Determination of total aerosol nitrogen by thermal evolution

An aerosol nitrogen analyzer has been developed for determination of total nitrogen (TN) content in aerosols collected on a filter substrate. It uses rapid thermoevolution of nitrogenous species at 800degreesC in a 2.5% O-2/97.5% He carrier gas. Evolved nitrogen is oxidized to nitrogen oxides on a manganese dioxide catalyst, converted to nitrogen monoxide on a molybdenum catalyst, and quantified with a chemiluminescence detector. The analyzer is built upon components of two commercially available instruments, a thermal aerosol carbon analyzer and a chemiluminescent NOx analyzer. This system is able to provide fast (3 min per sample) and highly sensitive (a detection limit of 26 ng N) TN measurements for aerosol samples of a small size (similar to1 x 1 cm(2)) without any pretreatment. When the aerosol nitrogen analyzer was properly calibrated by ammonium sulfate, excellent agreement was obtained between TN measurements of twelve aerosol samples by the aerosol nitrogen analyzer and those obtained from an elemental analyzer. This instrument provides a useful and convenient tool for characterization of the organic nitrogen (ON) pool in atmospheric aerosols. The ON concentrations in the twelve aerosol samples, calculated from the difference between TN and the sum of the inorganic nitrogen species determined by ion chromatographic analysis, were found comparable in magnitude to particulate nitrate-N. The contribution of ON to TN ranged from 1.7% to 54% for the limited set of test samples. While twelve samples are insufficient to draw general conclusions about the relative distribution of ambient aerosol nitrogen species, they have demonstrated the applicability of the new method to ambient aerosol characterization and exposed a potential limitation in deriving at ON concentrations by taking a sometime small difference between two large numbers (i.e., TN and inorganic nitrogen). Differentiation of organic and inorganic nitrogen using thermal methods is also explored and found not to be feasible.