Volatility primary organic aerosol

Of a total flux of non-methane reduced organic compounds into the atmosphere of about 1,350 Tg year [35, 36], rally 10% or so leads to organic aerosol [25, 37], However, less than 1 % of the primary organic emissions into the atmosphere have a sufficiently low volatility to remain in the condensed phase under ambient conditions, so SOA formation must be a huge part (90% or more) of the OA story [38]. The straightforward fact is that only a small fraction of all organic compoimds (by mass) in the atmosphere have what it takes to stay on or in a particle. That special property is low volatility, and most compounds acquire that low volatility via chemical transformation in the atmosphere. 

Figure 21.25. shows typical variations with time of primary and secondary pollutants. Initially, the concentration of NO2 is quite low. As soon as solar radiation penetrates the atmosphere, though, more NO2 is formed from NO and O2. The concentration of ozone remains fairly constant at a low level in the early morning hours. As the concentration of unbumed hydrocarbons and aldehydes increa.ses in the air, the concentrations of NO2 and O3 also rise rapidly. The actual amounts depend on the location, traffic, and weather conditions, but their presence is always accompanied by haze (Figure 21.26). The oxidation of hydrocarbons produces various organic intermediates, such as alcohols and carboxylic acids, which are all less volatile than the hydrocarbons themselves, These sub.stances eventually condense into small droplets of liquid. The dispersion of these droplets in air, called an aerosol, scatters sunlight and reduces visibility. This interaction also makes the air look hazy.

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