Aerosol Chemistry and Physics
Atmospheric aerosols have a significant influence on human health, chemical component deposition, cloud albedo, cloud condensation (CCN) and atmospheric turbidity. In addition to these effects aerosols play an important role in many chemical processes occurring in the atmosphere. Furthermore, the protuberant role of atmospheric aerosol particles in the radiative transfer within the atmosphere by scattering and absorbing electromagnetic radiation makes them an important parameter in modelling the Earth’s climate.
Since chemical and radiative effects of atmospheric aerosols are size and phase related, they are strongly influenced by the ambient relative humidity (RH) due to water absorbing hygroscopic components, changing both particle diameter and wavelength dependent refractive indices. Therefore, the net effect on chemistry and/or climate for a given atmospheric particle load will depend on the relative humidity and will hence be modified by either temperature or water partial pressure.
Phase and water content of particles govern their mass and hence their surface area and reactivity. The hygroscopic properties and phase changes of atmospheric aerosols as well as the surface morphology (of solid particles) and chemical surface composition must be understood and represented accurately in order to improve aerosol-climate models.
Sulphate aerosols are widely abundant in the atmosphere and represent the largest anthropogenic mass source for the accumulation mode of atmospheric particles. Being non-absorbing in the visible region of the electromagnetic spectrum, they provide the most significant anthropogenic cooling contribution to the global direct radiative forcing. Depending on location, aerosols can contain various ratios of inorganic to organic material. Results from field measurements indicate that organic material typically accounts for 10-50% of the fine particle mass, with the organic material originating from both anthropogenic and natural sources. Recent field data confirm these findings indicating that indeed up to 50% or more organic material may be present in atmospheric aerosols.
Dicarboxylic acids are amongst the chemical compounds found in atmospheric aerosol particles. Like many other polar organic substances they are predominantly present in condensed phases rather than in the gas phase, resulting from their low vapour pressures. In aqueous aerosol particles, dicarboxylic acids are found to be major constituents of the water soluble organic compounds (WSOCs). Composition measurements have shown that the organic material is internally mixed together with inorganic compounds in tropospheric particles. Dicarboxylic acids are found in many different environments including aerosols from the urban, marine, polar and tropical atmosphere. The sources of organic aerosols include fossil-fuel combustion and biomass burning.
Whilst the behaviour towards varying relative humidity, as normally described by deliquescence and efflorescence, of pure ammonium sulphate (AS) particles is well established, information about phase transitions and hygroscopic properties of organic and mixed organic/inorganic particles is not at the same level. Studies of pure organic systems have shown that their deliquescence behaviour strongly depends on the chemical nature of the organic substance. A number of groups have also studied the deliquescence and crystallisation of mixed organic/inorganic particles. Studies with such internally mixed organic / AS particles have shown that the organic component changes the deliquescence relative humidity (DRH) relative to pure ammonium sulphate. The water content of atmospheric aerosols is governed by the equilibrium with the ambient relative humidity. A low ambient relative humidities leads to a low water content of the aerosol. Hence, high aqueous phase concentrations can be attained at low relative humidities. These result in large deviations from ideal solution behaviour and make the properties of the system difficult to predict.