| Abstract : | The impacts of escalating emissions of air pollutants are now one of South Asia's most pressing
environmental concerns, particularly in Northern India. Additionally, the adjacent Himalayas lead to
the confinement of these pollutants and transport them to greater horizontal and vertical extents.. Along
the way, the primary emissions also get transformed into secondary pollutants such as ozone. However,
our understanding of episodic events and ozone variations are limited over such regions owing to the
sparsity of in-situ observations and lack of comprehensive validation of space-based retrievals. To
address this issue, we performed observations from ozonesonde and MAX-DOAS and analyzed satellite
data. Firstly, the ozone profile and total ozone retrieval from Indian satellite INSAT-3D at a high spatio-
temporal resolutions are assessed over the central Himalaya. Eventually the poor retrieval performance
is improved in INSAT-3DR using an improved radiance intercalibration and apriori information in
inverse modelling and machine learning techniques. Furthermore, AIRS/NASA, IASI/ESA,
CrIS/NOAA satellite ozone retrieval are compared against balloon-borne (ozonesonde) observations
convolved with satellite averaging kernel and a-priori. The larger differences are observed in the upper
troposphere and lower stratospheric regions. However, both balloon borne and AIRS satellite
observation over the subtropical Himalayas detected frequent tropopause folding during the winter and
spring leading to a 5 - 25% increase in ozone over the middle and upper troposphere. Whereas ozone
increased by 10 - 20% during biomass burning periods in the lower troposphere. The estimated ozone
UV radiative forcing over the Himalayas is increasing and matches well between ozonesonde (4.86
mW/m2) and OMI (4.04 mW/m2), while significant underestimation is seen in AIRS satellite RF
calculations (2.96 mW/m2) due to large uncertainties in the total ozone observations. The MAX-DOAS
and satellite observations (TROPOMI and GOME-2) captured systematic monthly variations in NO2,
SO2, HCHO, and CHOCHO vertical column densities (VCDs) depending on their sources and
photochemistry. Moreover, MAX-DOAS comparison with the satellite observation shows
underestimation up to 30% for satellite NO2 VCDs, while SO2, HCHO, and CHOCHO VCDs agree
well. Rgf sensitivity calculation shows prominent biogenic sources of VOCs during noon hours. Rfn
calculation mostly shows NOx limited ozone production regime over the Himalayan foothill. The
satellite remote sensing observations are also utilized to study the influence of the Indian lockdown on
the changes in vertical profile and columnar amounts of trace gases over the Indian region and various
contrary effects are observed.
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