Optical properties and molecular compositions of water-soluble and water-insoluble brown carbon (BrC) aerosols in northwest China
<p>Brown carbon (BrC) contributes significantly to aerosol light absorption and thus can affect the Earth's radiation balance and atmospheric photochemical processes. In this study, we examined the light absorption properties and molecular compositions of water-soluble (WS-BrC) and water-...
| 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/20/4889/2020/acp-20-4889-2020.pdf |
| Summary: | <p>Brown carbon (BrC) contributes significantly to aerosol light absorption and
thus can affect the Earth's radiation balance and atmospheric photochemical
processes. In this study, we examined the light absorption properties and
molecular compositions of water-soluble (WS-BrC) and water-insoluble
(WI-BrC) BrC in PM<span class="inline-formula"><sub>2.5</sub></span> collected from a rural site in the Guanzhong
Basin – a highly polluted region in northwest China. Both WS-BrC and WI-BrC
showed elevated light absorption coefficients (Abs) in winter (4–7 times those in summer) mainly attributed to enhanced emissions from residential
biomass burning (BB) for heating of homes. While the average mass absorption
coefficients (MACs) at 365 nm (MAC<span class="inline-formula"><sub>365</sub></span>) of WS-BrC were similar between daytime
and nighttime in summer (<span class="inline-formula">0.99±0.17</span> and <span class="inline-formula">1.01±0.18</span> m<span class="inline-formula"><sup>2</sup></span> g<span class="inline-formula"><sup>−1</sup></span>, respectively), the average MAC<span class="inline-formula"><sub>365</sub></span> of WI-BrC was more than a
factor of 2 higher during daytime (<span class="inline-formula">2.45±1.14</span> m<span class="inline-formula"><sup>2</sup></span> g<span class="inline-formula"><sup>−1</sup></span>)
than at night (<span class="inline-formula">1.18±0.36</span> m<span class="inline-formula"><sup>2</sup></span> g<span class="inline-formula"><sup>−1</sup></span>). This difference was
partly attributed to enhanced photochemical formation of WI-BrC species,
such as oxygenated polycyclic aromatic hydrocarbons (OPAHs). In contrast,
the MACs of WS-BrC and WI-BrC were generally similar in winter and both
showed few diel differences. The Abs of wintertime WS-BrC correlated
strongly with relative humidity, sulfate and <span class="inline-formula">NO<sub>2</sub></span>, suggesting that
aqueous-phase reaction is an important pathway for secondary BrC formation
during the winter season in northwest China. Nitrophenols on average
contributed <span class="inline-formula">2.44±1.78</span> % of the Abs of WS-BrC in winter but only
<span class="inline-formula">0.12±0.03</span> % in summer due to faster photodegradation reactions.
WS-BrC and WI-BrC were estimated to account for <span class="inline-formula">0.83±0.23</span> % and
<span class="inline-formula">0.53±0.33</span> %, respectively, of the total down-welling solar
radiation in the ultraviolet (UV) range in summer, and <span class="inline-formula">1.67±0.72</span> % and
<span class="inline-formula">2.07±1.24</span> %, respectively, in winter. The total absorption by BrC
in the UV region was about 55 %–79 % relative to the elemental carbon (EC)
absorption.</p> |
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| ISSN: | 1680-7316 1680-7324 |