[1] Measurements of ambient formaldehyde (HCHO), related gases and particulate matter were carried out from the SUNY Albany mobile platform at the Queens College site in New York City (NYC) from 15 July to 3 August 2009. Ambient HCHO was measured using a quantum cascade laser (QCL) trace gas detector. HCHO concentrations ranged from 0.4 to 7.5 ppb with a mean value of 2.2 AE 1.1 ppb. Daily HCHO peaks were nearly always found between 1100 EST (Eastern Standard Time) and noontime throughout the sampling period. HCHO correlated strongly with NOx and black carbon during the traffic rush hours, but around noontime HCHO correlated much better with total oxidants (Ox = O 3 + NO 2 ). Using the diurnal pattern of HCHO/BC ratios, we estimated that 70% of HCHO present between 1200 EST to 1500 EST was produced by photochemical reactions. Sources of photochemically produced HCHO were calculated using measured concentrations of hydrocarbons, their reaction kinetics with OH radicals, and HCHO yields. These calculations indicated that isoprene oxidation was the dominant source of HCHO for this period at this site, responsible for 44%, followed by methane (25%) and propene (18%). To assess the impact of HCHO as a radical source, the HOx production rates from HCHO, HONO, O 3 photolysis, and alkenes +O 3 were calculated as well. Daily averaged HOx production rates from HONO, HCHO, O 3 photolysis and alkenes +O 3 were 8.6 Â 10 6 , 2.3 Â 10 6 , 1.7 Â 10 6 , 2.1 Â 10 5 molecules cm À3 s À1 , respectively, contributing 67, 18, 13 and 2% to the overall daily HOx radical budget from these precursors.