Aircraft Instrument for Comprehensive Characterization of Aerosol Optical Properties, Part I: Wavelength-Dependent Optical Extinction and Its Relative Humidity Dependence Measured Using Cavity Ringdown Spectroscopy

Abstract

High-quality in situ observations of aerosol particle optical properties, namely extinction, scattering, and absorption, provide important information needed to constrain the role of aerosols in the climate system. This paper outlines the design and performance of an aircraft instrument utilizing cavity ringdown spectroscopy for the measurement of aerosol extinction. The 8-channel cavity ringdown spectrometer measures extinction at multiple wavelengths (405, 532, and 662 nm) and at multiple relative humidities (e.g., 10%, 70%, and 95%). Key performance characteristics include a 1-s detection limit better than 0.1 Mm⁻¹, accuracy of <2% for dry aerosol measurements, and a 1-s precision better than 40% for extinction levels of >10 Mm⁻¹. Laboratory and field data demonstrate that the 1-s precision is limited by the statistics of aerosol particles in the laser beam rather than the precision of the extinction measurement per se. The measurement precision improves with averaging to 5% at 60 s for extinction levels of >10 Mm⁻¹. Field data collected during a recent airborne campaign in California, which involved eighteen research flights during May and June 2010, are used to demonstrate the in-flight performance of new instrument.

Acknowledgments

This work was supported by NOAA climate and air quality funding. JML thanks CIRES for the award of a visiting postdoctoral fellowship. The authors thank Drs. Ilana Pollack and Tom Ryerson for supplying NO₂ measurement data and Dr. Charles Brock for useful discussions together with aerosol size distribution data from the WP-3D aircraft.