A review of optical measurements at the aerosol and cloud chamber AIDA

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Abstract

This paper provides a survey of recent studies on the optical properties of aerosol and cloud particles that have been conducted at the AIDA facility of Forschungszentrum Karlsruhe (Aerosol Interactions and Dynamics in the Atmosphere). Reflecting the broad accessible temperature range of the AIDA chamber which extends from ambient temperature down to 183 K, the investigations feature a broad diversity of research topics, such as the wavelength-dependence of the specific absorption cross sections of soot and mineral dust aerosols at room temperature, depolarization and infrared extinction measurements of ice crystal clouds generated at temperatures below 235 K, and the optical properties of polar stratospheric cloud constituents whose formation was studied in chamber experiments at temperatures well below 200 K. After reviewing the AIDA research activity of the past decade and introducing the optical instrumentation of the AIDA facility, this paper presents illustrative examples of ongoing and already published work on optical measurements of soot aerosols, mineral dust particles, and ice crystal clouds.

Introduction

The AIDA aerosol and cloud chamber of Forschungszentrum Karlsruhe (Aerosol Interactions and Dynamics in the Atmosphere) has been established during the past decade as a unique experimental facility to study the optical properties of complex aerosol particles and to investigate the formation of cloud particles under realistic atmospheric conditions [1]. As the AIDA aerosol vessel can be operated over a broad temperature range from ambient down to 183 K, the research activity includes the formation and optical characterization of supercooled liquid water clouds, ice crystal clouds (cirrus), and polar stratospheric clouds (PSCs). Apart from its excellent temperature control, another unique feature of the AIDA chamber is its huge size of 84.3 m3, thereby enabling experiments on typical tropospheric aerosol life times of several days.

As a prerequisite for the cloud formation experiments, the AIDA chamber can be evacuated with two vacuum pumps down to a final pressure of about 0.01 hPa. The mechanical pumps can be operated at variable pumping speeds, allowing for controlled expansion cooling experiments which mimic the adiabatic expansion cooling of rising air parcels in the atmosphere. The formation of supercooled liquid water and/or ice clouds is triggered when the relative humidity inside the vessel has exceeded a threshold value whose magnitude depends on the type of the pre-added seed aerosol particles and the temperature.

In the present contribution, we want to review the AIDA research activities on optical measurements of aerosol and cloud particles. In Section 2, we will present a brief summary of recent journal publications using AIDA and draw attention to the benefits of performing these experiments in the AIDA chamber. Section 3 will describe the optical measurement techniques and numerical models which are currently employed in the AIDA investigations, followed by a detailed discussion of selected optical measurements in Section 4.

Section snippets

Aerosol optics

The first investigations with AIDA focused on the optical properties of airborne soot particles [2], [3], combining broadband extinction measurements in the 230–1000 nm wavelength range [4] and scattering measurements with a three-color integrating nephelometer to derive accurate extinction, scattering, and absorption coefficients which are needed to assess the influence of soot aerosol on the atmospheric radiation budget. The experimental data were compared to model calculations with the

Optical instrumentation of the AIDA facility and adopted numerical models

The AIDA facility (Fig. 1) consists of an 84.3 m3 sized aluminum vessel of 4 m diameter that is located inside an isolating containment whose interior can be cooled to any temperature from ambient to 183 K. The mechanical pumps allow for an efficient cleaning of the aerosol vessel which results in background aerosol number concentrations of less than 0.1 cm−3 after refilling the evacuated aerosol vessel with particle-free synthetic air. Readers interested in detailed technical descriptions of the

Optics of pure and internally mixed soot aerosols

Soot is in an exceptional position in comparison with other atmospheric aerosols, given that: (i) it is the only aerosol that strongly absorbs visible solar radiation, (ii) it has a fractal-like agglomerate structure consisting of nanometer-sized primary particles, and (iii) it is mostly of anthropogenic origin. From the perspective of particle optics, the strong broadband absorption cross section, the agglomerate structure, and the atmospheric aging by internal mixing with other aerosol

Outlook

Studies on the optical properties of aerosol and cloud particles will continue to be a focus of AIDA research activities. Regarding the optical instrumentation of the chamber, particular emphasis was placed on extending the diagnostic techniques for the characterization of ice crystal clouds, as reflected in the development of the cloud particle imager PHIPS. The ice cloud diagnostics will soon be supplemented by the Small Ice Detector probe SID3, constructed at the University of Hertfordshire.

Acknowledgments

We thank the editors of the journal for inviting us to contribute this review to the Special Issue. We are grateful to all AIDA operators and technicians for their continuous support during the numerous measurement campaigns that have been conducted at the chamber facility in the past decade.

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