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 x 106cm(-3)) for OH and 0.7pptv 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 O-3, NO, NO2, CO, SO2, speciated volatile organic compounds (VOCs) and meteorological parameters, was used to study the HO2 chemistry in this environment. The model generally reproduced the daytime OH well, with a median measured-tomodel ratio of 0.98. However, HO2 was significantly under-predicted both at day and at night, with a median measured-tomodel 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 HO2 source during daytime; O-3 reactions with alkenes became the main calculated HO2 source at night. The main calculated HO, sink was the OH reaction with NO2. The discrepancy between measured and modeled HO2 may be caused by significant HO2 production that is missing in the model. An additional HO2 production of up to 3 x 10(7) cm(-3) S-1 (1.1 pptv s(-1)), which is three times the calculated HO, production, is needed. This HO2 production can come either from unknown new HO, production or from unknown HO2 recycling that does not go through OH. (c) 2006 Elsevier Ltd. All rights reserved.