Despite their minuscule size, they have strong influence on our everyday lives. In high enough concentrations, they affect visibility, a phenomenon clearly visible in today’s megacities. They can also be detrimental to human health. Most importantly, they affect the climate. Dark, soot containing particles absorb solar radiation and heat the atmosphere. In contrast, light coloured particles can scatter incoming sunlight, cooling the Earth. Possibly more importantly, aerosol particles enable the formation of clouds in the atmosphere. Clouds play a crucial role in Earth’s water cycle, and act to cool the climate. Increasing particle concentrations lead to more reflective and longer lived clouds, enhancing the cooling effect. As aerosol concentrations in the atmosphere have increased due to human activity, the resulting cooling effect has masked a part of the warming caused by greenhouse gas emissions. Aerosols remain one of the least understood parts of the climate system, and are the largest single source of uncertainty in projections of future climate.
Aerosols can end up in the atmosphere either directly as particles, known as primary particles, or be formed from low volatility vapours in the air. The latter are known as secondary particles. A large part of the atmospheric aerosol consists of so called secondary organic aerosol (SOA). This is organic material that has originally been emitted into the atmosphere as gases known as volatile organic compounds (VOCs). Globally, the majority of the VOCs originate from plants, but in cities anthropogenic sources like traffic can dominate. Once in the atmosphere, VOCs can get oxidized. Many of the reaction products are of lower volatility compared to the precursor VOCs. Products with low enough volatility can either contribute to the formation of new aerosol particles, or grow existing ones bigger.
Different VOCs form SOA with different efficiencies. For example monoterpenes, the largest group of VOCs emitted by boreal forests, are known as potent SOA precursors. In addition to the type of VOCs
The oxidation process itself is sensitive to environmental conditions: oxidation by different oxidants leads to products with differing properties, such as their volatility. Temperature and the presence of certain anthropogenic emissions, such as nitrogen oxides, also affect the process.