Isoprene Nitrate Chemistry in a Mixed Coniferous/Deciduous Forest during the 2016 PROPHET-AMOS Summer Field Study

Isoprene, the most abundant biogenic volatile organic compound emitted from forest ecosystems, represents a significant sink for atmospheric organic nitrogen. The formation of isoprene nitrates by the oxidation of isoprene in the presence of NOx influences global concentrations of NOx and ozone, and can be a major precursor to secondary organic aerosol. Measurements of isoprene nitrates in the ambient environment are sparse, however, and present an analytical challenge. In part, this is due to the influence of isomeric species on the sensitivity of commonly employed mass spectrometry techniques, including chemical ionization mass spectrometry (CIMS), and the availability of standards. Here, we present results from the 2016 PROPHET-AMOS summer field study in northern Michigan on the atmospheric chemistry of isoprene hydroxynitrates (IN) both at and below the canopy level using a single-quadrupole CIMS. The PROPHET research site is unique in that it is currently undergoing a succession from predominantly deciduous, and thus primarily isoprene emitting, to coniferous, primarily monoterpene emitting. Preliminary results indicate that IN production reaches a maximum in the early afternoon of approximately 40-50 ppt on average, driven by NO chemistry. This is relatively low compared to, for example, the SOAS site, owing to the relative availability of NO in this pristine atmosphere in northern Michigan. However, concentrations can remain high past midnight, possibly due to the influence of NO3 chemistry, poor vertical mixing in the evening, or slow deposition. Between the two inlets, the within-canopy inlet demonstrated higher [IN], indicative of canopy-scale isoprene-NOx chemistry; however, the vertical gradients can be more homogeneous depending on the meteorological conditions. Using supplemental measurements of OH, NOx, O3, HO2, and isoprene, we apply a 0-D box model to simulate the production of IN at the PROPHET site, to better understand the influence of local-scale chemistry on its production and destruction.