Temporal variations in the hygroscopicity and mixing state of black carbon aerosols in a polluted megacity area
<p>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 aerosol mixing states. It is well known that a...
Main Authors: | , , , , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2018-10-01
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Series: | Atmospheric Chemistry and Physics |
Online Access: | https://www.atmos-chem-phys.net/18/15201/2018/acp-18-15201-2018.pdf |
Summary: | <p>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 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 and the hygroscopicity and mixing state of BC
aerosol particles in polluted city areas are not well understood. Here, we
studied the temporal variation in hygroscopicity and its correlation with
the mixing state of ambient BC particles during the summer of 2017 in Shanghai,
China, using a hygroscopic tandem differential mobility analyzer inline
with a single-particle soot photometer (HTDMA–SP2 system) as well as a single-particle aerosol mass spectrometer (SPAMS). BC particles with 120, 240, and
360 nm in dry diameter were humidified at relative humidity (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 with a SP2 to
obtain their BC mixing states. The diurnal trends in 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 with lower
temperature and higher RH during nighttime, 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 because of 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 in the hygroscopic growth of BC particles and atmospheric
aging processes in polluted environments. These findings have significant
ramifications in understanding the aging process, wet removal, and
climate effects of BC particles.</p> |
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ISSN: | 1680-7316 1680-7324 |