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Radiometric observations of relative humidity profile at Kolkata show a significant fall at around 1 to 2 km height during convective rain events. An extensive investigation shows that the fall of relative humidity is not seen during calm conditions but is strongly related to the characteristics of temperature lapse rate profiles. Moreover, the phenomenon may have strong association with boundary layer structure. The reason for such anomalies in the planetary boundary layer humidity profile might be due to the release of latent heat at the mentioned altitude. The abundance of pollutant aerosols in urban regions has also been found to contribute to this relative humidity anomaly. It has also been reported that this boundary layer relative humidity is accompanied by high latent heat release and condensation of vapour to liquid which is not much prominent in other rain types as observed in stratiform rain. Hence, convective rain produces some unique boundary layer characteristics which have also been partially supported with allied satellite and multi-station observations. [ABSTRACT FROM AUTHOR]

• Pre-monsoon rainfall over Kolkata is categorized on the basis of time of occurrence. • Rain events are categorized as expected (14:30 IST-18:30 IST) and delayed (after 18:30 IST). • Small value of the IWV to LWP ratio results in significant precipitation for the delayed events. • Ground level temperature of the delayed events exhibits 1 K higher values than the expected ones. • Variation of CAPE shows discernible increasing trend prior to two types of rain events. The annual rainfall amount over Gangetic West Bengal is predominantly determined by monsoon (June to September) and pre-monsoon (March to May) rain events. Consequently, such seasonal precipitation is directly linked to the socio-economic advancement of the area. In this paper, some characteristics of pre-monsoon rainfall over a tropical urban location, Kolkata have been explored utilizing ground-based observations. Pre-monsoon rainfall over Kolkata is characterized by convective rain events and thunderstorms, mostly occurring from afternoon to night (15:30 IST − 21:30 IST) span. Emphasis has been given to the set-in time of pre-monsoon rain events, based upon which they are classified as expected (14:30 IST − 18:30 IST) and delayed (after 18:30 IST). Efforts have been made to assess the sustaining atmospheric conditions (in terms of temperature, water vapour, liquid water content and instability parameter) associated with such rain events prevailing over two non-overlapping time spans of pre-monsoon season. The mean diurnal variations of integrated water vapour (IWV), liquid water path (LWP), and instability parameter, namely, convective available potential energy (CAPE) during the expected and delayed rain event dates show distinct intensity in the increasing trends prior to the respective rainfall peaks. It has been observed that for the delayed events, the temperature difference between ground and 2000 m altitude begins to fall at a later part of the evening in comparison with the expected ones. Overall the investigated parameters exhibited distinguishable nature of variations for the two classes of rain events over the study location. [ABSTRACT FROM AUTHOR]

Near surface aerosol black carbon (BC) concentration data were collected using a seven channel Aethalometer (AE31) during June 2012–May 2013 in Kolkata (22° 34′E, 88° 22′N), a metropolitan city located near the land–ocean boundary in Eastern India. BC concentration shows a prominent seasonal and diurnal variation associated with the meteorological parameters. The mean BC concentration varied from 5 μg/m 3 to 27 μg/m 3 seasonally. The variation of BC mass concentration and its significant association with atmospheric parameters such as temperature profile, relative humidity and wind speed have been studied. Moreover, the influence of the transported air masses on BC concentration at different seasons has also been discussed. An estimation of Angstrom exponent discloses that fossil fuel combustion is a major source of BC at this location. [ABSTRACT FROM AUTHOR]

Atmospheric pollutants (NO 2 , SO 2 , PM10, BC, CO, surface O 3 ), emitted during fireworks display, have significant effects on meteorological parameters like air temperature, relative humidity, lapse rate and visibility in air over Kolkata (22°65′ N, 88°45′ E), a metropolitan city near the land–ocean boundary, on the eve of Deepawali festival when extensive fireworks are burnt. Long-term trend (2005–2013), indicates that the yearly average concentrations of both primary and secondary air pollutants have increased, exceeding the National Ambient Air Quality Standard (NAAQS) limit, on the respective Deepawali days. Short-term study (2012–2013) during the festival shows that the average pollutant concentrations have increased too compared to normal days. This study also reveals the immediate effects of the increased air pollutants on the boundary layer meteorology. PM10 has been found to be the most dominant atmospheric pollutants during this period. As a result of an increase in atmospheric heat content with elevated surface air temperature, a significant increase in the environmental lapse rate bears a signature of the influence of pollutants on the boundary layer temperature profile. A change in the diurnal pattern of relative humidity as well as in the vertical temperature profile is due to the change of the lapse rate during the festival days. Thus, the atmospheric pollutants during this festival over the urban region have significant effect on the boundary layer meteorology with bearings on environmental hazards. [ABSTRACT FROM AUTHOR]

Mass concentration of black carbon (BC) near the surface (within the planetary atmospheric boundary layer) was measured using a seven-channel aethalometer at Kolkata, a metropolitan city in the Indian tropical region, during the period from June 2012 to May 2013. The diurnal variation of BC concentration shows a prominent increase in the morning and evening hours, an usual feature seen over continents. However, an anomalous feature of the BC variation is observed subsequent to rain events. On normal days, the BC mass concentration during noon and early afternoon hours remains around 8000 ng/m³ at Kolkata. However, after the occurrence of isolated thundershowers, interestingly, the BC concentration increases (rather than decreasing due to washout) and at times reaches above 20,000 ng/m³ during noon and early afternoon hours. This increase is found to be associated with the formation of local temperature inversion within the atmospheric boundary layer during and after the occurrence of rain, which would suppress or inhibit vertical mixing and dispersion in contrast to non-rainy days. Results are presented to indicate the above-mentioned behaviour of BC concentration. [ABSTRACT FROM AUTHOR]

In recent times, a significant number of studies on the composition and sources of fine particulate matters and volatile organic compounds have been carried out over Delhi, either initiated by or in association with the researchers from the Indian Institute of Technology Kanpur (IIT Kanpur), in collaboration with researchers from within and outside India. All these studies utilized highly time-resolved, campaign-mode observations made with state-of-the-art instrumentation during the late winter months (mid-January to March) of 2018. Individually, each of these studies were rigorous in nature, containing explicit detailing about different types of ambient air pollutants in Delhi such as organic aerosols, inorganic elements, metals, carbonaceous aerosols, and volatile organic compounds. This study consolidates the extremely useful knowledge on source attribution of these air pollutants in the Delhi National Capital Region currently contained in these fragmented studies, which is vital to further enhancing our understanding of composition, characteristics, and sources of air pollutants over Delhi, as well as to designing appropriate mitigation measures tailored to local specifics. [ABSTRACT FROM AUTHOR]