Air mass physiochemical characteristics over New Delhi: impacts on aerosol hygroscopicity and cloud condensation nuclei (CCN) formation
<p>Delhi is a megacity subject to high local anthropogenic emissions and long-range transport of pollutants. This work presents for the first time time-resolved estimates of hygroscopicity parameter (<span class="inline-formula"><i>κ</i></span>) and cloud cond...
| Main Authors: | , , , , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-06-01
|
| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/20/6953/2020/acp-20-6953-2020.pdf |
| Summary: | <p>Delhi is a megacity subject to high local anthropogenic emissions and
long-range transport of pollutants. This work presents for the first time
time-resolved estimates of hygroscopicity parameter (<span class="inline-formula"><i>κ</i></span>) and cloud condensation nuclei (CCN), spanning for
more than a year, derived from chemical composition and size distribution
data. As a part of the Delhi Aerosol Supersite (DAS) campaign, the
characterization of aerosol composition and size distribution was conducted
from January 2017 to March 2018. Air masses originating from the Arabian Sea
(AS), Bay of Bengal (BB), and southern Asia (SA) exhibited distinct
characteristics of time-resolved sub-micron non-refractory PM<span class="inline-formula"><sub>1</sub></span> (NRPM<span class="inline-formula"><sub>1</sub></span>) species, size distributions, and CCN number concentrations.
The SA air mass had the highest NRPM<span class="inline-formula"><sub>1</sub></span> loading with high chloride and
organics, followed by the BB air mass, which was more contaminated
than AS, with a higher organic fraction and nitrate. The primary sources
were identified as biomass-burning, thermal power plant emissions,
industrial emissions, and vehicular emissions. The average hygroscopicity parameter
(<span class="inline-formula"><i>κ</i></span>), calculated by the mixing rule, was approximately 0.3 (varying between
0.13 and 0.77) for all the air masses (<span class="inline-formula">0.32±0.06</span> for AS, <span class="inline-formula">0.31±0.06</span> for BB, and <span class="inline-formula">0.32±0.10</span> for SA). The diurnal variations in <span class="inline-formula"><i>κ</i></span> were
impacted by the chemical properties and thus source activities. The total,
Aitken, and accumulation mode number concentrations were higher for SA,
followed by BB and AS. The mean values of estimated CCN number concentration
(<span class="inline-formula"><i>N</i><sub>CCN</sub></span>; 3669–28926 cm<span class="inline-formula"><sup>−3</sup></span>) and the activated fraction (<span class="inline-formula"><i>a</i><sub>f</sub></span>;
0.19–0.87), for supersaturations varying from 0.1 % to 0.8 %, also showed the
same trend, implying that these were highest in SA, followed by those in BB
and then those in AS. The size turned out to be more important than chemical
composition directly, and the <span class="inline-formula"><i>N</i><sub>CCN</sub></span> was governed by either the Aitken
or accumulation modes, depending upon the supersaturation (SS) and critical
diameter (<span class="inline-formula"><i>D</i><sub>c</sub></span>). <span class="inline-formula"><i>a</i><sub>f</sub></span> was governed mainly by the geometric mean
diameter (GMD), and such a high <span class="inline-formula"><i>a</i><sub>f</sub></span> (<span class="inline-formula">0.71±0.14</span> for the most
dominant sub-branch of the SA air mass – R1 – at 0.4 % SS) has not been seen
anywhere in the world for a continental site. The high <span class="inline-formula"><i>a</i><sub>f</sub></span> was a
consequence of very low <span class="inline-formula"><i>D</i><sub>c</sub></span> (25–130 nm, for SS ranging from
0.1 % to 0.8 %) observed for Delhi. Indirectly, the chemical properties
also impacted CCN and <span class="inline-formula"><i>a</i><sub>f</sub></span> by impacting the diurnal patterns of Aitken
and accumulation modes, <span class="inline-formula"><i>κ</i></span> and <span class="inline-formula"><i>D</i><sub>c</sub></span>. The high-hygroscopic nature of
aerosols, high <span class="inline-formula"><i>N</i><sub>CCN</sub></span>, and high <span class="inline-formula"><i>a</i><sub>f</sub></span> can severely impact the
precipitation patterns of the Indian monsoon in Delhi, impact the radiation budget,
and have indirect effects and need to be investigated to quantify this
impact.</p> |
|---|---|
| ISSN: | 1680-7316 1680-7324 |