ABSTRACT
Mid-infrared extinction spectroscopy has been established as an important tool to derive microphysical properties such as size, shape, and phase of aerosols and individual aerosol particles and to monitor multiphase processes, both in laboratory measurements as well as in remote sensing applications.1,2 The extinction of an incident infrared beam is the sum of light absorption in the particles and light scattering by the particles. Absorption is the dominant contribution for particle sizes small compared to the wavelength of the incident light. At mid-infrared wavelengths, this holds for particle diameters below approximately 200 nm. In its absorption contribution, the infrared spectrum is susceptible to the distinctive bands of organic and inorganic functional groups inherent in molecularly structured aerosol particles and can thus be a powerful tool for chemical characterization. Recent examples include the analysis of the chemical evolution of secondary organic aerosol in a smog chamber and the unique discrimination between different types of polar stratospheric cloud particles in satellite infrared measurements.3,4 Moreover, infrared spectroscopy is ideally suited to investigate the deliquescent and efŸorescent behavior of aerosol particles, identifying the phase transition by the appearance and disappearance of the broad liquid water absorption band at around 3300 cm−1.5−9 Exploiting the different spectral habitus of the absorption bands of liquid water droplets and ice crystals, infrared measurements are
1.1 Introduction ..............................................................................................................................3 1.2 Theory .......................................................................................................................................5
