The Influence of the Nocturnal Boundary Layer on Secondary Trace Species in the Atmosphere at Dorset, Ontario

The impact of the nocturnal boundary layer (NBL) on the concentrations of O3, H2O2, PAN and CH2O has been studied for two 4-day periods, one in the summer of 1989 and the other in the spring of 1990. Where the presence of the nocturnal layer is clear, O3, PAN and CH2O concentrations decrease rapidly and these species appear to be deposited to the surface. Break-up of the inversion in the morning returns concentrations to levels typical of the previous day. H2O2 is removed much faster than the other species and it is replenished much more slowly. This appears to be due to the H2O2 dissolving in water droplets resulting from the rapid cooling of the air. There is evidence for a morning maximum in PAN, likely due to the combination of high concentrations being brought downwards during the break-up of the NBL followed by thermal decomposition.