Cooperative Institute for Research in Environmental Sciences

Abstracts: 27

The Foliar Uptake of Peroxy Acetic Nitric Anhydride (PAN) and its Effects on the Cycling of Atmospheric Odd Nitrogen

Jed P. Sparks^1,3, James Roberts^2,3 and Russell K. Monson^1,3

1 Department of EPO Biology, University of Colorado, Boulder
2 NOAA Aeronomy Laboratory, Boulder [ About this Lab ]
3 Cooperative Institute for Research in Environmental Science (CIRES), University of Colorado, Boulder

The oxides of nitrogen (NOx = NO + NO2) play a central role in the chemistry of the troposphere, catalyzing ozone formation, producing HNO3, and moderating the oxidizing capacity of the atmosphere. NOx is rapidly converted in the atmosphere to product species, primarily HNO3 and peroxyacetic nitric anhydride, PAN, (a.k.a. peroxyacetyl nitrate, CH3C(O)OONO2). Deposition of these species is key to understanding the loss of odd-nitrogen from the atmosphere and the source of fixed nitrogen to the biosphere. We report here the first leaf-level measurements of the uptake of PAN conducted over the range of PAN mixing ratios characteristic of the continental lower troposphere. Seven plant species, characteristic of a number of different ecosystems, exhibited leaf-level PAN uptake rates between 20 and 40 pmol m-2 s-1 at an ambient PAN mixing ratio of 250 pptv. The uptake of PAN into the leaf appears to be strongly controlled by the stomatal aperture and at near stomatal closure (conductances < 10 mmol m-2 s-1) PAN uptake was not observed. When stomatal aperture was varied by drying the air stream around the leaf under constant light the same strong correlation between leaf PAN uptake and stomatal aperture was observed. Over the timescale of our measurements (20-30 minutes) simultaneous measurements of PAN and NOx revealed approximately 75% of the PAN is taken up by the leaf and ~ 25 % is transformed and reemitted as NOx.

Simple calculations suggest approximately 3% of the annual emission of NOx to the atmosphere could be removed by the direct foliar uptake of PAN. Using similar calculations and foliar uptake rates from the literature for other forms of oxidized nitrogen (most of which carry a high degree of uncertainty), 24% of the total annual atmospheric NOx budget could return to the biosphere through the direct foliar uptake of oxidized nitrogen. When translated into terms of the annual nitrogen requirement for tree growth in a temperate deciduous forest (the example provided here is a beech forest in Germany), the input of oxidized nitrogen through direct foliar uptake is ~3% of the annual N requirement. However, because most of the nitrogen used for annual growth is from the internal cycling of nitrogen within the ecosystem this number can be misleading. When compared to the total external input of nitrogen to the system (e.g. wet and non-assimilated dry deposition), the direct foliar uptake of oxidized nitrogen could account for ~ 14% of the annual external input of nitrogen to an ecosystem.