Field assessment of the dynamics of particulate nitrate vaporization using differential TEOM® and automated nitrate monitors

In this study, we report a comparison of the results from the differential tapered element oscillating microbalance (TEOM) and a semi-continuous nitrate monitor in an attempt to obtain insights into the dynamics of volatilization from the heated TEOM filter. Measurements were conducted in Claremont, California, a city downwind of Los Angeles and a region of high nitrate concentrations. Field results are compared with that obtained in the laboratory for sampling of pure ammonium nitrate aerosol, and with the vaporization losses predicted by theory. The primary hypothesis that we are testing is that the nitric acid and ammonia gas volatilization from particle-bound ammonium nitrate loaded on the TEOMʹs fiber filter occurs over time scales that are longer than the 5-min cycle time for the system. This is important to the use of the Differential TEOM, which utilizes the vaporization measured during alternate 5-min periods as a reference baseline value for its particle mass measurements. Our experiments showed that under ambient conditions, the mass lost from the Differential TEOM tracks well the ambient particulate nitrate concentration. The saturation ratio for nitrate vapor immediately downstream of the filter, calculated at the filter temperature assuming all volatilized mass is ammonium nitrate, ranged from 0.05 to 0.2. By comparison, for laboratory collection of ammonium nitrate aerosol the vaporization reached a maximum value corresponding to a saturation of nitrate vapor downstream of the heated filter. This difference is due to the relatively higher particle concentrations used for the laboratory experiments. For the ambient measurements, the particulate nitrate concentrations were consistently lower than the equilibrium vapor concentration at the TEOM filter temperature. In both cases, the nitrate vaporization is driven by the temperature of the TEOM filter and independent of the pressure drop across the filter.