OH, HO2, and OH reactivity during the PMTACS-NY Whiteface Mountain 2002 campaign: Observations and model comparison

Hydroxyl (OH), hydroperoxy (HO2) radicals, collectively known as HOx, and OH reactivity, were measured during the PMTACS–NY (PM2.5 Technology Assessment and Characteristics Study‐New York) summer 2002 intensive at Whiteface Mountain, Wilmington, New York. The measurement results of OH and HO2 for 4 weeks are presented. Diurnal cycles show that the average noontime maximum mixing ratios were about 0.11 pptv (2.6 × 106 cm−3) for OH and 20 pptv for HO2. Measured HO2 to OH ratios were typically between 40 and 400, which are greater than those obtained in polluted and semipolluted rural environments. Low but significant mixing ratios of OH and HO2 persisted into early evening and were frequently observed during nighttime, consistent with previous studies in different environments. Steady state OH and HO2 were calculated with a zero‐dimensional chemical model using a complete Regional Atmospheric Chemical Mechanism (RACM) and a parameterized RACM which was constrained to the measured OH reactivity. Good agreement was obtained between the complete RACM and the parameterized RACM models. On average, the complete RACM model reproduced the observed OH with a median measured‐to‐modeled OH ratio of 0.82 and daytime HO2 with a median measured‐to‐modeled HO2 ratio of 1.21. The reasonably good agreement in this study is inconsistent with the significant underestimation of OH in the Program for Research on Oxidants: Photochemistry, Emissions, and Transport in 1998 (PROPHET98) study at a similar forested site. HOx budget analysis indicates that OH was primarily from the photolysis of HONO and O3 during the day and from O3 + alkenes reactions at night. The main HOx loss was the self reaction of HO2. The good agreement between the measured and calculated OH reactivity in this environment contrasts with findings in the PROPHET2000 study, in which significant OH reactivity was missing and the missing OH reactivity was temperature‐dependent.