Factors controlling sea salt modification and dry deposition of nonsea-salt components to the ocean

摘要

Modification of sea salt aerosol particles (SSA particles) by HNO3 and SO2 is an important process for changing phase partitioning of acidic gases from industrial regions to the ocean. During 12–29 September 2005, size-segregated aerosol particles and acidic gases were sampled around the western part of the Japanese Islands to elucidate controlling factors of modification of SSA particles by acidic gases and to estimate dry deposition flux over the ocean. For coarse (>2 μm diameter) SSA particles, the amount of Cl deficiency from the seawater ratio was comparable to the sum of the equivalent concentrations of NO3 and nonsea-salt (nss) SO4 2, suggesting Cl displacement of SSA particles by acidic gases such as HNO3 and SO2. The Cldeficiency of SSA particles varied according to the size range and wind speed. Decreasing modification occurred with increasing wind speed for particles of 2–8 μm. Under high (low) wind conditions, the NO3 concentration per unit surface area of coarse SSA particles was lower (higher) for particles >8 μm than for those of 2–8 μm diameter. The respective dry deposition fluxes (F dry) of NO3, nss-SO4 2, HNO3, and SO2 were estimated according to the wind speed and size of aerosol particles. On average, the F dry of particulate NO3 was 10 times larger than that of HNO3, but F dry of nss-SO4 2 was almost equal to that of SO2. Phase partitioning of dry deposition for NO3 and nss-SO4 2 over the ocean differs from that of coastal marine areas.