Nighttime Oxidative Aging of Biomass Burning Brown Carbon

Abstract:

Brown carbon (BrC) from biomass burning sources affects overall climate by influencing global radiative forcing. BrC is known to absorb radiation, thus potentially contributing to warming effects. However, the evolution over time of its optical properties relative to various aging processes (photoreaction, oxidation) remains highly uncertain. Furthermore, with the increase in global mean temperatures and forest management practices, emissions from biomass burning sources are expected to increase. There are many different aging processes that can affect BrC’s optical properties, dependent upon the oxidative agent as well as the medium (water/in-cloud, particulate). In this study, heterogeneous nighttime aging of biomass burning brown carbon was investigated by laboratory generation of wood smoke and oxidation with NO₃ radicals in a chamber. Generation of these radicals was performed by addition of NO₂ and O₃. Changes in optical properties were monitored with a photoacoustic soot spectrometer (PASS) and chemical changes were observed with an aerosol mass spectrometer (AMS). A scanning mobility particle sizer (SMPS), an ozone monitor and a NOx monitor were also used to determine respectively the size distributions, the O₃ concentrations and the NO₂ concentrations. Initial results from 10 hours of aging in the atmosphere show a gradual increase in absorption along with an increase in particulate organic nitrogen content, indicative of chemical reactivity leading to the formation of more absorptive compounds.