Atmospheric deposition to oak forests along an urban-rural gradient

To determine the patterns of atmospheric deposition and throughfall in the vicinity of a large city, bulk deposition, oak forest throughfall, and particulate dust deposition were measured at sites along a transect within and to the north of New York City. Concentrations and fluxes of NO3-, MH4+, Ca2+, Mg2+, SO42-, and Cl- in throughfall all declined significantly with distance from the city, while hydrogen ion concentration and flux increased with distance from the city. Most of the change in concentrations and fluxes occurred within 45 km of the city. Throughfall deposition of inorganic N was twice as high in the urban sites as compared to the suburban and rural sites. Bulk deposition patterns were similar to those of throughfall, but changes along the transect were much less pronounced. The water-extractable component of dust deposition to Petri plates also was substantially higher in the urban sites for Ca2+,Mg2+, SO42-, NO3-, and Cl-. The dust particles had little alkalinity, suggesting that alkaline aerosols were neutralized by acidic gases in the atmosphere. We propose that dust emissions from New York City act like an "urban scrubber", removing acidic gases from the atmosphere and depositing them on the city as coarse particle dry deposition. Despite the urban scrubber effect, most of the dry deposition of nitrate was from gaseous nitrogen oxides, which were in much higher concentration in the city than in rural sites. Excess deposition of nutrients and pollutants could be important for the nutrient budgets of forests in and near urban areas.