Formation of atmospheric aerosol particles by chemical reaction and condensation

2 Formation of atmospheric aerosol particles by chemical reaction and condensation 

Particles formed by the dispersal of surface materials generally have radii larger than about 0.1 /im. This means that Aitken-size particles must be produced by another mechanism, namely by condensation of vapours, preceded in many cases by gaseous chemical reactions. These reactions are generally initiated by photochemical processes. 

a large set of data obtained by McWilliams (1969) in clean air (W. Ireland) by means of expansion chambers showed that the concentration of Aitken particles is lower in the winter than in the summertime. Furthermore, McWilliams observations also demonstrated that more aerosol particles can be detected during daylight than at night. This finding was confirmed by the investigations of Vohra et 

The formation of aerosol particles from gaseous components is appropriately investigated under laboratory conditions. In so-called aerosol chambers an artificial atmosphere is created to which small quantities of appropriate trace gases (e.g. SO, NO, H,0, NH, and organics) is added. It is also possible to use ambient air purified from particulate matter. The chamber may be illuminated to initiate photochemical processes, and the behaviour of particles formed is studied by the methods outlined in Subsection 4.1.2, e.g. by electrical mobility analyzers (Whitby et aL 1972). 

An important result from aerosol chamber studies was the discovery of the indirect photochemical process. Thus, Bricard et al. (1968) found that intense aerosol particle production can be observed in the chamber in the dark if ambient filtered air is sampled from a sunlit atmosphere. It is speculated that in the atmosphere some gaseous substance is excited by sunlight and is not collected by the filter used to obtain air which is free of aerosol particles. In the chamber these photochemically excited molecules initiate secondary thermal reactions leading to the formation of some supersaturated vapour (e.g. H,S04) which subsequently condenses (see also Subsection 3.6.3). 

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