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The second Arctic Gas and Aerosol Sampling Program (AGASP-II) was conducted across the Alaskan and Canadian Arctic in April 1986, to study the in situ aerosol, and the chemical and optical properties of Arctic haze. The NOAA WP-3D aircraft, with special instrumentation added, made six flights during AGASP-II. Measurements of wind, pressure, temperature, ozone, water vapor, condensation nuclei (CN) concentration, and aerosol scattering extinction (b~,) were used to determine the location of significant haze layers. The measurements rhade on the ftrst three flights, over the Arctic Ocean north of Barrow and over the Beaufort Sea north of Barter Island, Alaska are discussed in detail in this report of the first phase of AGASP II. In the Alaskan Arctic the WP-3D detected a large and persistent region of haze between 960 and 750 mb, in a thermally stable layer, on 2, 8, and 9 April 1986. At its most dense, the haze contained CN concentrations >10,000 cm -3 and bsp of 80 × 10 -6 m -1 suggesting active SO2 to H2SO4 gas-to-particle conversion. Calculations based upon observed SO2 concentrations and ambient relative humidities suggest that 104-105 small H2SO4 droplets 18 G.A. HERBERT ET AL.could have been produced in the haze layers. High concentrations of sub-micron H2SO 4 droplets were collected in haze. Ozone concentrations were 5-10 ppb higher in the haze layers than in the surrounding troposphere.Outside the regions of haze, CN concentrations ranged from 100 to 400 cm -3 and bsp values were about (20-40) × 10 -6 m-1. Air mass trajectories were computed to depict the air flow upwind of regions in which haze was observed. In two cases the back trajectories and ground measurements suggested the source to be in central Europe.