Wavelength-Resolved Optical Extinction Measurements of Aerosols Using Broad-Band Cavity-Enhanced Absorption Spectroscopy over the Spectral Range of 445–480 nm

Abstract

Despite the significant progress in the measurements of aerosol extinction and absorption using spectroscopy approaches such as cavity ring-down spectroscopy (CRDS) and photoacoustic spectroscopy (PAS), the widely used single-wavelength instruments may suffer from the interferences of gases absorption present in the real environment. A second instrument for simultaneous measurement of absorbing gases is required to characterize the effect of light extinction resulted from gases absorption. We present in this paper the development of a blue light-emitting diode (LED)-based incoherent broad-band cavity-enhanced spectroscopy (IBBCEAS) approach for broad-band measurements of wavelength-resolved aerosol extinction over the spectral range of 445-480 nm. This method also allows for simultaneous measurement of trace gases absorption present in the air sample using the same instrument. On the basis of the measured wavelength-dependent aerosol extinction cross section, the real part of the refractive index (RI) can be directly retrieved in a case where the RI does not vary strongly with the wavelength over the relevant spectral region. Laboratory-generated monodispersed aerosols, polystyrene latex spheres (PSL) and ammonium sulfate (AS), were employed for validation of the RI determination by IBBCEAS measurements. On the basis of a Mie scattering model, the real parts of the aerosol RI were retrieved from the measured wavelength-resolved extinction cross sections for both aerosol samples, which are in good agreement with the reported values. The developed IBBCEAS instrument was deployed for simultaneous measurements of aerosol extinction coefficient and NO(2) concentration in ambient air in a suburban site during two representative days.

Light extinction by secondary organic aerosol: an intercomparison of three broadband cavity spectrometers

“…Engineering improvements since the NO3Comp campaign, including more frequent re-calibration of the baseline spectrum, have produced performances much closer to the best case values (Kennedy et al, 2011). Zhao et al (2013) report a long-term stability of around 1 Mm −1 , which is only slightly worse than the detection limit of their instrument. The detection limits we report are well sufficient for monitoring aerosol extinction in polluted atmospheres (for instance, mean aerosol extinction coefficients are 121 Mm −1 in Atlanta, and over 300 Mm −1 in Beijing) and are possibly low enough for measurements in pristine environments (Carrico et al, 2003;He et al, 2009).…”

“…Based solely on the precision of the extinction coefficient measurements, the respective limits of detection (3σ ) of the three systems were respectively 1.2, 2.1, and 1.2 Mm −1 for the BBCRDS and CE-DOAS systems over 60 s, and for the IBBCEAS system over 5 s. Other recently developed broadband systems by Washenfelder et al (2013) and Zhao et al (2013) have reported somewhat lower precisions of around 0.2 Mm −1 (corresponding to a detection limit of 0.6 Mm −1 ). These figures represent the best case performance of the instruments and do not take into account any drift in the instruments' baselines.…”

“…Wilson et al (2013) reported the extinction coefficient from 320 to 410 nm of SOA formed by the photolysis of nitroaromatic compounds in an atmospheric simulation chamber. For field observations, Zhao et al (2013) have developed an IB-BCEAS system for the 445 to 480 nm spectral region and have demonstrated concurrent measurement of NO 2 concentration and aerosol extinction in a polluted urban environment. A key benefit of the Washenfelder, Wilson, and Zhoa instruments, like the broadband spectrometers in the present work, is their ability to acquire wavelength-resolved spectra of the atmosphere extinction and hence the explicit separation of the structured gas-phase and unstructured aerosol contributions.…”

“…The method is also readily extended to different wavelength regions -particularly to shorter wavelengths where aerosol absorption often increases (Andreae and Gelencser, 2006;Kirchstetter et al, 2004;Chen and Bond, 2010;Hoffer et al, 2006;Martins et al, 2009). Very recently, several groups have started to exploit the potential of broadband cavity systems to achieve both spectrally continuous measurements of aerosol extinction and to perform measurements at short visible and ultraviolet wavelengths (Zhao et al, 2013;Washenfelder et al, 2013;Wilson et al, 2013).…”

A complete parameterisation of the relative humidity and wavelength dependence of the refractive index of hygroscopic inorganic aerosol particles

“…Ensemble broadband cavity-enhanced spectroscopy involves σ ext measurements on a cloud of aerosol particles over a broad range of wavelengths. Zhao et al (2013) have developed an ensemble broadband cavity-enhanced spectrometer for measurements of σ ext over the wavelength range 445-480 nm and reported measurements of (NH 4 ) 2 SO 4 aerosol (Zhao et al, 2013). Rudich and co-workers developed an ensemble broadband cavity-enhanced spectrometer for the wavelength range 360-420 nm (Flores et al, 2014a, b;Washenfelder et al, 2013).…”