Secondary Organic Aerosol
Secondary organic aerosols represent a major component of atmospheric particulates, affecting the global radiation budget through the direct and indirect effects. However, our capability to accurately predict these is currently hampered by relative scarcity of the detailed measurements needed in many environments and a lack of mechanistic understanding of the formation processes. To address these scientific needs, a programme of coordinated atmospheric measurements, laboratory experiments and model development is being performed by NCAS staff at the University of Manchester.
Organic aerosols can form in a variety of different scenarios, with different environments around the world presenting different conditions (temperature, humidity, sunlight), precursors (biogenic and anthropogenic VOCs) and oxidants (ozone and radicals). In order to provide a more comprehensive characterisation and help constrain the many variables within the theoretical framework, measurements of organic aerosols are performed in many different locations around the world and at different times of the year in conjunction with major field campaigns. This has included biogenically-dominated regions such as remote forests ranging from boreal to tropical. It has also included polluted regions, such an inner cities and regions downwind, marine environments (ships and coastal measurements) and sites which could be expected to receive influences from a number of sources. The principle measurement technique used is the AMS, which provides a measurement of total organic matter and some information on functionality. However this is frequently performed in conjunction with other on- and offline techniques by other groups, for example HNMR and FTIR spectroscopy and gas- and liquid chromatography. These data are systematically compared across measurement techniques and sites to identify and characterise specific sources and processes. They are also reconciled with the model and chamber data to assess model performance and guide future work.
It has long been recognised that photochemical “smog”, or aerosol, chambers can provide valuable insights into the complex multiphase processes leading to the formation and transformation of secondary organic aerosols, which cannot be readily derived from more conventional laboratory studies. The Manchester aerosol chamber is currently the only facility in the UK where photochemical formation and transformation of aerosols is, and can be, studied under atmospherically realistic conditions. These chamber studies are central to a broader programme of chemical and physical aerosol property investigations, directly providing data and understanding to detailed aerosol models. The chamber studies are performed using i) selected individual precursors representative of specific environments ii) synthetic mixtures of precursors and iii) real emissions from selected sources (e.g. plants). The chamber programme ranges from formation and transformation of secondary organic aerosols (SOA) under seeded and un-seeded conditions.
Improving our ability to fully understand and predict the formation and impacts of secondary organic aerosol requires development of appropriate modelling tools. Accounting for composition dependent phenomena allows development of mechanistic understanding of process level phenomena while aiding development of meaningful routes to reduce chemical complexity within large-scale models. Tools at either end of this spectrum necessarily have vastly different levels of chemical and numerical complexity: from explicit consideration of intermolecular interactions to lumped representations of organic compounds in the atmosphere. At CAS, a holistic approach is taken such that models on both scales are used in conjunction with both field and laboratory studies (chamber and ambient). Targeted laboratory studies on fundamental properties of organic compounds enable improved representations of SOA across all model scales. The chemical composition of SOA is immensely complex, which renders manual calculation of aerosol properties near impossible. Informatic tools developed at Manchester negate this, allowing investigation of systems with any level of organic complexity. Convolving subsequent predictions of SOA loadings with appropriate instrument response functions allow direct comparisons between models and measurements. These results are used to directly inform future developments on state of the art models along with providing further guidance on experimental programmes.