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.