Atmospheric Dry Deposition of Gaseous and Particulate Nitrogen to Urban-Influenced Sonoran Desert Sites in Central Arizona

Dry deposition of atmospheric pollutants is a vector for the transfer

of anthropogenic nutrients to terrestrial ecosystems. Urbanization in

arid regions magnifies the impact of atmosphere-land exchange by dry

deposition. This additional atmospheric N is eventually deposited to

the biosphere where it may affect receptor ecosystems. Dry deposition

fluxes of gaseous and particulate nitrogen were measured in the

Phoenix, Arizona, metropolitan area in order to examine patterns of

nitrogen deposition at urban-influenced Sonoran desert sites.

Micrometeorological measurements were made at one Sonoran desert site

to estimate deposition velocities from meteorological data collected

at each sites. Concentrations of N-containing species were measured

periodically in January, May, and September at three Sonoran desert

sites located upwind, within, and downwind of the Phoenix urban core.

Gas phase nitric acid and ammonia were monitored using denuder

samplers. Nitrate and ammonium associated with fine and coarse

particles were monitored using filter samplers. Modeled deposition

velocities and measured concentrations were then used to infer dry

deposition fluxes. Characteristic inferred nitrogen deposition fluxes

were 0.92, 2.28, and 1.47 kg/(ha y) at the upwind, core, and downwind

sites, respectively. The main contributors to nitrogen flux were

nitric acid and ammonia. Total deposition was an order of magnitude

lower than previous estimates based on air quality modeling of high

pollution episodes.