Understanding the Sources, Atmospheric Evolution and Radiative Effect of Brown Carbon Aerosol Particles

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Zeng, Linghan
Weber, Rodney J.
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Organic aerosols (OA) have long been thought to only scatter incoming solar radiation and have a cooling effect on climate. However, a fraction of OA, referred to as brown carbon (BrC), absorbs light in the lower visible to ultraviolet range. BrC can be emitted from incomplete combustions and can also be generated through secondary processes. The radiative impact of BrC on climate is difficult to assess owing to the lack of knowledge about emissions and the evolution of BrC aerosol. As part of the thesis work, the light absorption of BrC aerosol was measured with filters sampled from research aircraft during the NASA ATom mission. The aircraft flew from near surface to up to ~ 13 km altitude nearly pole to pole along the central Pacific and Atlantic Ocean basins and across the southern and Arctic Oceans, providing the first direct measurement of BrC aerosol on a global scale. BrC concentrations were found to be highly spatially heterogeneous, and high BrC levels were associated with the long-range transport of biomass burning emissions. A radiative transfer model suggested that BrC could substantially affect the global climate. The characteristics and evolution of BrC emitted from wildfires in the western US as part of the FIREX-AQ study were investigated. An optical closure analysis was performed to compare the overall light absorption measured by a photoacoustic spectrometer and the sum of the light absorption by individual light absorbers, including black carbon (BC) and BrC. The evolution of BrC was examined in the first few hours after emissions, but no consistent fate of BrC was observed. We found that BrC did not behave as the bulk OA or as a single BrC compound (4-Nitrocatechol) in response to change with temperature increases. Evidence was found that oxidation of ozone could cause BrC enhancement under high NOx conditions, while BrC could be bleached by ozone when NOx levels were low. Additionally, an online water-soluble BrC measuring system was developed and deployed in the FIREX-AQ. The newly built system was compared to two other systems with similar detection methods but different aerosol collection methods. In general, all three instruments can make effective BrC measurements in airborne campaigns, but baseline drift and signal hysteresis were observed. A possible approach to correct the baseline drift and hysteresis effect was proposed, along with possible methods for future improvements for these systems.
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