A secondary organic aerosol (SOA) is a molecule produced via oxidation over several generations of a parent organic molecule. [1] In contrast to primary organic aerosols, which are emitted directly from the biosphere, SOAs are either formed via homogeneous nucleation through the successive oxidation of gas-phase organic compounds, or through condensation on pre-existing particles. [2] These gas-phase species exert high vapor pressures, meaning they are volatile and stable in the gas-phase.
Upon oxidation, the increased polarity, and thus reduced volatility, 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 (the particle phase of SOA). SOAs represent a significant proportion of aerosols contained in the troposphere. [1]
Biogenic volatile organic compounds (BVOCs) emitted by plants serve as a significant SOA precursor. BVOCs are oxidized by the hydroxyl radical (OH) during the day or nitrate (NO3) at night. Anthropogenically emitted volatile organic compounds (VOCs), such as aromatics from fossil fuel combustion, can also be oxidized by such regimes. Oxidation products that have sufficiently low vapor pressures allow for the condensation of these oxidation products onto aerosols. [2] These SOA particles have complex mixing states, meaning that often one SOA particle formed from a single precursor can consist of hundreds of different compounds. [2] Dimethylsulfide (DMS) emitted by marine phytoplankton represent another significant SOA source. The oxidation of DMS produces SO2, and subsequently, sulfuric acid, which then either condenses onto particles already present in the atmosphere or forms new secondary particles. [2]
