Broadband Cavity-enhanced Spectrometer Research Articles

Light absorption and scattering properties of indole secondary organic aerosol prepared under various oxidant and relative humidity conditions

Secondary organic aerosol (SOA) from indole was produced in a smog chamber under different relative humidity (RH = <2%, 25% and 50%) and with different oxidants (O3, OH, OH + NOx, and NO3). The mass absorption coefficients were measured by extracting SOA in methanol and taking absorption spectra in the 200-700 nm range. Chromophoric compounds were tentatively identified by liquid chromatography – spectrophotometry – mass spectrometry. One of the strongest chromophores was nitroindole, produced in high abundance by both NO3 and OH + NOx oxidation. Indole SOA was also prepared in an oxidation flow reactor (OFR) using the same set of oxidants under variable RH conditions. Densities of the OFR-generated SOA particles ranged from 1.05 g/cm3 for indole + O3 SOA to 1.33 g/cm3 for indole + OH SOA. The real (n) and imaginary (k) refractive indices were quantified using a broadband cavity-enhanced spectrometer (BBCES) over the 360–425 nm spectral range. The values of k ranged from 10−2 to 10−1 at 405 nm, which is comparable to those for humic-like substances and at least an order of magnitude larger than k values of SOA produced from other biogenic and anthropogenic volatile organic compounds. The k values for the indole + NO3 SOA had a strong and non-monotonic dependence on RH, suggesting two competing reaction mechanisms for the chromophores, highlighting the complex effect of water on the absorption coefficient of SOA from heterocyclic aromatic precursors. Strong absorption of indole SOA may contribute to the degradation of visibility near its emission sources, which include animal husbandry facilities and stressed plants.Copyright © 2023 American Association for Aerosol Research

Complex refractive indices in the ultraviolet and visible spectral region for highly absorbing non-spherical biomass burning aerosol

Abstract. Biomass burning aerosol is a major source of PM2.5, and significantly affects Earth's radiative budget. The magnitude of its radiative effect is poorly quantified due to uncertainty in the optical properties of aerosol formed from biomass burning. Using a broadband cavity-enhanced spectrometer with a recently increased spectral range (360–720 nm) coupled to a size-selecting aerosol inlet, we retrieve complex refractive indices of aerosol throughout the near-ultraviolet and visible spectral region. We demonstrate refractive index retrievals for two standard aerosol samples: polystyrene latex spheres and ammonium sulfate. We then retrieve refractive indices for biomass burning aerosol from 13 controlled fires during the 2016 Missoula Fire Science Laboratory Study. We demonstrate that the technique is highly sensitive to the accuracy of the aerosol size distribution method and find that while we can constrain the optical properties of brown carbon aerosol for many fires, fresh smoke dominated by fractal-like black carbon aerosol presents unique challenges and is not well-represented by Mie theory. For the 13 fires, we show that the accuracy of Mie theory retrievals decreases as the fraction of black carbon mass increases. At 475 nm, the average refractive index is 1.635 (±0.056) +0.06 (±0.12)i, and at 365 nm, the average refractive index is 1.605 (±0.041) +0.038 (±0.074)i.

Correcting micro-aethalometer absorption measurements for brown carbon aerosol

Portable aethalometers are commonly used for online measurements of light-absorbing carbonaceous particles (LAC). However, they require strict calibration. In this study, the performance of a micro-aethalometer (MA200 with polytetrafluoroethylene filter) in charactering brown carbon aerosol (BrC) absorption was evaluated in comparison with reference materials and techniques that included bulk solution absorbance and Mie-theory based particle extinction retrieval via broadband cavity enhanced spectrometer (BBCES). Continuous-wavelength resolved (300–650 nm) imaginary refractive index (kBrC) was derived with these methods for various BrC proxies and standard materials representing a wide range of sources and absorbing abilities, including the strongly absorbing nigrosin, pahokee peat fluvic acid (PPFA), tar aerosol from wood pyrolysis, humic-like substance (HULIS) separated from wood smoldering burning emissions, and secondary organic aerosols (SOA) from photochemical oxidation of indole and naphthalene in the presence of NOx. The BrC and nigrosin optical results by bulk solution absorption are comparable with the properties retrieved from BBCES. The MA200 raw measurements provide reliable absorption Ångström exponent (AAE) but overestimate kBrC largely. The parameterized overestimates against reference methods depend on light absorption strength, so that the MA200 overestimates more for the less absorbing BrC. The correction factor for MA200 can be expressed well as an exponential function of kBrC or particle single scattering albedo (SSA), and also as a power-law function of the MA200 raw results derived BrC mass absorption efficiency (MAE). The ensemble correction factor regressed for all these BrC and nigrosin is 2.8 based on bulk absorption and 2.7 using BBCES result as reference. Simple radiative forcing (SRF) calculations for different scenarios using the correction for MA200, show consistent SRF when using the aethalometer results after the kBrC-dependent correction.

Portable broadband cavity-enhanced spectrometer utilizing Kalman filtering: application to real-time, in situ monitoring of glyoxal and nitrogen dioxide

This article describes the development and field application of a portable broadband cavity enhanced spectrometer (BBCES) operating in the spectral range of 440-480 nm for sensitive, real-time, in situ measurement of ambient glyoxal (CHOCHO) and nitrogen dioxide (NO2). The instrument utilized a custom cage system in which the same SMA collimators were used in the transmitter and receiver units for coupling the LED light into the cavity and collecting the light transmitted through the cavity. This configuration realised a compact and stable optical system that could be easily aligned. The dimensions and mass of the optical layer were 676 × 74 × 86 mm3 and 4.5 kg, respectively. The cavity base length was about 42 cm. The mirror reflectivity at λ = 460 nm was determined to be 0.9998, giving an effective absorption pathlength of 2.26 km. The demonstrated measurement precisions (1σ) over 60 s were 28 and 50 pptv for CHOCHO and NO2 and the respective accuracies were 5% and 4%. By applying a Kalman adaptive filter to the retrieved concentrations, the measurement precisions of CHOCHO and NO2 were improved to 8 pptv and 40 pptv in 21 s.

A new approach for retrieving the UV–vis optical properties of ambient aerosols

Abstract. Atmospheric aerosols play an important part in the Earth's energy budget by scattering and absorbing incoming solar and outgoing terrestrial radiation. To quantify the effective radiative forcing due to aerosol–radiation interactions, researchers must obtain a detailed understanding of the spectrally dependent intensive and extensive optical properties of different aerosol types. Our new approach retrieves the optical coefficients and the single-scattering albedo of the total aerosol population over 300 to 650 nm wavelength, using extinction measurements from a broadband cavity-enhanced spectrometer at 315 to 345 nm and 390 to 420 nm, extinction and absorption measurements at 404 nm from a photoacoustic cell coupled to a cavity ring-down spectrometer, and scattering measurements from a three-wavelength integrating nephelometer. By combining these measurements with aerosol size distribution data, we retrieved the time- and wavelength-dependent effective complex refractive index of the aerosols. Retrieval simulations and laboratory measurements of brown carbon proxies showed low absolute errors and good agreement with expected and reported values. Finally, we implemented this new broadband method to achieve continuous spectral- and time-dependent monitoring of ambient aerosol population, including, for the first time, extinction measurements using cavity-enhanced spectrometry in the 315 to 345 nm UV range, in which significant light absorption may occur.