Aircraft observations of aerosols in the free marine troposphere over the North Pacific Ocean: Particle chemistry in relation to air mass origin

Abstract

Aircraft measurements of atmospheric aerosol particles were performed in the free troposphere up to 5.0 km altitude over the North Pacific Ocean in August 1983 and 1984. Particles were analyzed for a single‐particle base with the regent thin‐film technique in conjunction with a transmission electron microscope. The calcium thin‐film method was used for the specific determination of the molecular form of sulfate particles. Nitrate and sulfate in particles was identified by the nitron and barium chloride thin‐film method, respectively. Ammonium was examined by the thin‐film method of sodium tetraphenyl boron. Collected aerosols were related to the origin of air parcels. Sulfuric acid particles in submicron size range formed by the photochemical oxidation of sulfur‐bearing gases and subsequent gas‐to‐particle conversion are the basic aerosols in the unpolluted free marine troposphere. They contain a small amount of NH4+ and more acidic than NH4HSO4. The chemical composition of the predominant aerosol changed drastically according to the origin of air parcels. Sulfuric acid aerosols were predominant in the air parcels originating from the midlatitude maritime atmosphere. On the other hand, for air parcels that have passed over the continent within several days, sulfate aerosols neutralized fully by ammonia are predominant. The extension of the continental air mass supplies ammoniated sulfate aerosols to remote marine atmosphere about 1200 km from land. Mineral dust particles are transported from Asian continent over the northwestern Pacific Ocean via the free troposphere. Nitrate was heterogeneously formed on the mineral dust particles and they were present in the air parcels passed over mainland of Japan, whereas few nitrate particles were found in the midlatitude marine atmosphere. Ammoniated sulfate particles, nitrate‐containing particles, and mineral dust particles are the tracers of the continental air mass.