Temporal variations of the hygroscopicity and mixing state of black carbon aerosols in a polluted megacity area
Abstract. Black carbon (BC) aerosols in the atmosphere strongly affect radiative forcing. They are mainly removed from the air by wet deposition, and their lifetime is controlled by their water uptake ability or hygroscopicity, which is a function of the aerosol mixing states. It is well known that atmospheric aging processes coat various materials on BC aerosols and affect their mixing states and hygroscopicity. However, detailed relations between the aging processes, the hygroscopicity and mixing state of BC aerosol particles in polluted city areas are not well understood. Here, we studied the temporal variation of the hygroscopicity and its correlation with the mixing state of ambient BC particles during 2017 summer in Shanghai, China using a hygroscopicity tandem differential mobility analyzer in-line with a single particle soot photometer (HTDMA-SP2 system) as well as a single particle aerosol mass spectrometer (SPAMS). BC particles with 120 nm, 240 nm and 360 nm dry diameter were humidified at RH = 85 %. After humidification, particles with growth factors (GFs) of 1.0, 1.2 and 1.4, representing the BC particles with different hygroscopicities (hydrophobic, transition and hydrophilic modes, respectively), were analyzed by a SP2 to obtain their BC mixing states. The diurnal trends of coating thickness and chemical mixing state show that coating materials of BC particles were distinct between daytime and nighttime. The differences were associated with the hygroscopicity of BC particles. Single particle mass spectrometry and other chemical characterization techniques revealed that during nighttime with lower temperature and higher relative humidity (RH), formation or condensation of nitrates resulted in an enhanced hygroscopicity of BC particles. During daytime, secondary organic carbon formation was mainly responsible for the change of hygroscopicity of BC particles. Due to the high hygroscopicity of inorganic nitrate, a thinner nitrate coating on BC particles could convert fresh BC particles to aged hygroscopic ones during nighttime while a thicker coating layer of secondary materials was required to reach the same overall hygroscopicity during daytime since the participation of secondary organic carbon. Different atmospheric aging processes between daytime and nighttime led to the change of BC particles' mixing states, which play a fundamental role in determining their hygroscopicity. To our knowledge, this is the first report of links between temporal variations of the hygroscopic growth of BC particles and atmospheric aging processes in polluted environment. These findings have significant ramification of understanding the aging process, wet removal as well as climate effects of BC particles.