Ultrafine nitrate particle events in Baltimore observed by real-time single particle mass spectrometry

Ambient particles in Baltimore, Maryland were characterized from April through November 2002 using the real-time single particle mass spectrometer, RSMS III. When particles containing nitrate were examined, two types of ultrafine particle events were revealed: a large burst of nominally “pure” nitrate particles in the 50–90 nm size range, and a smaller (and less frequent) burst of “pure” particles in the 50–90 nm size range that grew to 110–220 nm with time. Coincident with both of these events was an increase in the number of mixed composition particles containing nitrate, suggesting that they were formed by condensation of ammonium nitrate onto pre-existing particles. Meteorological variables, particle number concentrations and continuous nitrate mass measurements were compared to the single particle data. Number and mass concentrations estimated from RSMS III correlated well with similar measurements with other techniques. Ultrafine nitrate particle events were observed during periods of low temperature and high relative humidity as expected from ammonium nitrate equilibrium considerations. During these events, the partitioning of ammonium nitrate to the particle phase strongly influenced the particle number concentration as well as the chemical composition.