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

Abstract Hydroxyl (OH) and hydroperxy (HO 2 ) radicals, collectively known as HO x , were measured during an intensive field study in January and February 2004 in New York City. Much less OH and HO 2 levels were observed than in the summer of 2001 at the same site. On average, the maximum daytime mixing ratios were 0.05 pptv (1.4×10 6  cm −3 ) for OH and 0.7 pptv for HO 2 , 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, NO 2 , CO, SO 2 , speciated volatile organic compounds (VOCs) and meteorological parameters, was used to study the HO x chemistry in this environment. The model generally reproduced the daytime OH well, with a median measured-to-model ratio of 0.98. However, HO 2 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 HO x source during daytime; O 3 reactions with alkenes became the main calculated HO x source at night. The main calculated HO x sink was the OH reaction with NO 2 . The discrepancy between measured and modeled HO 2 may be caused by significant HO x production that is missing in the model. An additional HO 2 production of up to 3×10 7  cm −3  s −1 (1.1 pptv s −1 ), which is three times the calculated HO x production, is needed. This HO 2 production can come either from unknown new HO x production or from unknown HO 2 recycling that does not go through OH.