No Evidence for Brown Carbon Formation in Ambient Particles Undergoing Atmospherically Relevant Drying

Recent laboratory studies have reported the formation of light-absorbing organic carbon compounds (brown carbon, BrC) in aqueous particles undergoing drying.  Atmospheric particles undergo cycles of humidification and drying during vertical transport and through daily variations in temperature and humidity, which implies particle drying could potentially be an important source of BrC globally.  In this work, we investigated BrC formation in ambient particles undergoing drying at a site in the eastern United States during summer.  Aerosol BrC concentrations were linked to secondary organic aerosol (SOA) formation, consistent with seasonal expectations for this region.  Measurements of water-soluble organic aerosol concentrations and light absorption (365 nm) were alternated between an unperturbed channel and a channel that dried particles to 41% or 35% relative humidity (RH), depending on the system configuration. The RH maintained in the dry channels was below most ambient RH levels observed throughout the study.  We did not observe BrC formation in particles that were dried to either RH level. The results were consistent across two summers, spanning ~5 weeks of measurements that included a wide range of RH conditions and organic and inorganic aerosol loadings.  This work suggests that mechanisms aside from humidification-drying cycles are more important contributors to ambient particle BrC loadings.  The implications of this work on the atmospheric budget of BrC are discussed.