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

Abstract

Abstract Hydroxyl (OH) and hydroperoxy (HO 2 ) radicals (collectively called HO x ) 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 HO 2 are presented for the month-long study. The detection limits were about 3.0×10 5 cm −3 for OH and 2.5×10 6 cm −3 (∼0.1 ppt) for HO 2 with a 1-min integration time and a 2 σ confidence level. The daytime maximum concentrations were 5–20×10 6 cm −3 for OH and 0.4–6×10 8 cm −3 (2–24 pptv) for HO 2 , usually appearing later than the peak of ozone photolysis frequency, J (O 1 D). Relative high OH and HO 2 persisted into early evening and were frequently observed during nighttime. The ratios of HO 2 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 HO 2 with model calculations is given in a companion paper (OH and HO 2 chemistry in the urban atmosphere of New York City, Atmospheric Environment (2003a) this issue).