Estimation of the particle and gas scavenging contributions to wet deposition of organic nitrogen

We estimated the wet deposition flux of dissolved organic nitrogen (DON) to Tampa Bay, Florida, using 24-h integrated aerosol and rainwater samples collected simultaneously on days with rainfall between July and September 2005. In rainwater, dissolved inorganic nitrogen (DIN) and DON average concentrations were 54.7±44.0 and 4.7±2.7 μM-N, respectively, and DON represented 8.9±5.8% of the total dissolved nitrogen (TDN=DIN+DON). Our estimates of wet deposition fluxes for NH4+, NO3- and DON were 1.40, 3.18 and 0.34 kg-N ha−1 yr−1, respectively. In aerosols (PM10), DIN and DON concentrations were 78.5±56.2 and 6.3±2.6 nmol m−3, respectively, and DON represented 10.3±7.3% of TDN. Particle scavenging rates were calculated assuming a lognormal size distribution for particles and lognormal, gamma and Marshall-Palmer size distributions for raindrops. For the range of precipitation rates and measured aerosol concentrations, below-cloud scavenging of aerosol-phase DON contributed only 1±0.7% to rainwater N concentrations. Dimethylamine (DMA) was observed in aqueous extracts of fine (PM2.5) and coarse (PM10−2.5) aerosol samples, but could be quantitatively measured only in fine particles with an average concentration of 688±615 pmol-N m−3, representing an average contribution of 12.8±6.7% of the total DON concentration measured in the same particle fraction. AIM 2 vapor-liquid equilibrium modeling predicted an average gas-phase DMA concentration of 107.4±176.9 pmol-N m−3. Although DMA concentrations were below our analytical detection limit on all rainwater samples, the average modeled gas and particle DMA contribution to DON concentration in rainwater was 0.4±0.7%.