Atmospheric ammonia measurement with an ion mobility spectrometer

An ion mobility spectrometry system was deployed during a field experiment to measure ambient ammonia (NH3). A dual-cell AirSentry ion mobility spectrometer (ASIMS) was utilized to measure ambient NH3 in suburban Tampa, FL from 1 to 16 October 2003. Statistical analysis of half-hourly averaged data showed that the concentration distribution was bimodal. The vicinity of the first mode of the data set had a mean and standard error of 1.4±0.1 ppb, which was ascribed to background levels of NH3, while the mean concentration of the second mode, 3.5±0.1 ppb, probably represented the contribution from local emission sources. Joint frequency distributions of NH3 concentration and 16 wind direction sectors were developed into pollution roses. The largest joint frequency values were from the NNW through NE sectors as well as the SW and SE sectors. Spikes in NH3 concentrations were observed in winds emanating from the N, NNE, S, and NW sectors. A comparison of a cumulative pollution rose and a map of known NH3 emitters in the vicinity of the sampling site showed a direct relationship between winds emanating from the sectors where fertilizer manufacturing facilities were located and high NH3 concentrations.