An intensive field campaign, Program of Regional Integrated Experiments of Air Quality over Pear River Delta (PRIDE-PRD2004), was carried out in Pearl River Delta (PRD) in October 2004 to provide an in-depth understanding and a comprehensive record of O3, PM2.5 and other air pollutants in this quickly developing region of China. Two super sites, one in Guangzhou urban center and the other at a downwind rural site Xinken, were extensively equipped with research-grade instruments. In addition, 16 stations of the PRD air quality monitoring network participated in the campaign to provide regional scale spatial coverage. We found high levels of O3 exceeding China's National Standard throughout the PRD region and clear evidence of regional scale photochemical production of O3. Furthermore, the data suggested that in situ photochemical production was the primary source of high concentrations of O3 in the upwind area, and that the ozone photochemical production efficiency was highly non-linear with greater value at lower NOx. This non-linear relationship between photochemical production of O3 and its precursors has an important implication for the O3 control strategy. The ozone-precursor relationship was examined by using an observation-based model (OBM). Large positive relative incremental reactivity (RIR) values for VOCs and negative values for NOx were found, suggesting that O3 production was usually in VOC-limited regime in both Guangzhou and Xinken sites, while reducing NOx could increase O3. However, we caution that this conclusion was reached in a campaign of limited time period and the OBM results could differ in other seasons. Finally, high concentrations of HONO were observed at the two super sites. Current known gas phase source of HONO could account for only a fraction of the observed values, particularly at the Guangzhou site. An unknown and presumed heterogeneous source of HONO would be required to sustain the observed concentrations. At these high levels of HONO, the OH concentration and ozone production would be 2-4 times higher than the case with gas phase source only.