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| Titel |
Source apportionment of light absorbing WSOC in South Asian outflow |
| VerfasserIn |
Carme Bosch, Elena Kirillova, August Andersson, Martin Kruså, Krishnakant Budhavant, Suresh Tiwari, Örjan Gustafsson |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250075954
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| Zusammenfassung |
| Carbonaceous aerosols (CA) formed over South Asia are of special concern for
human health and regional climate impacts. Anthropogenic emissions forming CA
are generally high throughout the region and particularly over the Indo-Gangetic
Plain.
The net effects of CA on radiative climate forcing are still uncertain. One of the
components of CA is black carbon (BC), dominated by soot-like elemental carbon, a strong
absorber of solar radiation. Another component is organic carbon (OC), traditionally
considered as a light scattering particle. However, recent field studies have shown OC to
absorb at lower wavelengths. Thus OC, in addition to BC, may also contribute to light
absorption and have a positive direct radiative effect on climate. Light absorbing
organic aerosol is usually termed brown carbon (BrC). A significant fraction of BrC is
water-soluble, therefore its dissolution into clouds could result in absorbing droplets that
affect the cloud absorption and thus contributing to the indirect aerosol climate
effects.
In this study, light absorption and δ13C + Δ14C isotopic measurements of WSOC were
studied in fine aerosols (PM 2.5) at two sites during early pre-monsoon season. New Delhi,
one of the most densely populated and industrialized urban megacities in South Asia, was
chosen to represent a strong source and Maldives Climate Observatory at Hanimaadhoo
(MCOH) was chosen as a regional receptor which in wintertime is located downwind of the
Indian subcontinent. Sampling in Delhi was done from mid-February to mid-March 2011 and
in MCOH during March 2012.
WSOC concentrations were 12±4.5 and 0.71±0.30 μg m-3 in Delhi and MCOH
respectively. Whereas in Delhi WSOC contributed 31±4% of total organic carbon, this
contribution was slightly higher in MCOH (40±12%). Light absorption by WSOC exhibited
strong wavelength (λ) dependence. In Maldives, WSOC Absorption Ångström Exponent
(AAE) was found to be 6.9±0.4 and Mass Absorption Efficiency (MAE) measured at 365 nm
was 0.38±0.09 m2g-1. MAE increased sharply from long to short wavelengths. This
evidences that WSOC is constituted of compounds that are light absorbing near ultraviolet
wavelengths. WSOC in Maldives would contribute 19% of the total absorption at
wavelengths below 400 nm.
Since a substantially absorbing effect by WSOC has been shown here and in
other studies, knowledge about WSOC sources is needed. The sources of WSOC in
atmospheric aerosols, which may be both of primary and secondary origins, are in
general poorly constrained. Biomass burning is a significant primary source of
WSOC whereas secondary organic aerosol formation also takes place. Isotopic
measurements (stable (δ13C) and radiocarbon (Δ14C)) were applied to inform on the
origin of WSOC for both sites. Δ14C allows quantitative apportionment between
fossil fuel versus biogenic and biomass combustion sources. δ13C is informative
of the atmospheric processing of WSOC during long-range transport of aerosols. |
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