We performed airborne measurements of sulfur dioxide (SO2), carbon monoxide (CO), and aerosol chemical/microphysical properties over the Aegean Sea. The data were collected on board the Met Office C-130 research aircraft in August 1998. High SO2 mixing ratios up to 18 ppb and aerosol sulfate levels up to 500 ppt were measured near Thessaloniki in northern Greece. The highest concentrations were observed at altitudes between 1 and 2 km, while near the surface much lower SO2 mixing ratios between 1 and 4 ppb were found. The pollution was transported southward over the Aegean Sea as far south as Crete, where SO2 mixing ratios up to 5 ppb were observed. The combined results of the SO2, aerosol sulfate, and particle size distribution measurements indicate air masses containing emissions with different ages, which range from a few hours to several days. Fossil fuel pollution observed over northern Greece is transported from eastern Europe (Bulgaria, Romania, and Turkey). Additionally, our measurements indicate significant contributions from forest fires, in particular from haze layers that originated ten days earlier from fires over the Northwestern Canadian territories, and had crossed the Atlantic and passed over Europe. CO layers 1 to 2 km deep were observed above 1 km. Often a double-layer structure with a secondary CO layer above 3.5 km was also observed. We estimate that the contribution of biomass burning to the CO measured over Greece is fourfold that of fossil fuel. The lower layer had a significant number of particles in the accumulation mode (Np, size range 0.1--1 ¿m, ratio Np/CO of the order of 2--6 cm-3 (STP) ppb-1), while the upper layers were particle-depleted by convection and wet scavenging. The Aitken particle mode (size range 5--100 nm) was depleted both in the lower and in the upper layers. This suggests that the Aitken particles originally present in the lower layers had already grown to the accumulation mode size when detected.