Automated measurement of the size and concentration of airborne particulate nitrate

A three-stage, cascaded integrated collection and vaporization system has been developed to provide automated, 10 min resolution monitoring of the size and concentration of fine particulate nitrate in the atmosphere. Particles are collected (7 min) by a humidified impaction process, and analyzed in place (3 min) by flash vaporization and chemiluminescent detection of the evolved nitrogen oxides. The three size fractions, < 0.45 μm, 0.45-1.0 μm, and 1.0-2.5 μm, are chosen to distinguish the condensation, droplet, and coarse components of PM_2.5. The size precut at 2.5 μm is done at ambient conditions, while the size fractionation at 1.0 μm and 0.45 μm is done at a constant relative humidity of 65%. The system is calibrated with laboratory aerosols, including comparison of sizing for hygroscopic salt and hydrophobic organic aerosols. The complete system is tested with monodisperse ammonium nitrate aerosol generated with a high-flow differential mobility analyzer coupled with an impactor precut and yields results consistent with the calibration of the individual stages.