Variabilities in Trace Gases and Meteorological Parameters over the Himalayan and Associated Regions

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. Among the various pollutants, atmospheric ozone has a unique vertical distribution that plays different roles at different altitudes. The useful stratospheric ozone absorbs harmful UV radiation, while tropospheric ozone, a potent greenhouse gas, adversely affects living beings and vegetation. Its role in atmospheric chemistry, radiative forcing, and the processes that regulate its variation are distinct at different altitudes. However, our understanding of episodic air pollution and ozone variations over these regions are limited owing to the sparsity of in-situ observations fraught with difficulties due to the complex terrains and lack of comprehensive validation of data retrievals from space-based sensors. In this thesis, we performed observations from ozonesonde and MAX-DOAS and analyzed satellite data to characterize the emission sources, transport pathways, vertical profiles, and interannual variabilities of ozone and other trace gases over Northern India, in the particular Himalayas and foothill region. In recent decades, monitoring trace gases via satellite-based remote sensing has gained vast importance owing to their near-global and higher temporal coverage. The Indian geostationary satellite INSAT-3D/3DR has capability in this direction for India. Apart from the meteorological sounding INSAT-3D/3DR sounder is also incorporated with a 9.7 μm strong ozone absorption channel, which can provide very high spatio-temporal data of ozone over India. Firstly, we assessed the INSAT-3D retrieved total ozone and ozone profiles with respect to the ozonesonde observations from Nainital in the central Himalayas. Apart from some nominal differences, INSAT-3D is able to capture the ozone peak and gradient successfully with negligible bias in the stratosphere and a somewhat larger bias in the troposphere. The total ozone column from INSAT-3D showed a maximum difference of 22 DU with ozonesonde-derived total ozone column and OMI. In continuation, we developed an improved retrieval algorithm for INSAT-3D/DR ozone to mitigate such differences. The algorithm uses a modified scheme for radiance bias correction and improved a-priori information in the optimal estimation algorithm. The newly retrieved INSAT-3D/DR ozone product shows very high statistical corroboration with TROPOMI and IASI ozone and can produce seasonal and latitudinal variation effectively compared to old retrieval. In the successive work, we validated satellite retrieved ozone profile (i.e., AIRS/AQUA, IASI/MetOp, CrIS/NPP) against balloon-borne (ozonesonde) observations convolved with satellite averaging kernel and a-priori information. Our assessment shows that the AIRS, IASI, and CrIS retrieved ozone profiles over the central Himalayas show higher statistical errors in the upper and lower stratosphere (UTLS) regions, especially during the winter and spring seasons. At the same time, ozonesonde and satellite ozone observation over the subtropical Himalayas detected frequent tropopause folding during the winter and spring leading to a 5 - 25% increase in ozone over the 2 - 16 km altitude range. In contrast, ozone increased by 10 - 20% during biomass burning periods in the 2 - 6 km altitude range is observed. The analysis of ozone UV radiative forcing over the Himalayan surface shows an increase of RF since the Anthropocene, that matches well between ozonesonde (4.86 mW/m2) and OMI (4.04 mW/m2), while significant underestimation is seen in AIRS RF estimate (2.96 mW/m2) due to large uncertainties in the total ozone data. In addition, NO2, SO2, HCHO, and CHOCHO vertical column densities (VCDs) observations are performed using MAX-DOAS over Pantnagar, a semi-urban Himalayan foothill site and compared with satellite observations. The MAX-DOAS and satellite observations (TROPOMI/S-5P and GOME-2/MetOp) reasonably captured systematic monthly variations of these trace gases. In general, larger VCDs are observed during the spring and winter months. MAX-DOAS VCDs comparison with the TROPOMI and GOME-2 shows underestimation up to 30% for satellite NO2 VCDs, while SO2, HCHO, and CHOCHO VCDs agree reasonably well. Rgf sensitivity calculation over Himalayan foothills shows prominent biogenic sources of VOCs during noon hours. Rfn calculation mostly shows NOx limited ozone production regime except during the winter when a signature of transition and VOCs limited regime is observed. 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.