Photochemical formation of organic aerosols: Growth laws and mechanisms

Smog chamber experiments were conducted to study aerosol formation by chemical reactions involving organics (o-cresol, nitrocresol, l-heptene, and methyl sulfide), oxides of nitrogen and ammonia in air. Aerosol size distributions were measured as a function of time with an electrical aerosol analyzer (0.01 μm < particle diameter < 0.56 μm) and with a single particle optical counter (0.48 μm < particle diameter < 3 μm). The aerosol data were analyzed with the 'growth law' technique to make inferences about chemical mechanisms of gas-to-particle conversion in these model chemical systems. Particle diameter growth rate measurements were made after the production of new panicles had stopped (i.e. while number concentrations were decreasing), but while aerosol mass concentrations were steadily increasing. The growth rates of 'small' particles (particle diameter < 0.02-0.1 μm) were typically negative, suggesting that these particles were evaporating. Growth rates of larger panicles tended to increase with particle size within the measured size range. This behavior is consistent with condensation, provided that the dependence of particle curvature on vapor pressure (the Kelvin effect) is taken into account. It is concluded that gas phase chemical reactions (rather than reactions on or within aerosol particles) are responsible for the formation of the aerosol. Saturation vapor concentrations for the condensable species that were formed in these systems were estimated to range from 0.04 to 2 ppb, depending on the chemical system.