A secondary organic aerosol (SOA) is a molecule produced via oxidation over several generations of a parent organic molecule. 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.