The Chemistry of Organic Aerosols in the Atmosphere
Introduction
Organic aerosols are a significant component of the atmospheric particulate matter. They are complex mixtures of thousands of organic compounds, emitted directly into the atmosphere or formed in-situ by the oxidation of volatile organic compounds (VOCs). The chemistry of organic aerosols in the atmosphere is a complex and dynamic field, involving a variety of chemical reactions and physical processes.
Formation of Organic Aerosols
Organic aerosols can be formed through both primary and secondary processes. Primary organic aerosols (POA) are emitted directly into the atmosphere from various sources, including biomass burning, fossil fuel combustion, and biogenic emissions. Secondary organic aerosols (SOA) are formed in the atmosphere through the oxidation of VOCs, which can be emitted from both anthropogenic and biogenic sources.
Primary Organic Aerosols
POA are typically composed of complex mixtures of organic compounds, including hydrocarbons, fatty acids, and polycyclic aromatic hydrocarbons (PAHs). The exact composition of POA can vary widely depending on the source and the conditions of emission. For example, biomass burning can produce a wide range of organic compounds, including levoglucosan, a tracer for wood burning, and various PAHs.
Secondary Organic Aerosols
SOA formation is a complex process that involves the oxidation of VOCs, followed by the condensation of the oxidation products onto existing aerosol particles. The oxidation of VOCs can occur through various pathways, including reaction with oxidants such as the hydroxyl radical (OH), ozone (O3), and the nitrate radical (NO3). The resulting oxidation products can then undergo further reactions, including functionalization, fragmentation, and oligomerization, leading to the formation of a complex mixture of organic compounds.
Chemical Composition of Organic Aerosols
The chemical composition of organic aerosols is highly complex and can vary widely depending on the source of the aerosols and the atmospheric conditions. Organic aerosols can contain a wide range of organic compounds, including hydrocarbons, alcohols, carboxylic acids, and more complex compounds such as oligomers and organosulfates.
Hydrocarbons
Hydrocarbons are a major component of organic aerosols, particularly those formed from anthropogenic sources such as fossil fuel combustion. These can include both aliphatic and aromatic hydrocarbons, with the latter being particularly important due to their potential to form SOA through photochemical reactions.
Alcohols and Carboxylic Acids
Alcohols and carboxylic acids are common components of organic aerosols, particularly those formed from biogenic sources. These compounds can be formed through the oxidation of VOCs, and can contribute to the water-soluble fraction of the aerosols, influencing their hygroscopic properties and their ability to act as cloud condensation nuclei.
Oligomers and Organosulfates
Oligomers and organosulfates are more complex compounds that can be formed in organic aerosols through various chemical reactions. Oligomers are formed through the reaction of smaller organic compounds, while organosulfates can be formed through the reaction of organic compounds with sulfate ions. These compounds can contribute to the overall mass of the aerosols and can influence their physical and chemical properties.
Atmospheric Processes Involving Organic Aerosols
Organic aerosols can undergo a variety of atmospheric processes, including chemical reactions, physical transformations, and interactions with other atmospheric components. These processes can influence the properties of the aerosols, their distribution in the atmosphere, and their impacts on climate and air quality.
Chemical Reactions
Organic aerosols can undergo a variety of chemical reactions in the atmosphere, including oxidation, functionalization, and fragmentation. These reactions can alter the chemical composition of the aerosols, leading to changes in their physical properties and their ability to interact with other atmospheric components.
Physical Transformations
Physical transformations of organic aerosols can include processes such as condensation, evaporation, and coagulation. These processes can influence the size distribution of the aerosols, their phase state, and their ability to act as cloud condensation nuclei.
Interactions with Other Atmospheric Components
Organic aerosols can interact with other atmospheric components, including inorganic aerosols, water vapor, and radiation. These interactions can influence the properties of the aerosols and their impacts on climate and air quality. For example, the interaction of organic aerosols with inorganic aerosols can lead to the formation of mixed-phase particles, which can have different properties than the individual components.
Impacts of Organic Aerosols
Organic aerosols can have significant impacts on climate and air quality. They can influence the radiative balance of the Earth by scattering and absorbing solar and terrestrial radiation, and they can act as cloud condensation nuclei, influencing cloud formation and precipitation processes. In addition, organic aerosols can have adverse effects on air quality and human health, due to their ability to carry toxic compounds and to penetrate deep into the respiratory system.