Synergistic effect of water-soluble species and relative humidity on morphological changes in aerosol particles in the Beijing megacity during severe pollution episodes
<p>Depolarization ratio (<span class="inline-formula"><i>δ</i></span>) of backscattered light is an applicable parameter for distinguishing the sphericity of particles in real time, which has been widely adopted by ground-based lidar observation systems. In th...
| Main Authors: | , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-01-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/19/219/2019/acp-19-219-2019.pdf |
| Summary: | <p>Depolarization ratio (<span class="inline-formula"><i>δ</i></span>) of backscattered light is an
applicable parameter for distinguishing the sphericity of particles in real time, which
has been widely adopted by ground-based lidar observation systems. In this study, <span class="inline-formula"><i>δ</i></span> values of particles and chemical compositions in both PM<span class="inline-formula"><sub>2.5</sub></span> (aerodynamic diameter
less than 2.5 <span class="inline-formula">µ</span>m) and PM<span class="inline-formula"><sub>10</sub></span> (aerodynamic diameter less than
10 <span class="inline-formula">µ</span>m) were concurrently measured on the basis of a bench-top optical particle
counter with a polarization detection module (POPC) and a continuous dichotomous aerosol
chemical speciation analyzer (ACSA-14) from November 2016 to February 2017 at an urban
site in Beijing megacity. In general, measured <span class="inline-formula"><i>δ</i></span> values depended on both size and
sphericity of the particles. During the observation period, mass concentrations of
<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="8a872e45f44a0fc3c08e466e371cfb3a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-219-2019-ie00001.svg" width="25pt" height="16pt" src="acp-19-219-2019-ie00001.png"/></svg:svg></span></span> in PM<span class="inline-formula"><sub>2.5</sub></span> (<span class="inline-formula"><i>f</i>NO<sub>3</sub></span>) were about an order of
magnitude higher than that in PM<span class="inline-formula"><sub>2.5−10</sub></span> (<span class="inline-formula"><i>c</i>NO<sub>3</sub></span>) with a mean
<span class="inline-formula"><i>f</i>NO<sub>3</sub>∕<i>c</i>NO<sub>3</sub></span> ratio of <span class="inline-formula">14±10</span>. A relatively low
<span class="inline-formula"><i>f</i>NO<sub>3</sub>∕<i>c</i>NO<sub>3</sub></span> ratio (<span class="inline-formula">∼5</span>) was also observed under higher
relative humidity conditions, mostly due to heterogeneous processes and particles in the
coarse mode. We found that <span class="inline-formula"><i>δ</i></span> values of ambient particles in both PM<span class="inline-formula"><sub>2.5</sub></span> and
PM<span class="inline-formula"><sub>2.5−10</sub></span> obviously decreased as mass concentration of water-soluble species
increased at unfavorable meteorological conditions. This indicated that the morphology of
particles was changed as a result of water-absorbing processes. The particles
with optical size (Dp) of Dp <span class="inline-formula">=</span> 5 <span class="inline-formula">µ</span>m were used to represent mineral dust
particles, and its <span class="inline-formula"><i>δ</i></span> values (<span class="inline-formula"><i>δ</i><sub>Dp=5</sub></span>) decreased by 50 % as
the mass fraction of <span class="inline-formula"><i>c</i>NO<sub>3</sub></span> increased from 2 % to 8 % and
ambient relative humidity increased up to 80 %, suggesting that mineral dust
particles were likely to be spherical during humid pollution episodes. During the
observation, relative humidity inside the POPC measuring chamber was stable at <span class="inline-formula">34±2</span> %, lower than the ambient condition. Its influence on the morphology was estimated
to be limited and did not change our major conclusion. This study highlights the evident
alteration of non-sphericity of mineral dust particles during their transport owing to a
synergistic effect of both pollutant coatings and hygroscopic processes, which plays an
important role in the evaluation of its environmental effect.</p> |
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| ISSN: | 1680-7316 1680-7324 |