Interactive comment on “Nighttime observation and chemistry of HOx in the Pearl River Delta and Beijing in summer 2006” by K. D. Lu et al

The authors discuss three different mechanisms that could explain the observations. These include as yet unknown HOx recycling reactions, similar to that used to explain large daytime concentrations of HOx from the same campaigns; a source of HOx from nighttime ozonolysis reactions of highly reactive biogenic VOCs; and transport of radical reservoirs from a layer aloft to the surface. None of the three mechanisms satisfactorily explain the magnitude or time dependences of the observations, but they do suggest useful tests that can be applied in future investigations, especially with regard to the measurements of nighttime vertical gradients.