Composition of atmospheric aerosol

See also: Ultrafine Organic TDMA and Aerosol Mass Spectrometry.

• Ari Laaksonen, Ph.D., Professor
• Jorma Joutsensaari, Dr.Tech, Researcher
• Pasi Miettinen, M.Sc, Researcher
• Petri Tiitta, M.Sc, Researcher
• Petri Vaattovaara, M.Sc, Researcher

Background

Atmospheric aerosols can act as the cloud condensation nuclei for cloud droplet formation, they can efficiently scatter incoming solar radiation and modify cloud radiative properties, and participate in heterogeneous chemical reactions. Hence, they have an important role in atmospheric chemistry and they affect the radiation balance of the Earth both directly and indirectly. The chemical composition of aerosol particles is, however, difficult to determine because of very small mass of the particles, especially for the ultrafine particles.

The atmospheric aerosols contain both inorganic, e.g. ammonium sulphate and sodium chloride, and organic matter, e.g. PAH and organic acids. Frequently organic matter dominates the composition of submicron atmospheric aerosol particles, especially over continental region. Obviously, organic particulate matter can also influence the radiation balance of the Earth.

In this study, we are developing a novel tandem differential mobility analyzer (TDMA) method for measuring the organic composition of fine aerosol particles. This work is a part of the project "Origin and Formation of Secondary Organic Aerosol" funded by EU 5th Frame Programme.

Organic TDMA Method

The tandem differential mobility analyzer method (TDMA) has been used to characterize the composition and the properties of atmospheric aerosol particles, e.g., hygroscopic properties as a function of relative humidity and volatility of the particles as function on temperature. In the TDMA system, the particles are size-classified by a DMA and the second DMA is used to study a change of the particle size. TDMA provides an indirect method to determine the composition of fine particles that are hard to analyze with conventional methods. We are developing a novel TDMA method for measuring the organic composition of fine aerosol particles. The instrument is named organic tandem differential mobility analyzer (OTDMA).

Schematic presentation of OTDMA system setup.

The OTDMA is based on the same system as the hycroscopicity TDMA with the difference that the size-classified aerosol is carried through organic solvent vapor, for example ethanol, instead of water vapor. The first DMA (DMA-1) is used to classify the particles based on their electrical mobility. Then they are passed through the ethanol vapor. During a treatment of the ethanol vapor, the size of the particles containing ethanol-soluble substances changes (mostly organic matter) whereas the size of the particles containing no ethanol-soluble substances is stable (mostly inorganic matter). Similarly, the hygroscopic particles grow during a treatment of the water vapor. The change of particle size is determined with the second DMA (DMA-2) by measuring the particle size distribution. Results can be used to determine the organic content of the aerosol particles. Organic (e.g. pinonic acid) and inorganic (e.g. sodium chloride) particles as well as their mixtures will be studied in a laboratory scale. In addition, the method will be used to study origin and chemical character of rural and urban aerosols.