Abstract-The impact ofthe nocturnal boundary layer (NBL) on the concentrations of03' H 2 0 2 , PAN and CH20 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, 03' PAN and CH 2 0 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 ofthe previous day. H 2 0 2 is removed much faster than the other species and it is replenished much more slowly. This appears to be due to the H 2 0 2 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.
Key word index:Ozone, hydrogen peroxide, formaldehyde, PAN, nocturnal boundary layer, deposition, nighttime chemistry.
INTRODUCfIONField measurement studies are an important part of the atmospheric chemists' arsenal. They are performed to give data to assist in understanding specific questions in atmospheric chemistry, and for the development and evaluation of photochemical models.For convenience, most field measurements are made at the surface. However, the presence of the ground, and other local effects, can often significantly impact the observations, masking the larger scale chemical features one wishes to study. One such factor is the occurrence of a strong nocturnal boundary layer (NBL). By isolating the air being sampled from the bulk of the air aloft, for several hours each night, it often plays the principal role in determining the diurnal variation of the surface concentrations of a number of the more reactive species.In this paper we look in detail at the measurements of a number of secondary, reactive, trace gases, made at a rural site in Ontario, to see the effect of the NBL on their concentrations. The gases under study were 0 3 , HzO z , CHzO and PAN. These species have no direct emission sources, with the exception of a small amount of CHzO from combustion, and originate from atmospheric oxidation chemistry. By concentrating on these species, it is possible to focus on the chemistry that is occurring, rather than on the emission or transport processes. Furthermore, using the NBL to isolate a relatively small section of the atmosphere accentuates the observable consequences of any of these processes. Although the concentrations of this suite of species would be expected to be related, they have unique atmospheric lifetimes and decay mechanisms, and therefore they constitute a diverse set of molecules with which to probe our understanding of the system.The behaviour of ozone under the NBL is well documented (e.g. Cox et al., 1975;Harrison et al., 1978;Oltmans, 1981; Kelly et al., 1984; Broder and Gygax, 1985;Broder et al., 1981; Fehsenfeld et al., 1983;Lifshitz et al., 1988;Shepson et al., 1992b). However, the previously reported diurnal behavi...