Behavior of OH and HO2 in the winter atmosphere in New York City

Hydroxyl (OH) and hydroperxy (HO2) radicals, collectively known as HOx, were measured during an intensive field study in January and February 2004 in New York City. Much less OH and HO2 levels were observed than in the summer of 2001 at the same site. On average, the maximum daytime mixing ratios were 0.05 pptv (1.4×106 cm−3) for OH and 0.7 pptv for HO2, which were about one fifth of the levels in the summer of 2001. A zero-dimensional chemical model, based on the regional atmospheric chemical mechanism (RACM) and constrained by the measured concentrations of O3, NO, NO2, CO, SO2, speciated volatile organic compounds (VOCs) and meteorological parameters, was used to study the HOx chemistry in this environment. The model generally reproduced the daytime OH well, with a median measured-to-model ratio of 0.98. However, HO2 was significantly under-predicted both at day and at night, with a median measured-to-model ratio of 6.0 during daytime. The discrepancy is pronounced when NO concentrations were high, a result that is consistent with some previous studies in urban environments. Photolysis of HONO was the dominant calculated HOx source during daytime; O3 reactions with alkenes became the main calculated HOx source at night. The main calculated HOx sink was the OH reaction with NO2. The discrepancy between measured and modeled HO2 may be caused by significant HOx production that is missing in the model. An additional HO2 production of up to 3×107 cm−3 s−1 (1.1 pptv s−1), which is three times the calculated HOx production, is needed. This HO2 production can come either from unknown new HOx production or from unknown HO2 recycling that does not go through OH.