Vertical and horizontal distribution of submicron aerosol chemical composition and physical characteristics across northern India during pre-monsoon and monsoon seasons
Authors: Brooks James , Allan James D. , Williams Paul I. , Liu Dantong , Fox Cathryn , Haywood Jim , Langridge Justin M. , Highwood Ellie J. , Kompalli Sobhan K. , O&amp;apos;Sullivan Debbie , Babu Suresh S. , Satheesh Sreedharan K. , Turner Andrew G. , Coe Hugh 
Published: April 2019

<p><strong>Abstract.</strong> The vertical distribution in the physical and chemical properties of
submicron aerosol has been characterised across northern India for the first
time using airborne in situ measurements. This study focusses primarily on
the Indo-Gangetic Plain, a low-lying area in the north of India which
commonly experiences high aerosol mass concentrations prior to the monsoon
season. Data presented are from the UK Facility for Airborne Atmospheric
Measurements BAe-146 research aircraft that performed flights in the region
during the 2016 pre-monsoon (11 and 12 June) and monsoon
(30 June to 11 July) seasons.</p> <p>Inside the Indo-Gangetic Plain boundary layer, organic matter dominated the
submicron aerosol mass (43&amp;thinsp;%) followed by sulfate (29&amp;thinsp;%), ammonium
(14&amp;thinsp;%), nitrate (7&amp;thinsp;%) and black carbon (7&amp;thinsp;%). However, outside the
Indo-Gangetic Plain, sulfate was the dominant species, contributing 44&amp;thinsp;%
to the total submicron aerosol mass in the boundary layer, followed by
organic matter (30&amp;thinsp;%), ammonium (14&amp;thinsp;%), nitrate (6&amp;thinsp;%) and black carbon
(6&amp;thinsp;%). Chlorine mass concentrations were negligible throughout the
campaign. Black carbon mass concentrations were higher inside the
Indo-Gangetic Plain (2&amp;thinsp;<span class="inline-formula">µ</span>g&amp;thinsp;m<span class="inline-formula"><sup>−3</sup></span>) compared to outside (1&amp;thinsp;<span class="inline-formula">µ</span>g&amp;thinsp;m<span class="inline-formula"><sup>−3</sup></span>).
Nitrate appeared to be controlled by thermodynamic processes,
with increased mass concentration in conditions of lower temperature and
higher relative humidity. Increased mass and number concentrations were
observed inside the Indo-Gangetic Plain and the aerosol was more absorbing
in this region, whereas outside the Indo-Gangetic Plain the aerosol was
larger in size and more scattered in nature, suggesting greater dust
presence, especially in north-western India. The aerosol composition remained
largely similar as the monsoon season progressed, but the total aerosol mass
concentrations decreased by <span class="inline-formula">∼50</span>&amp;thinsp;% as the rainfall arrived;
the pre-monsoon average total mass concentration was 30&amp;thinsp;<span class="inline-formula">µ</span>g&amp;thinsp;m<span class="inline-formula"><sup>−3</sup></span>
compared to a monsoon average total mass concentration of 10–20&amp;thinsp;<span class="inline-formula">µ</span>g&amp;thinsp;m<span class="inline-formula"><sup>−3</sup></span>.
However, this mass concentration decrease was less noteworthy
(<span class="inline-formula">∼20</span>&amp;thinsp;%–30&amp;thinsp;%) over the Indo-Gangetic Plain, likely due to the
strength of emission sources in this region. Decreases occurred in coarse
mode aerosol, with the fine mode fraction increasing with monsoon arrival.
In the aerosol vertical profile, inside the Indo-Gangetic Plain during the
pre-monsoon, organic aerosol and absorbing aerosol species dominated in the
lower atmosphere (<span class="inline-formula">&amp;lt;1.5</span>&amp;thinsp;km), with sulfate, dust and other scattering
aerosol species enhanced in an elevated aerosol layer above 1.5&amp;thinsp;km with
maximum aerosol height <span class="inline-formula">∼6</span>&amp;thinsp;km. The elevated concentration of
dust at altitudes <span class="inline-formula">&amp;gt;1.5</span>&amp;thinsp;km is<span id="page5616"/> a clear indication of dust
transport from the Great Indian Desert, also called the Thar Desert, in
north-western India. As the monsoon progressed into this region, the
elevated aerosol layer diminished, the aerosol maximum height reduced to
<span class="inline-formula">∼2</span>&amp;thinsp;km. The dust and sulfate-dominated aerosol layer aloft
was removed upon monsoon arrival, highlighted by an increase in fine mode
fraction throughout the profile.</p>