Measurements of isoprene nitrates above a forest canopy

Measurements of atmospheric organic nitrates derived from isoprene, i.e., " isoprene nitrates", were conducted from July 14 to August 19, 1998, as part of the 1998 summer intensive measurement campaign of the Program for Resea rch on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) at the University of Michigan Biological Station in Pellston, Michigan. The measur ements were conducted using on-line chromatography in conjunction with a ni trate-selective detection scheme. Measured concentrations of isoprene nitra tes ranged from 0.5 parts per trillion (ppt), the detection limit of the me thod employed, to 35 ppt. In this paper we discuss the contribution of the isoprene nitrates to NOy which was typically 0.5 - 1.5% of total odd nitrog en, but up to similar to4% for well-aged air. Concentrations of isoprene ni trates exhibited a strong diurnal variation consistent with their expected chemical and physical removal rates. In this work we also discuss the chemi stry of the precursor peroxy radicals and the NOx dependence of isoprene ni trate formation.