Source-diagnostic dual-isotope composition and optical properties of water-soluble organic carbon and elemental carbon in the South Asian outflow intercepted over the Indian Ocean

Abstract

The dual carbon isotope signatures and optical properties of carbonaceous aerosols have been investigated simultaneously for the first time in the South Asian outflow during an intensive campaign at the Maldives Climate Observatory on Hanimaadhoo (MCOH) (February and March 2012). As one component of the Cloud Aerosol Radiative Forcing Dynamics Experiment, this paper reports on the sources and the atmospheric processing of elemental carbon (EC) and water-soluble organic carbon (WSOC) as examined by a dual carbon isotope approach. The radiocarbon (Δ 14 C) data show that WSOC has a significantly higher biomass/biogenic contribution (86 ± 5%) compared to EC (59 ± 4%). The more 13 C-enriched signature of MCOH-WSOC (À20.8 ± 0.7‰) compared to MCOH-EC (À25.8 ± 0.3‰) and megacity Delhi WSOC (À24.1 ± 0.9‰) suggests that WSOC is significantly more affected by aging during long-range transport than EC. The δ 13 C-Δ 14 C signal suggests that the wintertime WSOC intercepted over the Indian Ocean largely represents aged primary biomass burning aerosols. Since light-absorbing organic carbon aerosols (Brown Carbon (BrC)) have recently been identified as potential contributors to positive radiative forcing, optical properties of WSOC were also investigated. The mass absorption cross section of WSOC (MAC 365 ) was 0.5 ± 0.2 m 2 g À1 which is lower than what has been observed at near-source sites, indicating a net decrease of WSOC light-absorption character during long-range transport. Near-surface WSOC at MCOH accounted for~1% of the total direct solar absorbance relative to EC, which is lower than the BrC absorption inferred from solar spectral observations of ambient aerosols, suggesting that a significant portion of BrC might be included in the water-insoluble portion of organic aerosols.

The impact of residential combustion emissions on atmospheric aerosol, human health, and climate

“…We do not analyse the nonfossil fraction of OC since OC arises from a larger range of sources including primary emissions and secondary organic aerosol (SOA). Nevertheless, non-fossil water-soluble organic carbon at Hanimaadhoo is dominated (∼ 80 %) by biomass and biogenic sources but the relative enrichment in the stable (δ 13 C) carbon isotope points largely to aged primary biomass emissions sources (Bosch et al, 2014). We estimate the simulated biomass contribution to OC at Hanimaadhoo to be ∼ 50-70 % for baseline simulations (res_base and res_monthly) and ∼ 70-80 % for simulations where residential carbonaceous emissions have been doubled.…”

“…For example, during the same measurement period Gustafsson et al (2009) found that 46 ± 8 % of EC and 68 ± 6 % of BC originated from non-fossil biomass (January-March). Bosch et al (2014) estimate that 59 ± 8 % of EC is from non-fossil biomass (February-March). Sheesley et al (2012) estimated that 73 ± 6 % of EC originated from non-fossil biomass during the dry season (November-February).…”

“…We further evaluate the simulated contribution of residential emissions to BC concentrations using 14 C source apportionment studies on the island of Hanimaadhoo (Gustafsson et al, 2009;Sheesley et al, 2012;Bosch et al, 2014), which is influenced by pollution transported from the Indian subcontinent. The model simulates well both BC and OC concentrations observed at this location (Sect.…”

“…We also use information on BC fossil and non-fossil fractions as obtained from three separate source apportionment studies (Gustafsson et al, 2009;Sheesley et al, 2012;Bosch et al, 2014) that use 14 C analysis of carbonaceous aerosol taken at Hanimaadhoo in the Indian Ocean. This technique determines the fossil and non-fossil fractions of carbonaceous aerosol, since 14 C is depleted in fossil fuel aerosol (half-life 5730 years), whereas non-fossil aerosol (e.g.…”

Variation of Stable Carbon and Nitrogen Isotopic Composition of PM10 at Urban Sites of Indo Gangetic Plain (IGP) of India

“…The average TC/TN ratio of PM 10 was 11.1 ± 5.7 with a range of 4.4-25.9 over Kolkata. Bosch et al (2014) studied the dual C isotope signatures and optical properties of CA simultaneously for the South Asian outflow during an intensive campaign at the Maldives Climate Observatory on Hanimaadhoo (MCOH). The radiocarbon (D 14 C) data showed that water soluble organic carbon (WSOC) had a significantly higher biomass/biogenic contribution (86 ± 5 %) compared to EC (59 ± 4 %).…”