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| Titel |
Atmospheric aerosol brown carbon in the high Himalayas |
| VerfasserIn |
Elena Kirillova, Stefano Decesari, Angela Marinoni, Paolo Bonasoni, Elisa Vuillermoz, M. Cristina Facchini, Sandro Fuzzi |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250127542
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| Publikation (Nr.) |
 EGU/EGU2016-7433.pdf |
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| Zusammenfassung |
| Anthropogenic light-absorbing atmospheric aerosol can reach very high concentrations in the
planetary boundary layer in South-East Asia ("brown clouds"), affecting atmospheric
transparency and generating spatial gradients of temperature over land with a possible impact
on atmospheric dynamics and monsoon circulation. Besides black carbon (BC), an important
light-absorbing component of anthropogenic aerosols is the organic carbon component
known as ‘brown carbon’ (BrC). In this research, we provided first measurements of
atmospheric aerosol BrC in the high Himalayas during different seasons. Aerosol sampling
was conducted at the GAW-WMO Global station “Nepal Climate Observatory-Pyramid”
(NCO-P) located in the high Khumbu valley at 5079 m a.s.l. in the foothills of Mt.
Everest. PM10 aerosol samples were collected from July 2013 to November 2014. The
sampling strategy was set up in order to discriminate the daytime valley breeze
bringing polluted air masses up to the observatory and free tropospheric air during
nighttime. Water-soluble BrC (WS-BrC) and methanol-soluble BrC (MeS-BrC) were
extracted and analyzed using a UV/VIS spectrophotometer equipped with a 50
cm liquid waveguide capillary cell. In the polluted air masses, the highest levels
of the BrC light absorption coefficient at 365 nm (babs365) were observed during
the pre-monsoon season (1.83±1.46 Mm−1 for WS-BrC and 2.86±2.49 Mm−1
for MeS-BrC) and the lowest during the monsoon season (0.21±0.22 Mm−1 for
WS-BrC and 0.32±0.29 Mm−1 for MeS-BrC). The pre-monsoon season is the
most frequently influenced by a strong atmospheric brown cloud (ABC) transport
to NCO-P due to increased convection and mixing layer height over South Asia
combined with the highest up-valley wind speed and the increase of the emissions from
open fires due to the agricultural practice along the Himalayas foothills and the
Indo-Gangetic Plain. In contrast, the monsoon season is characterized by a weakened
valley wind regime and an efficient removal of aerosols by wet scavenging. The
wavelength dependence of the light absorption by BrC expressed as Ångström
Exponent (AAE) within 330-500 nm was on average lower for MeS-BrC (3.9±1.1)
compared to WS-BrC (4.8±0.8) and exhibited no diurnal or seasonal trend. The light
absorption coefficient of BrC at 365 nm was about 13-17% (WS-BrC) and about
21-29% (MeS-BrC) of that of BC (AAEBC=1). Relative light absorption of BrC and
BC considering the whole solar spectrum showed that at NCO-P WS-BrC absorbs
5±2% and MeS-BrC absorbs 12±7% compared to equivalent BC, as measured by
Absorption Photometer (MAAP). These results are in line with previous in situ
measurements at low altitude stations in South-East Asia, and do not support the strong
enhancements of brown carbon absorption contribution in the upper part of the
boundary layer and in the free troposphere suggested by remote sensing observations. |
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