Toward closure of absorption and scattering by complex aerosols at high humidities: equipment benchmarking results

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Description

This project entails closure studies between predictions of optical properties, including absorption, for biomass combustion aerosols mixed with inorganic material. Novel components of this project include: (1) changes in both absorption and scattering with humidity; (2) optical properties at relative humidities up to 95%, which are usually extrapolated rather than being confirmed; and (3) examination of aerosol from wood pyrolysis—representative of most of the primary atmospheric organic carbon—mixed with inorganic particles. This year’s activities focused on benchmarking the optical instrumentation, iterating those tests with improvements in optical instrumentation, obtaining stable humidification, developing automated interfaces for determining closure, and exploring measurements to characterize organic aerosol. This report outlines the benchmarking of the instrumentation under dry (8% RH) and wet conditions (up to 92% RH). The dry benchmarking results with ammonium sulfate aerosols show promising signal stability and an approximate light absorption detection limit of 42 Mm-1 at 532 nm. The wet benchmarking results show good performance of the extinction cell at high humidities. Measurement of scattering at high humidities has been hindered by a previously reported challenge: heating of the sample volume by the light source, limiting our measurement of scattering response to 78% RH. Remedies for this heating are currently under investigation. Work in the immediate future (remainder of the second project year) will include benchmarking and closure studies with well-characterized absorbing aerosol in addition to the first measurements of organic aerosols from biomass combustion.