The area around Darjeeling consists of medium grade metamorphic rocks and provides a classic example of inverted Himalayan metamorphism. The area under investigation shows upper amphibolite facies metamorphism (sillimanite-muscovite subfacies), rocks are intimately associated with the migmatites and granites. The presence of quartzite, calc-silicate rocks, graphitic schist and abundance of aluminous minerals like kyanite or sillimanite in these rocks indicate their metasedimentary character. Granetsillimanite bearing gneisses occupy most of the area of Darjeeling but not persistent throughout. Textural relationship suggests sequential growth of progressively higher-grade metamorphic minerals during D₁ and D₂ deformation. The relative X_Mg in the minerals varies in the order: biotite>staurolite>garnet, and the X_Mn decreases in the order: garnet>staurolite>biotite. The P–T evolution of these garnetsillimanite gneiss has been constrained through the use of conventional geothermobarometry, internally consistent TWEEQU programme and Perple_X software in the KFMASH model system, the combination of these three approaches demonstrates that the Darjeeling gneisses experienced peak pressure and temperature at 7.0 ± 0.3 kbar and 700 ± 30°C. The observation in this study has important bearing on the inverted metamorphism in the Himalayan metamorphic belt.

The current study discourses the impact of variation in PM$_{2.5}$ concentration on the ambient air quality of Delhi. The 24-hourly PM$_{2.5}$ concentration dataset was obtained from air quality measurement site (Anand Vihar) of Delhi Pollution Control Committee (DPCC) for the duration of April 2015 to December 2018. The annual and seasonal variability in the trend of ambient PM$_{2.5}$ along with cumulative impact of meteorological parameters have been analyzed. The overall percentage increase in annual PM$_{2.5}$ concentration, compared to National Ambient Air Quality Standards (NAAQS) guidelines, is observed to be 286.09%. The maximum concentration of fine particulate matter was recorded to be 788.6 $\mu$g/m$^{3}$ during post-monsoon season and it was found to be associated with lower ambient temperature of 21.34${\circ}$C and wind speed of 0.33 m/sec. Further, PM$_{2.5}$ concentration was found to be correlated with CO (R= 0.6515) and NH$_{3}$ (R= 0.6396) indicating similar sources of emission. Further, backward trajectory analysis revealed contribution in PM$_{2.5}$ concentration from the states of Punjab and Haryana. The results indicated that particulate pollution is likely to occur in urban atmospheric environments with low temperatures and low wind speeds.

$\bf{Highlights}$

$\bullet$ PM$_{2.5}$/PM$_{10}$ ratio was observed to be highest in November, December and January, attributing aggravated levels of particle pollution to anthropogenic sources.

$\bullet$ Seasonal analysis of PM$_{2.5}$ concentration indicated that particulate pollution was severe during post monsoon and winter months.

$\bullet$ Carbon monoxide (R = 0.6515; $R^{2}$ = 0.4244) and Ammonia (R = 0.6396; $R^{2}$ = 0.4088) were found to be correlated with PM$_{2.5}$.