HOx concentrations and OH reactivity observations in New York City during PMTACS-NY2001

Hydroxyl (OH) and hydroperoxy (HO2) radicals (collectively called HOx) were measured by a laser-induced fluorescence instrument during the PMTACS-NY (PM2.5 Technology Assessment and Characterization Study–New York) intensive campaign in New York City in summer 2001. Measurement results for OH and HO2 are presented for the month-long study. The detection limits were about 3.0×105 cm−3 for OH and 2.5×106 cm−3 ( 0.1 ppt) for HO2 with a 1-min integration time and a 2σ confidence level. The daytime maximum concentrations were 5–20×106 cm−3 for OH and 0.4–6×108 cm−3 (2–24 pptv) for HO2, usually appearing later than the peak of ozone photolysis frequency, J(O1D). Relative high OH and HO2 persisted into early evening and were frequently observed during nighttime. The ratios of HO2 to OH were typically between 5 and 40, which are smaller than those obtained in relatively clean environments. The OH reactivity, measured by an instrument named total OH loss rate measurement was on average 19±3 s−1 in this urban environment. It was the highest in the morning and the lowest in the afternoon. The comparison of measured OH and HO2 with model calculations is given in a companion paper (OH and HO2 chemistry in the urban atmosphere of New York City, Atmospheric Environment (2003a) this issue).