Calibration and evaluation of a broad supersaturation scanning (BS2) cloud condensation nuclei counter for rapid measurement of particle hygroscopicity and cloud condensation nuclei (CCN) activity
<p>For understanding and assessing aerosol–cloud interactions and their impact on climate, reliable measurement data on aerosol particle hygroscopicity and cloud condensation nuclei (CCN) activity are required. The CCN activity of aerosol particles can be determined by scanning particle size a...
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| Autores principales: | , , , , , , , , |
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| Formato: | article |
| Lenguaje: | EN |
| Publicado: |
Copernicus Publications
2021
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| Materias: | |
| Acceso en línea: | https://doaj.org/article/f674095d1c3e4c6cbc5edbe1183e9c25 |
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| Sumario: | <p>For understanding and assessing aerosol–cloud
interactions and their impact on climate, reliable measurement data on
aerosol particle hygroscopicity and cloud condensation nuclei (CCN) activity
are required. The CCN activity of aerosol particles can be determined by
scanning particle size and supersaturation (<span class="inline-formula"><i>S</i></span>) in CCN measurements. Compared
to an existing differential mobility analyzer (DMA) with CCN activity
measurement, a broad supersaturation scanning CCN (BS2-CCN) system, in which
particles are exposed to a range of <span class="inline-formula"><i>S</i></span> simultaneously, can measure the CCN
activity with a high time resolution. Based on a monotonic relation between
the activation supersaturation of aerosol particles (<span class="inline-formula"><i>S</i><sub>aerosol</sub></span>) and the
activated fraction (<span class="inline-formula"><i>F</i><sub>act</sub></span>) of the BS2-CCN measurement, we can derive
<span class="inline-formula"><i>κ</i></span>, a single hygroscopicity parameter, directly. Here, we describe
how the BS2-CCN system can be effectively calibrated and which factors can
affect the calibration curve (<span class="inline-formula"><i>F</i><sub>act</sub>−<i>S</i><sub>aerosol</sub></span>). For calibration,
size-resolved CCN measurements with ammonium sulfate and sodium chloride
particles are performed under three different thermal gradient (<span class="inline-formula">d<i>T</i></span>)
conditions (<span class="inline-formula">d<i>T</i>=6</span>, 8, and 10 K). We point out key processes that can affect
the calibration curve and thereby need to be considered as follows: first,
the shape of the calibration curve is primarily influenced by <span class="inline-formula"><i>S</i><sub>max</sub></span>, the
maximum <span class="inline-formula"><i>S</i></span> in the activation tube. We need to determine appropriate <span class="inline-formula"><i>S</i><sub>max</sub></span>
depending on the particle size and <span class="inline-formula"><i>κ</i></span> to be investigated. To minimize the
effect of multiply charged particles, a small geometric mean diameter
(<span class="inline-formula"><i>D</i><sub>g</sub></span>) and geometric standard deviation (<span class="inline-formula"><i>σ</i><sub>g</sub></span>) in
number size distribution are recommended when generating the calibration
aerosols. Last, <span class="inline-formula"><i>F</i><sub>act</sub></span> is affected by particle number concentration and
has a decreasing rate of 0.02 per <span class="inline-formula">100 cm<sup>−3</sup></span> due to
the water consumption in the activation tube. For evaluating the BS2-CCN
system, intercomparison experiments between typical DMA-CCN and BS2-CCN
measurements were performed with a laboratory-generated aerosol mixture and
ambient aerosols. Good agreement of <span class="inline-formula"><i>κ</i></span> values between DMA-CCN and
BS2-CCN measurements for both experiments shows that the BS2-CCN system can
measure CCN activity well compared to the existing measurement method and can
measure a broad range of hygroscopicity distributions with a high
time resolution (<span class="inline-formula">∼1</span> s vs. a few minutes for a standard CCN
activity measurement). As the hygroscopicity can be used as a proxy for the
chemical composition, our method can also serve as a complementary approach
for fast and size-resolved detection and estimation of aerosol chemical
composition.</p> |
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