Role of Peroxyacyl Nitrates (PANs) Species in Tropospheric Chemistry in the Southeastern U.S. during SOAS 2013

Abstract:

Measurements of PANs, mainly focused on peroxyacetyl nitrate (PAN), peroxypropinoyl nitrate (PPN), and peroxymethacryloyl nitrate (MPAN), were made during the Southern Oxidant and Aerosol Study (SOAS) from June 1 to July 15, 2013 at the SEARCH ground site in Centerville, Alabama. Grab samples were measured at ground level by GC­ECD every 20 min. Since emissions of biogenic volatile organic compounds in the Southeast are high and there are a variety of anthropogenic pollution sources in the area, the location allows the study of biogenic/anthropogenic interactions.

PANs showed a diurnal behavior with double peaks around noon and again at the end of the afternoon. Mean mixing ratios during the daytime were 178, 5, and 9 pptv for PAN, PPN, and MPAN respectively with highest levels in air from the North. The ratios of PPN or MPAN with PAN are used to predict the relative importance of biogenic and anthropogenic VOC sources. It appears that biogenic VOC sources were slightly more important.

The concentrations measured in 2013 are lower than levels at similar sites in the Southeastern U.S. made over the last 20 years. A comparison is made to assess the role of changes in NOx emissions in the Southeastern U.S and resultant photoproducts like PANs and ozone. While absolute levels of PANs are lower, the relative amount MPAN is similar at Centreville while the relative amount of PPN decreased dramatically. This suggests that the impact of anthropogenic VOC emissions on the atmospheric chemistry seen in Centerville, 2013 was lower than seen in the SE previously.

Recent laboratory experiments have suggested that the hydroxynitrate of MPAN could be a main precursor to formation of secondary organic aerosol. Field data of speciated MPAN along with particle values, including particulate nitrogen, will allow the relationship between gas phase MPAN and aerosol composition to be explored.