Abstract
Abstract. We introduce a new instrument for the measurement of in situ ambient aerosol extinction over the 300–700 nm wavelength range, the spectral aerosol extinction (SpEx) instrument. This measurement capability is envisioned to complement existing in situ instrumentation, allowing for simultaneous measurement of the evolution of aerosol optical, chemical, and physical characteristics in the ambient environment. In this work, a detailed description of the instrument is provided along with characterization tests performed in the laboratory. Measured spectra of NO2 and polystyrene latex spheres (PSLs) agreed well with theoretical calculations. Good agreement was also found with simultaneous aerosol extinction measurements at 450, 530, and 630 nm using CAPS PMex instruments in a series of 22 tests including nonabsorbing compounds, dusts, soot, and black and brown carbon analogs. SpEx measurements are expected to help identify the presence of ambient brown carbon due to its 300 nm lower wavelength limit compared to measurements limited to longer UV and visible wavelengths. Extinction spectra obtained with SpEx contain more information than can be conveyed by a simple power law fit (typically represented by Ångström exponents). Planned future improvements aim to lower detection limits and ruggedize the instrument for mobile operation.
Highlights
Studies of aerosol optical properties have become an important topic of research motivated by large uncertainties that limited our predictive capabilities of climate (IPCC, 2007)
Laboratory tests were designed to document the accuracy of spectral aerosol extinction (SpEx) extinction spectra by assessing (1) whether the filter used to remove the aerosols removed NO2 such that measured spectral characteristics include gas-phase differences unrelated to aerosol extinction, (2) whether the intense UV lamp light photolyzed NO2 to O3 in the optical cell, and (3) whether the measured spectral shapes were consistent with expectations from theoretical calculations
The advantage of the difference method employed in the SpEx measurement is that gas-phase losses in the optical cell affect both the sample and reference lines in the same way, canceling each other out as long as ambient mixing ratios are constant throughout the full sample/background cycle
Summary
Studies of aerosol optical properties have become an important topic of research motivated by large uncertainties that limited our predictive capabilities of climate (IPCC, 2007). Substituted O or N molecules on a single ring, or multiple rings (i.e., polycyclic aromatic hydrocarbons, PAHs) shift the absorption into the UV/visible range, leading to a characteristic wavelength dependence for these BrC aerosols that features strong UV absorption such that αabs > 1 (Andreae and Gelencsér, 2006; Moosmüller et al, 2009; Chen and Bond, 2010; Desyaterik et al, 2013). This results in BrC exhibiting a yellow-to-brown color, the term “brown carbon”.
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