A SEASONAL CYCLE IN THE SOUTHWEST PACIFIC FREE TROPOSPHERIC AEROSOL CONCENTRATION

Measurements made of the concentration of aerosol particles in the dia meter range 0.15 - 3mum in the free troposphere over the southwest Pac ific show evidence of a seasonal cycle. The maximum concentration occu rs in spring and the minimum in autumn. The amplitude of the cycle is greatest at 30-degrees-S where, in early spring, the mean aerosol conc entration is 18 cm-3, more than 10 times the mean autumn value. In the vicinity of the equator die seasonality disappears and concentrations throughout die year apparently remain close to the autumn levels at o ther latitudes. A number of different mechanisms could account for the observed seasonality but it is probable that die peak in early spring is due to biomass burning through the tropical southern hemisphere dr y season. Continued high values through late spring and early summer a re thought to be the result of die seasonal production of aerosol prec ursors, such as dimethyl sulfide (DMS), at middle and high latitudes.