Chemistry and Microphysics of Atmospheric Aerosol Surfaces: Laboratory Techniques and Applications

Probing the Equilibrium Size and Hydrogen Bonding Structure in Aqueous Aerosol Droplets

Comparative measurements on two or more optically trapped aerosol droplets can allow the hygroscopic properties of aerosol to be investigated in exquisite detail. In this paper we consider two methods for assessing the dependence on relative humidity of the water activity and solute concentration in the condensed phase. We demonstrate in this paper that using a control droplet as a microprobe of relative humidity in the vicinity of a second droplet of interest can allow the RH to be determined with an accuracy of between 0.01 to 0.1 %. Not only is such a probe highly responsive, but the accuracy of the RH determination improves with increase in GF or RH, counter to the performance of macroscopic RH probes. We also demonstrate that spontaneous Raman scattering recorded from excitation of the OH stretching vibration in supersaturated solution droplets of sodium chloride shows the considerable perturbation induced by the chloride ion on the hydrogen bonding network. Chloride is recognised as a hydrogen bonding structure breaker and these observations are shown to be consistent with measurements on more dilute solutions.

Time Resolved Infrared Spectroscopy of Formation and Processing of Secondary Organic Aerosol

An aerosol flow reactor was coupled to an infrared absorption cell to study aerosol formation processes with high temporal resolution. The recorded infrared spectra were referenced using aerosol smog chamber experiments. Evaluation was done by studying the formation of secondary organic aerosol from α-pinene and catechol as precursors and ozone as oxidant. Three main infrared absorptions: ν(O-H), ν(C-H) and ν(C=O) were considered, and humic like properties of the secondary organic aerosol are mainly interpreted according to the formation and variations of carbonyl bands in the region between 1850 and 1600 cm−1, especially the ν(C=O) of aryl carbonyls from catechol oxidation products below 1700 cm−1. The relative intensities of two major ν(C=O) stretching vibrations at 1690 cm−1 and 1755 cm−1 were observed to depend strongly on the available ozone concentration. At high precursor/ozone ratios (2:1 or 1:1) the vibration at 1690 cm−1 predominates, indicating aryl carbonyl vibrations. With increasing ozone concentrations this vibration is replaced by the higher carbonyl vibration at 1755 cm−1 indicating unsaturated carbonyl-containing compounds. This is a strong hint at ring opening processes leading to unsaturated aliphatic compounds in the resulting particle. Aryl carbonyls and aromatic or olefinic ν(C=C) at 1620 cm−1 in aged particles remain visible, as aerosol smog chamber studies exhibit – thus a strong hint at humic like properties of the SOA from the spectroscopic point of view.