Amplification of black carbon light absorption induced by atmospheric aging: temporal variation at seasonal and diel scales in urban Guangzhou
<p>Black carbon (BC) aerosols have been widely recognized as a vital climate forcer in the atmosphere. Amplification of light absorption can occur due to coatings on BC during atmospheric aging, an effect that remains uncertain in accessing the radiative forcing of BC. Existing studies on the...
Main Authors: | , , , , , , , , , , , , , , , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2020-02-01
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Series: | Atmospheric Chemistry and Physics |
Online Access: | https://www.atmos-chem-phys.net/20/2445/2020/acp-20-2445-2020.pdf |
Summary: | <p>Black carbon (BC) aerosols have been widely recognized as
a vital climate forcer in the atmosphere. Amplification of light absorption
can occur due to coatings on BC during atmospheric aging, an effect that
remains uncertain in accessing the radiative forcing of BC. Existing studies
on the absorption enhancement factor (<span class="inline-formula"><i>E</i><sub>abs</sub></span>) have poor coverage on both
seasonal and diurnal scales. In this study, we applied a recently developed
minimum <span class="inline-formula"><i>R</i></span> squared (MRS) method, which can cover both seasonal and diurnal
scales, for <span class="inline-formula"><i>E</i><sub>abs</sub></span> quantification. Using field measurement data in
Guangzhou, the aims of this study are to explore (1) the temporal dynamics of
BC optical properties at seasonal (wet season, 31 July–10 September; dry
season, 15 November 2017–15 January 2018) and diel scales (1 h time
resolution) in the typical urban environment and (2) the influencing factors on
<span class="inline-formula"><i>E</i><sub>abs</sub></span> temporal variability. Mass absorption efficiency at 520 nm by
primary aerosols (MAE<span class="inline-formula"><sub>p520</sub></span>) determined by the MRS method exhibited a strong
seasonality (8.6 m<span class="inline-formula"><sup>2</sup></span> g<span class="inline-formula"><sup>−1</sup></span> in the wet season and 16.8 m<span class="inline-formula"><sup>2</sup></span> g<span class="inline-formula"><sup>−1</sup></span>
in the dry season). <span class="inline-formula"><i>E</i><sub>abs520</sub></span> was higher in the wet season (<span class="inline-formula">1.51±0.50</span>) and lower in the dry season (<span class="inline-formula">1.29±0.28</span>). Absorption
Ångström exponent (AAE<span class="inline-formula"><sub>470–660</sub></span>) in the dry season (<span class="inline-formula">1.46±0.12</span>) was higher than that in the wet season (<span class="inline-formula">1.37±0.10</span>). Collective
evidence showed that the active biomass burning (BB) in the dry season
effectively altered the optical properties of BC, leading to elevated MAE,
MAE<span class="inline-formula"><sub>p</sub></span> and AAE in the dry season compared to those in the wet season.
Diurnal <span class="inline-formula"><i>E</i><sub>abs520</sub></span> was positively correlated with AAE<span class="inline-formula"><sub>470–660</sub></span> (<span class="inline-formula"><i>R</i><sup>2</sup>=0.71</span>) and negatively correlated with the AE33 aerosol loading
compensation parameter (<span class="inline-formula"><i>k</i></span>) (<span class="inline-formula"><i>R</i><sup>2</sup>=0.74</span>) in the wet season, but these
correlations were significantly weaker in the dry season, which may be
related to the impact of BB. This result suggests that during<span id="page2446"/> the wet
season, the lensing effect was more likely dominating the AAE diurnal
variability rather than the contribution from brown carbon (BrC). Secondary
processing can affect <span class="inline-formula"><i>E</i><sub>abs</sub></span> diurnal dynamics. The <span class="inline-formula"><i>E</i><sub>abs520</sub></span> exhibited
a clear dependency on the ratio of secondary organic carbon to organic carbon (<span class="inline-formula">SOC∕OC</span>), confirming the contribution of secondary organic aerosols to
<span class="inline-formula"><i>E</i><sub>abs</sub></span>; <span class="inline-formula"><i>E</i><sub>abs520</sub></span> correlated well with nitrate and showed a clear
dependence on temperature. This new finding implies that gas–particle
partitioning of semivolatile compounds may potentially play an important
role in steering the diurnal fluctuation of <span class="inline-formula"><i>E</i><sub>abs520</sub></span>. In the dry season,
the diurnal variability in <span class="inline-formula"><i>E</i><sub>abs520</sub></span> was associated with photochemical
aging as evidenced by the good correlation (<span class="inline-formula"><i>R</i><sup>2</sup>=</span>0.69) between oxidant
concentrations (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M30" display="inline" overflow="scroll" dspmath="mathml"><mrow><mrow class="chem"><msub><mi mathvariant="normal">O</mi><mi>x</mi></msub></mrow><mo>=</mo><mrow class="chem"><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">3</mn></msub></mrow><mo>+</mo><mrow class="chem"><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">2</mn></msub></mrow></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="74pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="28a7a81edb8f463e7f73110978604198"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-2445-2020-ie00001.svg" width="74pt" height="13pt" src="acp-20-2445-2020-ie00001.png"/></svg:svg></span></span>) and <span class="inline-formula"><i>E</i><sub>abs520</sub></span>.</p> |
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ISSN: | 1680-7316 1680-7324 |