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 × 104 − 1 × 106 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 × 104 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 © 2008 Royal Meteorological Society