UC Irvine

We employed a fast response thermal dissociation-chemical ionization mass spectrometer (TD-CIMS) system to measure eddy covariance fluxes of peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN) and peroxymethacryloyl nitrate (MPAN). Fluxes were measured for eight consecutive days in July 2003 at a Loblolly pine forest in North Carolina along with eddy covariance NOy fluxes. Covariances between PAN concentration and vertical wind velocity indicated consistent deposition fluxes that ranged up to approximately -14 ng N m-2 s-1. The average daytime flux peaked at -6.0 ng N m-2 s-1 and accounted for ∼20% of the daytime NOy flux. Calculations suggest minimum daytime surface resistances for PAN in the range of 70-130 s m-1. It was estimated that approximately half of daytime uptake was through plant stomates. Average PAN deposition velocities, Vd(PAN), showed a daytime maximum of ∼10.0 mm s-1; however, deposition did not cease during nighttime periods. Vd(PAN) was highly variable at night and increased when canopy elements were wet from either precipitation or dew formation. Diel patterns of deposition velocity of MPAN and PPN were similar to that of PAN. These results suggest that deposition of PAN, at least to coniferous forest canopies, is much faster than predicted with current deposition algorithms. Although deposition of PAN is unlikely to compete with thermal dissociation during warm summer periods, it will likely play an important role in removing PAN from the atmosphere in colder regions or during winter. The fate of PAN at the surface and within the plants remains unknown, but may present a previously ignored source of nitrogen to ecosystems. Copyright 2006 by the American Geophysical Union.