ConspectusThe atmosphere–biosphere exchange of
nitrogen oxides plays
a key role in determining the composition of reactive nitrogen in
terrestrial vegetated environments. The emission of nitric oxide (NO)
from soils is an important atmospheric source of reactive nitrogen.
NO is rapidly interconverted with NO2, making up the chemical
family NOx(NOx≡ NO2+ NO). NOxfurther
reacts with the oxidation products of volatile organic compounds (VOCs)
to form the functionalized nitrogen oxide groups acyl peroxynitrates
(APNs = R(O)O2NO2) and alkyl nitrates (ANs =
RONO2). Both canopy-level field measurements and laboratory
studies suggest that the absorption of nitrogen dioxide NO2and APNs by vegetation is a significant sink of atmospheric NOx, removing a large fraction of global soil-emitted
NOxand providing key control on the
amounts and lifetimes of NOxand reactive
nitrogen in the atmosphere. Nitrogen oxides influence the production
of surface O3and secondary aerosols. The balance of the
emission and uptake of nitrogen oxides thus provides a mechanism for
the regulation of regional air quality. The biosphere, via this biogeochemical
cycling of nitrogen oxides, is becoming an increasingly important
determining factor for airborne pollutants as much of the world continues
to reduce the amount of combustion-related nitrogen oxide emissions.
Understanding the function of the biosphere as a source and sink of
reactive nitrogen is therefore ever more critical in evaluating the
effects of future and current emissions of nitrogen oxides on human
and ecosystem health.Laboratory measurements of the foliar
deposition of NO2and other reactive nitrogen species suggest
that there is a substantial
diversity of uptake rates under varying environmental conditions and
for different species of vegetation that is not currently reflected
in the widely utilized chemical transport models. Our branch chamber
measurements on a wide variety of North American tree species highlight
the variability in the rates of both photosynthesis and nitrogen oxide
deposition among several different nitrogen oxide compounds. Box-modeling
and satellite measurement approaches demonstrate how disparities between
our understanding of nitrogen oxide foliar exchange in the laboratory
and what is represented in models can lead to misrepresentations of
the net ecosystem exchange of nitrogen. This has important implications
for assumptions of in-canopy chemistry, soil emissions of NO, canopy
reductions of NOx, lifetimes of trace
gases, and the impact of the biosphere on air quality.