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 x 10(5) cm(-3) for OH and 2.5 x 10(6) cm(-3) (similar to0.1 ppt) for HO2 with a 1-min integration time and a 2sigma confidence level. The daytime maximum concentrations were 5-20 x 10(6) cm(-3) for OH and 0.4-6 x 10(8) cm(-3) (2-24 pptv) for HO2, usually appearing later than the peak of ozone photolysis frequency, J((OD)-D-1). 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). (C) 2003 Elsevier Ltd. All rights reserved.