Secondary Organic Aerosol Chemistry

How the oxidation of volatile organic compounds produces low-volatility products that condense to form a major fraction of fine particulate matter.

Definition

Secondary organic aerosol is particulate matter formed in the atmosphere when the oxidation products of volatile organic compounds partition into the condensed phase.

Scope

Covers the oxidation of biogenic and anthropogenic volatile organic compounds, the generation of semivolatile and low-volatility products, gas-particle partitioning, the multigenerational aging of organic aerosol, the volatility basis set framework, and the contribution of secondary organic aerosol to global particulate burdens.

Core questions

• How does oxidation convert volatile organics into condensable products?
• What controls the partitioning of organic compounds between gas and particle phases?
• Why is secondary organic aerosol so abundant and chemically complex?

Key theories

Gas-particle partitioning of organics

Oxidation lowers the volatility of organic compounds, allowing them to partition into the particle phase in proportion to existing organic mass, with the volatility basis set describing the continuum of product volatilities.

Mechanisms

Volatile organic compounds emitted by vegetation and human activity are oxidized by the hydroxyl radical, ozone and the nitrate radical, producing a spectrum of products with reduced volatility. Semivolatile and low-volatility species condense onto existing particles according to their saturation concentrations, a process captured by the volatility basis set. Continued oxidation, or aging, of both gas- and particle-phase organics adds oxygen and further lowers volatility, so organic aerosol becomes more oxidized as air masses age.

Clinical relevance

Secondary organic aerosol makes up a large share of fine particulate matter affecting health and climate, and its complex chemistry is a major challenge for air-quality and climate models.

History

The importance of secondary organic aerosol grew clear through chamber experiments and field measurements from the 1990s onward, with the advent of aerosol mass spectrometry, summarized by Jimenez and colleagues and Hallquist and colleagues, transforming understanding of organic particle chemistry.

Key figures

• Mattias Hallquist
• Jose-Luis Jimenez

Seminal works

• hallquist2009
• jimenez2009

Frequently asked questions

How does secondary organic aerosol differ from primary aerosol?

Primary organic aerosol is emitted directly as particles, while secondary organic aerosol forms in the atmosphere when gaseous organic compounds are oxidized into low-volatility products that condense.