Organic nitrogen in PM2.5 aerosol at a forest site in the Southeast US
There is growing evidence that organo-nitrogen compounds may constitute a significant fraction of the aerosol nitrogen (N) budget. However, very little is known about the abundance and origin of this aerosol fraction. In this study, the concentration of organic nitrogen (ON) and major inorganic ions in PM2.5 aerosol were measured at the Duke Forest Research Facility near Chapel Hill, NC, dur- ing January and June of 2007. A novel on-line instrument was used, which is based on the Steam Jet Aerosol Collector (SJAC) coupled to an on-line total carbon/total nitrogen an- alyzer and two on-line ion chromatographs. The concentra- tion of ON was determined by tracking the difference in con- centrations of total nitrogen and of inorganic nitrogen (deter- mined as the sum of N-ammonium and N-nitrate). The time resolution of the instrumentwas 30 min with a detection limit for major aerosol components of ∼0.1 µgm−3. Nitrogen in organic compounds contributed ∼33% on av- erage to the total nitrogen concentration inPM2.5, illustrating the importance of this aerosol component. Absolute concen- trations of ON, however, were relatively low (<1.0 µgm−3) with an average of 0.16 µgm−3. The absolute and relative contribution of ON to the total aerosol nitrogen budget was practically the same in January and June. In January, the concentration of ON tended to be higher during the night and early morning, while in June it tended to be higher during the late afternoon and evening. Back-trajectories and correlation with wind direction indicate that higher concentrations ofON occur in air masses originating over the continental US, while marine air masses are characterized by lower ON concentra- tions. The data presented in this study suggests that ON has a variety of sources, which are very difficult to quantify with- out information on chemical composition of this important aerosol fraction.