Seasonal contrast in size distributions and mixing state of black carbon and its association with PM<sub>1.0</sub> chemical composition from the eastern coast of India
<p><span id="page3966"/>Over the Indian region, aerosol absorption is considered to have a potential impact on the regional climate, monsoon and hydrological cycle. Black carbon (BC) is the dominant absorbing aerosol, whose absorption potential is determined mainly by its micro...
| Main Authors: | , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-04-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/20/3965/2020/acp-20-3965-2020.pdf |
| Summary: | <p><span id="page3966"/>Over the Indian region, aerosol absorption is considered to have a potential
impact on the regional climate, monsoon and hydrological cycle. Black carbon
(BC) is the dominant absorbing aerosol, whose absorption potential is
determined mainly by its microphysical properties, including its
concentration, size and mixing state with other aerosol components. The
Indo-Gangetic Plain (IGP) is one of the regional aerosol hot spots with
diverse sources, both natural and anthropogenic, but still the information
on the mixing state of the IGP aerosols, especially BC, is limited and a
significant source of uncertainty in understanding their climatic
implications. In this context, we present the results from intensive
measurements of refractory BC (<span class="inline-formula"><i>r</i><sub>BC</sub></span>) carried out over Bhubaneswar, an urban
site in the eastern coast of India, which experiences contrasting air masses
(the IGP outflow or coastal/marine air masses) in different seasons. This
study helps to elucidate the microphysical characteristics of BC over this
region and delineates the IGP outflow from the other air masses. The
observations were carried out as part of South West Asian Aerosol Monsoon
Interactions (SWAAMI) collaborative field experiment during July 2016–May 2017, using a single-particle soot photometer (SP2) that uses a
laser-induced incandescence technique to measure the mass and mixing state
of individual BC particles and an aerosol chemical speciation monitor (ACSM)
to infer the possible coating material. Results highlighted the distinctiveness
in aerosol microphysical properties in the IGP air masses. BC mass
concentration was highest during winter (December–February) (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>∼</mo><mn mathvariant="normal">1.94</mn><mo>±</mo><mn mathvariant="normal">1.58</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="66pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="d05d0a131b7330f12bd2049f56cff7af"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-3965-2020-ie00001.svg" width="66pt" height="10pt" src="acp-20-3965-2020-ie00001.png"/></svg:svg></span></span> <span class="inline-formula">µ</span>g m<span class="inline-formula"><sup>−3</sup></span>), when the prevailing air masses were
mostly of IGP origin, followed by post-monsoon (October–November) (mean
<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>∼</mo><mn mathvariant="normal">1.34</mn><mo>±</mo><mn mathvariant="normal">1.40</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="66pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="bb337c0054b1e0325853f2f8fe6d05bc"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-3965-2020-ie00002.svg" width="66pt" height="10pt" src="acp-20-3965-2020-ie00002.png"/></svg:svg></span></span> <span class="inline-formula">µ</span>g m<span class="inline-formula"><sup>−3</sup></span>). The mass median diameter
(MMD) of the BC mass size distributions was in the range 0.190–0.195 <span class="inline-formula">µ</span>m, suggesting mixed sources of BC, and, further, higher values
(<span class="inline-formula">∼</span> 1.3–1.8) of bulk relative coating thickness (RCT) (ratio of
optical and core diameters) were seen, indicating a significant fraction of
highly coated BC aerosols in the IGP outflow. During the pre-monsoon
(March–May), when marine/coastal air masses prevailed, BC mass concentration
was lowest (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>∼</mo><mn mathvariant="normal">0.82</mn><mo>±</mo><mn mathvariant="normal">0.84</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="66pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="cbe8b26a729e315fd128e192f1e7f5a1"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-3965-2020-ie00003.svg" width="66pt" height="10pt" src="acp-20-3965-2020-ie00003.png"/></svg:svg></span></span> <span class="inline-formula">µ</span>g m<span class="inline-formula"><sup>−3</sup></span>), and larger
BC cores (MMD <span class="inline-formula">></span> 0.210 <span class="inline-formula">µ</span>m) were seen, suggesting distinct
source processes, while RCT was <span class="inline-formula">∼</span> 1.2–1.3, which may translate
into higher extent of absolute coating on BC cores, which may have crucial
regional climate implications. During the summer monsoon (July–September),
BC size distributions were dominated by smaller cores (MMD <span class="inline-formula">≤</span> 0.185 <span class="inline-formula">µ</span>m), with the lowest coating indicating fresher BC, likely from
fossil fuel sources. A clear diurnal variation pattern of BC and RCT was
noticed in all the seasons, and daytime peak in RCT suggested enhanced
coating on BC due to the condensable coating material originating from
photochemistry. Examination of submicrometre aerosol chemical composition
highlighted that the IGP outflow was dominated by organics (47 %–49 %), and
marine/coastal air masses contained higher amounts of sulfate (41 %–47 %),
while ammonium and nitrate were seen in minor amounts, with significant
concentrations only during the IGP air mass periods. The diurnal pattern of
sulfate resembled that of the RCT of <span class="inline-formula"><i>r</i><sub>BC</sub></span> particles, whereas organic mass
showed a pattern similar to that of the <span class="inline-formula"><i>r</i><sub>BC</sub></span> mass concentration. Seasonally,
the coating on BC showed a negative association with the mass concentration
of sulfate during the pre-monsoon season and with organics during the
post-monsoon season. These are the first experimental data on the mixing state
of BC from a long time series over the Indian region and include new
information on black carbon in the IGP outflow region. These data help in
improving the understanding of regional BC microphysical characteristics and
their climate implications.</p> |
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| ISSN: | 1680-7316 1680-7324 |