CAPS PMSSA Monitors for the In Situ Measurement of Particle Absorption

Absorbing atmospheric aerosols, such as black carbon from combustion and brown carbon (absorbing organics), directly impact the Earth’s atmospheric radiation balance and indirectly can affect clouds formation and extent. Aerosol absorption remains a significant driver of uncertainty in our understanding of climate change and weather patterns.

Direct absorption-based instrumentation exists for measuring in situ aerosol absorption, though these systems tend to be expensive, require frequent calibrations, and be challenging to achieve sensitive detection limits. Current, popular instruments rely on filter-based attenuation techniques to obtains aerosol absorption. The benefits of filter-based systems include high precision, multiple wavelength measurements, and ease of operation. However, these systems are susceptible to filter-based measurement artifacts, which can significantly affect accuracy.

CAPS PMssa monitors measure particle extinction and scattering, and hence the single scattering albedo (SSA; i.e., the ratio of scattering to extinction), on the same sample volume and derive aerosol absorption by subtracting scattering from extinction. The accuracy of CAPS PMssa monitors can be tested with Rayleigh scattering from gases and standard-traceable Polysytrene latex (PSL) spheres. When used to measure aerosol absorption, the uncertainties related to accuracy increase with higher SSA values (i.e., subtraction of two large numbers). However, the measurement accuracy achieved by CAPS PMssa monitors for atmospheric aerosol absorption may rival filter-based attenuation techniques, especially when filter-based biases are suspected, while simultaneously providing extinction, scattering, and SSA measurements.

We will summarize the current capabilities of CAPS PMssa monitors and discuss recent aerosol absorption measurements.