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

Hydroxyl (OH) and hydroperxy (HO₂) 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 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 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, 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 HO_x source during daytime; O₃ reactions with alkenes became the main calculated HO_x source at night. The main calculated HO_x sink was the OH reaction with NO₂. The discrepancy between measured and modeled HO₂ may be caused by significant HO_x 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 HO_x production, is needed. This HO₂ production can come either from unknown new HO_x production or from unknown HO₂ recycling that does not go through OH.