Knowledge Bank

With increasing concern about the environment and the effects of pollution there has developed a growing need for methods of studying the effects of human activities on the atmosphere, and in particular for means to study atmospheric effects in situ. In particular, one of the most important problem has been the determination of the properties of particulate species or aerosols in the atmosphere. We describe in situ absorption measurements of airborne submicron sized aqueous ammonium sulfate aerosol droplets with superior sensitivity ($\ge 10-8cm-1$) by employing a $CO2$ laser to photothermally modulate Mie scattered light. This modulated scattering signal is proportional to the particle infrared absorption strength and consequently, use of a tunable infrared laser (such as $CO2$) allows accurate relative absorption spectra to be obtained directly from the data without the necessity of complex inverse scattering calculations. Measurements based on this technique are found to be independent of the background gas absorption and we have achieved a sulfate detection limit of $\sim 5\mu g/m3$, sufficient for ambient sulfate analysis. In addition, we have visually observed the photothermally modulated light scattering from a single submicron droplet which is in fact the detection limit of this technique. This technique, like conventional Mie scattering, is sensitive, in situ, and convenient. In addition, photothermal modulation allows highly sensitive particle absorption spectroscopy to be performed. We will discuss the essential physics and the important applications of this technique. We will also present experimental results on ammonium sulfate aerosols in situ and compare with complimentary ``PFLOH”$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ spectroscopy data taken in our laboratory.