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The nationwide lockdown in India to curb the spread of Coronavirus disease 2019 (COVID-19) led to colossal reduction in anthropogenic emissions. Here, we investigated the impact of lockdown on surface ozone (O3) and nitrogen dioxide (NO2) over a tropical coastal station – Thumba, Thiruvananthapuram (8.5°N, 76.9°E). Daytime as well as night-time NO2 showed reduction by 0.8 (40%) and 2.3 (35%) ppbv, respectively during the lockdown period of 25–30 March 2020 as compared with the same period of previous 3 years. Unlike many urban locations, daytime surface O3 is found to be dramatically reduced by 15 ppbv (36%) with O3 production rate being lower by a factor of 3 during the lockdown. Interestingly, a feature of O3-hump during the onset of land breeze typically observed during 1997–1998 has reappeared with magnitude of 5–10 ppbv. A photochemical box model, capturing this feature, revealed that significant O3 sustained till onset of land breeze over the land due to weaker titration with NOx during lockdown. It is suggested that the transport of this O3 rich air with onset of land breeze led to the observed hump. Our measurements unravel a remarkable impact of the COVID-19 lockdown on the chemistry and dynamics of O3 over this tropical coastal environment. Supplementary Information The online version contains supplementary material available at 10.1007/s12040-021-01666-3.

Physical characterisation of $$\hbox {PM}_{2.5}$$ , $$\hbox {PM}_{10}$$ and surface ozone measured during the period from 17 July to 21 August 2012 at four strategic locations in and around the Lumbini Protected Zone, Nepal, is done to assess air quality of the region and understand qualitatively source mechanisms of these pollutants. The measurement locations are Panditarama Lumbini International Vipassana Meditation Centre, Parsahawa, Bhairahawa and Tilaurakot, representing monastic, industrial, urban and control areas, respectively. The overall average concentration of $$\hbox {PM}_{2.5}$$ at these locations is $${\sim }19\pm 12$$ , $$35\pm 13$$ , $$35\pm 11$$ and $$25\pm 6~ \upmu \hbox {g/m}^{3}$$ and of $$\hbox {PM}_{10}$$ is $${\sim }25\pm 11$$ , $$103\pm 41$$ , $$58\pm 15$$ and $$32\pm 7~ \upmu \hbox {g/m}^{3}$$ , respectively. $$\hbox {PM}_{2.5}$$ never crosses the safe limit of the National Ambient Air Quality Standards of Nepal (NNAAQS) in the monastic and control areas but either crosses the NNAAQS occasionally or remains in its vicinity at the other two locations. The $$\hbox {PM}_{10}$$ concentration frequently exceeds the safe limit in the industrial area but not in the other remaining areas. The analysis indicates the dominance of the impact of local sources and boundary layer thickness on the atmospheric loadings of the particulate matter. The daily average mixing ratio of surface ozone remains normally low at all the four observational sites although the mixing ratio of ozone at Panditarama Lumbini International Vipassana Meditation Centre is much lower than the NNAAQS but higher than that observed at Tilaurakot.

In the present study, measurements of surface ozone ( $$\hbox {O}_{3}$$ ) and its precursors (NO and $$\hbox {NO}_{2}$$ ) were carried out at a sub-urban site of Agra ( $$27{^{\circ }}10'\hbox {N}$$ , $$78{^{\circ }}05'\hbox {E}$$ ), India during May 2012–May 2013. During the study period, average concentrations of $$\hbox {O}_{3}$$ , NO, and $$\hbox {NO}_{2}$$ were $$39.6 \pm 25.3$$ , $$0.8 \pm 0.8$$ and $$9.1 \pm 6.6 \, \hbox {ppb}$$ , respectively. $$\hbox {O}_{3}$$ showed distinct seasonal variation in peak value of diurnal variation: summer $${>}$$ post-monsoon $${>}$$ winter $${>}$$ monsoon. However, $$\hbox {NO}_{2}$$ showed highest levels in winter and lowest in monsoon. The average positive rate of change of $$\hbox {O}_{3}$$ (08:00–11:00 hr) was highest in April (16.3 ppb/hr) and lowest in August (1.1 ppb/hr), while average negative rate of change of $$\hbox {O}_{3}$$ (17:00–19:00 hr) was highest in December (–13.2 ppb/hr) and lowest in July (–1.1 ppb/hr). An attempt was made to identify the $$\hbox {VOC--NO}_{\mathrm{x}}$$ sensitivity of the site using $$\hbox {O}_{3}/\hbox {HNO}_{3}$$ ratio as photochemical indicator. Most of the days this ratio was above the threshold value (12–16), which suggests $$\hbox {NO}_{\mathrm{x}}$$ sensitivity of the site. The episodic event of ozone was characterized through meteorological parameters and precursors concentration. Fine particles ( $$\hbox {PM}_{2.5}$$ ) cause loss of ozone through heterogeneous reactions on their surface and reduction in solar radiation. In the study, statistical analyses were used to estimate the amount of ozone loss.

Critical analysis of experimental surface ozone data and cloud coverage is reported over Kolkata during the period January 2011 to December 2011. Significant relationship between these two parameters is observed. Analysis shows that the trend of surface ozone concentration and cloud coverage follow opposite tendency. Some exceptional observations are also reported. Apart from this, seasonal variation of surface ozone with columnar ozone concentration, NO2 and temperature is also examined in relation with cloud cover to obtain a possible explanation of this type of variation.

Collocated measurements of the boundary layer evolution and surface ozone, made for the first time at a tropical rural site (Gadanki 13.5 ◦ N, 79.2 ◦ E, 375 m amsl) in India, are presented here. The boundary layer related observations were made utilizing a lower atmospheric wind profiler and surface ozone observations were made using a UV analyzer simultaneously in April month. Daytime average boundary layer height varied from 1.5 km (on a rainy day) to a maximum of 2.5 km (on a sunny day). Correlated day-to-day variability in the daytime boundary layer height and ozone mixing ratios is observed. Days of higher ozone mixing ratios are associated with the higher boundary layer height and vice versa .I t is shown that higher height of the boundary layer can lead to the mixing of near surface air with the ozone rich air aloft, resulting in the observed enhancements in surface ozone. A chemical box model simulation indicates about 17% reduction in the daytime ozone levels during the conditions of suppressed PBL in comparison with those of higher PBL conditions. On a few occasions, substantially elevated ozone levels (as high as 90 ppbv) were observed during late evening hours, when photochemistry is not intense. These events are shown to be due to southwesterly wind with uplifting and northeasterly winds with downward motions bringing ozone rich air from nearby urban centers. This was further corroborated by backward trajectory simulations.