Urban photochemistry in central Tokyo: 1. Observed and modeled OH and HO₂ radical concentrations during the winter and summer of 2004

We used laser‐induced fluorescence to measure the concentrations of OH and HO 2 radicals in central Tokyo during two intensive campaigns (IMPACT IV and IMPACT L) in January–February and July–August 2004. The estimated detection limit for the 10‐min data was 1.3 × 10 5 cm −3 for the nighttime and 5.2 × 10 5 cm −3 for the daytime. The median values of the daytime peak concentrations of HO 2 were 1.1 and 5.7 pptv for the winter and summer periods, respectively, while the values for OH were 1.5 × 10 6 and 6.3 × 10 6 cm −3 . High HO 2 mixing ratios (>50 pptv) were observed on a day in summer when O 3 mixing ratios exceeded 100 ppbv. The average nighttime concentrations of HO 2 were 0.7 and 2.6 pptv for the winter and summer periods, respectively, while the values for OH were 1.8 × 10 5 and 3.7 × 10 5 cm −3 . A photochemical box model constrained by ancillary observations was able to reproduce daytime OH concentrations reasonably well for both periods, although daytime HO 2 concentrations were underestimated in winter and overestimated in summer. Increasing the wintertime hydrocarbon concentrations in the model led to an increase in daytime HO 2 concentrations, thereby showing better agreement with observations; however, the model continued to underestimate HO 2 concentrations at high NO mixing ratios. This underestimate was most pronounced in the mornings of both periods and during the daytime in winter. We studied processes that are capable of explaining this discrepancy, including unknown reactions of HNO 4 or an unidentified HO x source that is linearly scalable to the NO mixing ratio. The important processes in terms of producing radicals were the olefin + O 3 reactions in the nighttime of both periods and during the daytime in winter, the photolysis of carbonyls in the daytime for both periods, and the photolysis of HONO during the daytime in winter (using measured HONO concentrations) and during mornings in summer (using estimated HONO concentrations).