Observations of reactive nitrogen and ozone at a rural site in Michigan

Measurements of NO_y, NO_x, PAN, PPN, MPAN, C₃–C₅ alkyl nitrates, total isoprene nitrates, HNO₃, HONO, and aerosol NO₃− and NO₂− made during the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) Summer 1998 intensive were used to investigate the reactive nitrogen budget at a forested, rural site in northern Michigan.; Mixing ratios of NO_y, NO_x, PAN, PPN, and alkyl nitrates were observed to have a strong dependence on the direction of transport to the site, as was expected from the geographical distribution of major urban and industrial sources. A peak in NO_x and NO_x/NO _y during the morning in southerly flow provided evidence for the transport of relatively unprocessed emissions in layers above the nocturnal boundary layer. Differences in PAN, PPN, and alkyl nitrate levels between northerly and southerly flow indicated a net transport of reactive nitrogen in the form of these species to regions farther north. Isoprene nitrates and MPAN were each typically observed to comprise less than 1% of NO_y, despite the dominance of isoprene in local VOC chemistry. Due to the rapid losses of isoprene nitrates through reaction, vertical mixing, and deposition, their role in the processing of NO_x may be significant.; Agreement within experimental uncertainty was observed between NO _y and the sum of the individually measured constituents during ten 24-hour periods, although a linear regression indicated the potential for an interference in one or more of the individual measurements and a possible shortfall in NO_y.; Measurements in both Summer 1998 and Summer 2000 intensives indicated that HONO may play a larger role in both the reactive nitrogen and radical budgets than expected for a rural site. Elevated levels of HONO both at night and during the day imply the presence of a significant heterogeneous production pathway.; Measurements of NO, NO₂, O₃, photolysis frequencies, OH, HO₂, and isoprene made during the PROPHET Summer 2000 intensive were used to evaluate the contribution of in situ photochemical ozone production to the local ozone budget. Typical calculated daytime O₃ production rates were 2–5 ppbv hr−1. A comparison of these values with the observed changes in O₃ implies that the near-surface layer was a source of ozone to the boundary layer.