Measurement of ambient ammonia using a tunable diode laser absorption spectrometer: Characteristics of ambient ammonia emissions in an urban area of New York City

Abstract

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 CO 2 /H 2 O analyzer was also collocated with the TDLAS to measure ambient CO 2 and H 2 O 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 NH 3 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 NH 3 spikes spanning over a less than 1-min duration were observed during the high traffic periods. The occurrence of the NH 3 spikes was closely correlated with observed CO 2 spikes, a good marker of traffic exhaust. This correlation yielded an NH 3 emission ratio of 0.12 ppbv/ppmv, which can be used to estimate an NH 3 emission factor of 35.5 mg/km. The [NH 3 ]/[CO 2 ] ratios over the entire field study was also obtained and added into the best NH 3 emission estimates. On a snowy day, no obvious drop of NH 3 and CO 2 concentrations was measured as the ambient H 2 O vapor increased. The observed dramatic decrease in the ambient NH 3 and CO 2 concentrations on a rainy day resulted from a quick air mass switch. Two similar bimodal diumal patterns associated with the rush hour traffic were observed during school holidays and school days of Queens College, New York. More NH 3 emissions from cold start vehicles might contribute to a higher peak in the late afternoon hours. Such observations suggest that the NH 3 emissions from the traffic exhaust could be a major source of the ambient NH 3 in urban areas. Copyright 2006 by the American Geophysical Union.