Research ArticleATMOSPHERIC SCIENCE

Photochemical degradation affects the light absorption of water-soluble brown carbon in the South Asian outflow

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Science Advances  30 Jan 2019:
Vol. 5, no. 1, eaau8066
DOI: 10.1126/sciadv.aau8066
  • Fig. 1 Meteorology and general aerosol characteristics during the SAPOEX-16.

    The average AOD at 550 nm during January to March 2016 over the South Asian region and sampling sites of Delhi, BCOB, and MCOH are shown. The trail (dashed line) is the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model–based mean air mass back trajectory (BT) of the IGP cluster showing the dominating air mass transport from Delhi to BCOB to MCOH (shown by arrows). The pie charts depict the particulate matter (PM) composition in terms of mean relative abundances of (i) TCA mass (black; i.e., TCA = OM + EC), (ii) anthropogenic WS inorganic species (green; i.e., WSISanth: Cl + NO3 + SO42− + NH4+ + K+), and (iii) largely nonanthropogenic fraction [red; i.e., NAF: PMtot – (TCA + WSISanth)]. The AOD data were obtained from NASA Moderate Resolution Imaging Spectroradiometer (MODIS) (https://giovanni.gsfc.nasa.gov/giovanni/).

  • Fig. 2 Aerosol chemical composition during SAPOEX-16.

    Fractional mass contribution of measured chemical species in PM2.5 over Delhi (n = 15) and BCOB (n = 24) and PM1 over MCOH (n = 43) during the SAPOEX-16 campaign. Synoptic period refers to the period when air masses from the IGP exiting Bay of Bengal passed over the northern region of Maldives (MCOH), as evidenced in the BTs (fig. S1). Relative air mass cluster contribution at each site revealed this period to be between 2 and 31 January 2016 (fig. S2). The mass fraction of the WS inorganic ions is corrected for contribution from sea salt.

  • Fig. 3 Evolution of WS-BrC in the South Asian outflow.

    (A) The fractional contribution of WSOC, water-insoluble OC (WIOC), and stable carbon isotope ratio of WSOC (δ13CWSOC) at Delhi, BCOB, and MCOH for the synoptic period of the SAPOEX-16 campaign when air masses were overwhelmingly from IGP and over the Bay of Bengal to MCOH (and thus similar air masses connecting the three stations, see figs. S1 and S2). (B) Absorption coefficient (babs) and mass absorption cross section of WS-BrC at 365 nm (MACWS-BrC 365) and AAE for 330 to 400 nm at Delhi, BCOB, and MCOH during the SAPOEX-16 campaign. Note the order of magnitude difference in scale for babs of MCOH compared to Delhi and BCOB.

  • Fig. 4 Atmospheric dynamics of WS-BrC optical properties.

    Measurements of the two–optical parameter mass absorption cross section at 365 nm (MACWS-BrC 365) and AAE for 330 to 400 nm (AAE330–400nm) of WS-BrC are shown. MACWS-BrC 365 (bottom panel) and AAE330–400nm (top panel) from SAPOEX-16 are shown with suffix “16,” and previous campaigns measured separately at Delhi (26) and MCOH (23) are shown with suffix “12” (studies performed in the year 2012). A first-order aging model fit (gray line) elucidates the interdependence of MACWS-BrC 365, AAE330–400nm, and atmospheric processing (i.e., bleaching) during source-to-receptor transport in the South Asian outflow (see note S3 for mathematical formulation). δ13CWSOC is used as proxy for aging. The goodness of fit for MACWS-BrC 365 was root mean square deviation (RMSD) of 0.34 and normalized RMSD (NRMSD) of 0.27, with the corresponding values for AAE330–400nm being RMSD of 0.80 and NRMSD of 0.13.

  • Fig. 5 Bounding the light absorption of WS-BrC in the South Asian outflow.

    The decay/bleaching of the mass absorption cross section at 365 nm (MACWS-BrC 365) (vertical bars in black) of WS-BrC between source-to-receptor sites in the South Asian outflow is shown. The first-order rate of bleaching (k) is constrained (using Eq. 1) for the over-ocean transport of the BrC plume between BCOB and MCOH (solid line). The transport times of BrC plume evolution (horizontal bars in red) are calculated from BCOB as forward trajectories to MCOH and BTs to Delhi using the HYSPLIT model. The AOD at 550 nm is shown in combination with distance from BCOB. The AOD was obtained from NASA MODIS for the SAPOEX-16 campaign period.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/1/eaau8066/DC1

    Note S1. A theoretical model for ruling out marine-biogenic source contribution.

    Note S2. A theoretical model for the degradation of WSOC and WIOC in the South Asian outflow.

    Note S3. A conceptual aging model for the joint time and wavelength dependence of MACWS-BrC.

    Note S4. Absorption measurements of MACWS-BrC and AAE.

    Note S5. Estimating the imaginary part of the refractive index of WS-BrC.

    Note S6. Testing a putative effect of pH on WS-BrC optical properties over the South Asian region.

    Fig. S1. Air mass clusters during SAPOEX-16.

    Fig. S2. Fractional contribution of air mass clusters during SAPOEX-16.

    Fig. S3. Concentrations of EC, OC, and WSOC during SAPOEX-16.

    Fig. S4. Imaginary part of the refractive index (KWS-BrC) at 365 nm.

    Fig. S5. Asserting the peripheral contribution of marine-biogenic sources at MCOH during SAPOEX-16.

    Fig. S6. Testing a putative effect of pH on WS-BrC optical properties.

    Fig. S7. Constraining the mixing of WSOC sources on WS-BrC light absorption in the South Asian outflow.

    Fig. S8. Degradation of WSOC and WIOC during long-range transport in the South Asian outflow.

    Table S1. Concentrations (mean ± SD) and element mass ratios of carbonaceous species during SAPOEX-16.

    Table S2. pH of aerosol WSOC extracts during SAPOEX-16.

  • Supplementary Materials

    This PDF file includes:

    • Note S1. A theoretical model for ruling out marine-biogenic source contribution.
    • Note S2. A theoretical model for the degradation of WSOC and WIOC in the South Asian outflow.
    • Note S3. A conceptual aging model for the joint time and wavelength dependence of MACWS-BrC.
    • Note S4. Absorption measurements of MACWS-BrC and AAE.
    • Note S5. Estimating the imaginary part of the refractive index of WS-BrC.
    • Note S6. Testing a putative effect of pH on WS-BrC optical properties over the South Asian region.
    • Fig. S1. Air mass clusters during SAPOEX-16.
    • Fig. S2. Fractional contribution of air mass clusters during SAPOEX-16.
    • Fig. S3. Concentrations of EC, OC, and WSOC during SAPOEX-16.
    • Fig. S4. Imaginary part of the refractive index (KWS-BrC) at 365 nm.
    • Fig. S5. Asserting the peripheral contribution of marine-biogenic sources at MCOH during SAPOEX-16.
    • Fig. S6. Testing a putative effect of pH on WS-BrC optical properties.
    • Fig. S7. Constraining the mixing of WSOC sources on WS-BrC light absorption in the South Asian outflow.
    • Fig. S8. Degradation of WSOC and WIOC during long-range transport in the South Asian outflow.
    • Table S1. Concentrations (mean ± SD) and element mass ratios of carbonaceous species during SAPOEX-16.
    • Table S2. pH of aerosol WSOC extracts during SAPOEX-16.

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