Peroxycarboxylic Nitric Anhydrides as Markers of Anthropogenic and Biogenic VOC Photo-oxidation in the Alberta Oil Sands

Abstract:

The peroxycarboxylic nitric anhydrides (molecular formula RC(O)O₂NO₂) are well-known byproducts of the photo-oxidation chemistry between NO_x and volatile organic compounds (VOCs) that produces ozone (O₃) and photochemical smog. More than 43 different PAN species are known; their relative abundances are chemical markers of the types and quantities of the VOCs involved in the O₃-formation process. For example, MPAN (R: CH₂=C(CH₃)-) is primarily derived from isoprene and thus a marker of biogenic VOC oxidation, whereas PPN (R: C₂H₅-) is a photo-oxidation byproduct of anthropogenic VOCs.

In the summer of 2013 an intensive air quality measurement campaign was conducted to investigate the impacts of emissions from the Alberta oil sands mining operations on the chemical composition of ambient air. As part of this effort, several peroxycarboxylic nitric anhydrides, specifically PAN (R: CH₃-), PPN, MPAN, APAN (R: CH₂=CH-), and PiBN (R: iC₃H₇-), were quantified by gas chromatography with electron capture detection at the AMS13 ground site near Fort McKay, Alberta. Furthermore, total peroxyacyl nitrates (ΣPAN) were quantified by thermal dissociation cavity ring-down spectroscopy (TD-CRDS).

PAN mixing ratios typically peaked in the mid-afternoon (maximum PAN mixing ratio of 0.85 ppbv), constituting up to 25% of total odd nitrogen (NO_y), and were usually below detection limits at night. ΣPAN was generally greater than the amount calculated by summation of individually measured PANs (SPAN_i) suggesting the presence of PAN species not measured by GC. During times of active photo-oxidation chemistry, the PPN:PAN and MPAN:PAN ratios varied considerably between days, depending on air mass origin and VOC composition. A linear combination model (LCM) was used to assess regional O₃ production from the oxidation of biogenic hydrocarbons (via MPAN) relative to that of anthropogenic hydrocarbons (via PPN). The relative contribution of anthropogenic VOCs to regional O₃ production varied between 20% and 80%. A box model using a subset of the Master Chemical Mechanism was used to investigate how measurements of APAN and PiBN may be incorporated in the LCM framework.