From HandWiki - Reading time: 2 min
Secondary Organic Aerosol or SOA are molecules produced via multigeneration oxidation of a parent organic molecule.[1] In contrast to primary organic aerosols, which are emitted directly from the biosphere, secondary organic aerosols are formed via homogeneous nucleation through the successive oxidation of gas-phase organic compounds. These gas-phase species exert high vapor pressures, meaning they are volatile and stable in the gas-phase, however, upon oxidation, the increased polarity of the molecules results in a reduction of vapor pressure. After sufficient oxidation, the vapor pressure is sufficiently low that the gas-phase compound partitions into the solid-phase, producing secondary organic matter.
A common misconception is that the aerosol refers to the solid phase of the compound, where in reality, by definition, it is the combination of the gas- and solid-phases which constitute the aerosol.