A tunable diode laser absorption spectrometer (TDLAS) was deployed during the PMTACS-NY Supersite winter 2004 intensive field campaign at Queens College in New York City to measure the ambient gaseous ammonia. For the characterization of ammonia emissions from the mobile sources, a LI-7000 CO₂/H₂O analyzer was also collocated with the TDLAS to measure ambient CO₂ and H₂O vapor. The field measurements and laboratory calibration with certified ammonia standard have been used to evaluate the performance of the TDLAS system. High time resolved TDLAS ambient ammonia measurements performed at Queens College from 10 January to 6 February showed high variability, with NH₃ concentrations ranging from below the detection limit (0.1 ppb) to maxima of 197.4 ppb and a mean value of 0.8 ppb over the entire campaign. Many high-frequency NH₃ spikes spanning over a less than 1-min duration were observed during the high traffic periods. The occurrence of the NH₃ spikes was closely correlated with observed CO₂ spikes, a good marker of traffic exhaust. This correlation yielded an NH₃ emission ratio of 0.12 ppbv/ppmv, which can be used to estimate an NH₃ emission factor of 35.5 mg/km. The [NH₃]/[CO₂] ratios over the entire field study was also obtained and added into the best NH₃ emission estimates. On a snowy day, no obvious drop of NH₃ and CO₂ concentrations was measured as the ambient H₂O vapor increased. The observed dramatic decrease in the ambient NH₃ and CO₂ concentrations on a rainy day resulted from a quick air mass switch. Two similar bimodal diurnal patterns associated with the rush hour traffic were observed during school holidays and school days of Queens College, New York. More NH₃ emissions from cold start vehicles might contribute to a higher peak in the late afternoon hours. Such observations suggest that the NH₃ emissions from the traffic exhaust could be a major source of the ambient NH₃ in urban areas.