The impact of sea-salt aerosols on particulate inorganic nitrogen deposition in the western Taiwan Strait region, China

Abstract

Sea-salt aerosol (SSA) has significant impact on the formation of secondary nitrate and its dry deposition in the coastal region. In this study, size-segregated aerosols were collected for water-soluble ions analysis in the coastal cities and sea cruises in the western Taiwan Strait of China from 2016 to 2017 to investigate the impact of SSA on particulate inorganic N deposition. The size distributions of NH4+ were characterized by a unimodal pattern peaking at 0.44–1.0 μm while NO3− exhibited typical bimodal distributions peaking at 0.44–1.0 μm and 2.5–10 μm. Furthermore, the fine mode peaks of NO3− became insignificant or disappeared in summer and fall due to the volatilization and dissociation of NH4NO3 at higher temperatures. Elevated sea-salt contributions in the largest size bin (>16 μm) during sea cruises were attributed to vessel induced wave breaking, while there was no peak for NO3− at this range due to its short atmospheric lifetime. Although the concentrations of particulate NH4+-N showed higher levels than those of NO3−-N for both coastal and marine aerosols, the dry deposition fluxes of the particulate NO3−-N were significant higher than those of the particulate NH4+-N due to their different size distributions. The contribution of SSA to particulate inorganic N deposition was estimated on average to be 20.33% and 36.91% in the coastal and offshore region, respectively, assuming all of the chlorine depletion was caused by the reaction between nitric acid and SSA in the coarse mode. In addition, dry deposition fluxes using constant deposition velocities for fine/coarse particles and NH4+/NO3− were higher than those using size-dependent dry deposition velocities. Considering the increasing human activities at the coastal urban zone in the western Taiwan Strait, the atmospheric N deposition could have an important ecological impact on the marine environment that is already under pressure from riverine inputs and urban waste discharges.