| The PRIDE-PRD 2006 campaign took place close to the mega-city Guangzhou in the Pearl
River Delta (PRD) in South China in July 2006. It offered, for the first time, an opportunity to
investigate the tropospheric photochemistry in a densely populated region in China by direct
radical measurements. Concentrations of OH and HO2 were simultaneously measured by
laser-induced fluorescence (LIF) with high time resolution, together with the OH reactivity
(reciprocal OH lifetime), which was recorded by a newly developed UV-pump LIF-probe
technique. In addition, a comprehensive set of other important trace gases (e.g.,
O3, H2O, CO, CH4, NMHCs, isoprene, NO, NO2, HONO etc.) and photolysis
frequencies were measured. The experimental data indicate that the photochemistry was
very active under the polluted conditions encountered in PRD, with high OH levels
reaching 2 x 107 cm-3 during daytime. In this presentation, the chemical OH budget
will be analyzed with respect to its chemical sources and sinks, and HOx radical
concentrations will be compared with box model calculations that are constrained by
measurements. Consequences for the photochemical formation of ozone will be discussed. |