the answer to the disparity between the two methods. Copyright (c) 2008 Royal Meteorological Society