Abstract. The nationwide lockdown was imposed over India from 25 March to 31 May 2020 with varied relaxations from phase I to phase IV to contain the spread of COVID-19. Thus, emissions from industrial and transport sectors were halted during lockdown (LD), which has resulted in a significant reduction of anthropogenic pollutants. The first two lockdown phases were strictly implemented (phase I and phase II) and hence were considered to be total lockdown (TLD) in this study. Satellite-based tropospheric columnar nitrogen dioxide (TCN) from the years 2015 to 2020, tropospheric columnar carbon monoxide (TCC) during 2019/20, and aerosol optical depth (AOD550) from the years 2014 to 2020 during phase I and phase II LD and pre-LD periods were investigated with observations from Aura OMI, Sentinel-5P TROPOMI, and Aqua and Terra MODIS. To quantify lockdown-induced changes in TCN, TCC, and AOD550, detailed statistical analysis was performed on de-trended data using the Student paired statistical t test. Results indicate that mean TCN levels over India showed a dip of 18 % compared to the previous year and also against the 5-year mean TCN levels during the phase I lockdown, which was found to be statistically significant (p value < 0.05) against the respective period. Furthermore, drastic changes in TCN levels were observed over hotspots, namely eastern region and urban cities. For example, there was a sharp decrease of 62 % and 54 % in TCN levels compared to 2019 and against 5-year mean TCN levels over New Delhi with a p value of 0.0002 (which is statistically significant) during total LD. The TCC levels were high in the northeast (NE) region during the phase I LD period, which is mainly attributed to the active fire counts in this region. However, lower TCC levels are observed in the same region due to the diminished fire counts during phase II. Further, AOD550 is reduced over the country by ∼ 16 % (Aqua and Terra) from the 6-year (2014–2019) mean AOD550 levels, with a significant reduction (Aqua MODIS 28 %) observed over the Indo-Gangetic Plain (IGP) region with a p value of ≪ 0.05. However, an increase in AOD550 levels (25 % for Terra MODIS, 15 % for Aqua MODIS) was also observed over central India during LD compared to the preceding year and found significant with a p value of 0.03. This study also reports the rate of change of TCN levels and AOD550 along with statistical metrics during the LD period.

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Temporally broadened ultrashort laser pulses impart thermal effects in the system under study even with nJ energies at high repetition rates (HRRs). The absorption induced accumulated thermal effects are prominent in materials with higher absorption. A numerical and experimental study with the help of a Z-scan has been carried out to estimate the nonlinear thermo-optic parameters of graphene by varying the input pulse duration ranging from sub-ps (100 fs) to a few ps (2.3 ps) at 780 nm. It is well known that for longer input pulses (in ns range), the thermal effects are high. In this study, we have found that the thermal effects in the sub-ps to few ps regime also result in a higher value of the thermo-optic nonlinear refractive index. Finite difference method calculations have been used to simulate the effect on the nonlinear phase shift under HRR fs pulses. The study has been found to be consistent with experiments. The experimental results along with the numerical simulation provide the importance of the contribution of the thermally induced refractive index change under HRRs within a few ms after excitation.