Aerosol concentrations and spectral characteristics critically influence the air quality and hence mortality rates(Apte et al. 2015), as well as profoundly impact the Earth’s radiation budget and climate(Lelieveld et al. 2015, Ramanathan et al. 2001). The tropical Indian region is a global hotspot of enhanced aerosol loadings and among few regions of the world where aerosol concentration has been constantly increasing(Babu et al. 2013, Krishna Moorthy et al. 2013, Ramachandran et al. 2020). Aerosol dynamics over India manifest complex influences of strong anthropogenic and biomass burning emissions and diverse natural processes(Babu et al. 2016, Ojha et al. 2020, Singh et al. 2020a). Additionally, warm and humid tropical climatic conditions and mesoscale coastal meteorology also have considerable impacts on the spatial and temporal distributions of aerosols and their spectral characteristics(Moorthy et al. 2003, Ratnam et al. 2018). Despite numerous experimental efforts, the role of coastal meteorology and regional emissions remained unclear over this part of the world owing to large biases between chemical transport models and in-situ observations resulting from lack of observations, which will delineate the impact of anthropogenic emissions. Further, the aerosol concentrations remain typically elevated over urban megacities and downwind, which inhibit the sampling of variations in the regional background in order to observationally constrain the effects of emissions. At times, the high-altitude sites have been recommended as a proxy to the background/natural pollutant concentrations. The differences between emission rates, boundary layer dynamics and impacts, rate of transport and removal, and very strong differences between meteorology, however, precludes such an assumption. Therefore, to understand the influence of human-induced pollution on climate, near-natural condition measurements are essential.
While the need for baseline measurements is extremely important, it is, however, very impractical to shut down anthropogenic emissions significantly under the business-as-usual scenario. Recently, an unprecedented lockdown imposed to contain the spread of the COVID-19 pandemic over various regions of the globe, including India, offered such a rare situation. Several studies reported that comprehensive lockdowns implemented for extended periods dramatically impacted the air quality globally(Kroll et al. 2020, Rodríguez-Urrego &Rodríguez-Urrego 2020, Venter et al. 2020) and regionally(Agarwal et al. 2020, Berman &Ebisu 2020, Menut et al. 2020, Sharma et al. 2020, Singh et al. 2020b). Stringent lockdown forced the public to stay indoors, resulting in a near-zeroing of the emissions from fossil-fuels and other anthropogenic burning sources over different regions of the globe. India also observed a very comprehensive and stringent lockdown restricting anthropogenic activities for an extended period: 24th March to 30th June 2020. This lockdown offered a unique opportunity to investigate the near baseline variations in various pollutants over the tropical Indian conditions with negligible modulations by anthropogenic factors. Several studies have been carried out to study the lockdown impacts on surface-level air quality as well as the tropospheric columns of pollution loadings over the Indian region(Dhaka et al. 2020, Kumari &Toshniwal 2020, Lokhandwala &Gautam 2020, Mahato et al. 2020, Naqvi et al. 2020, Pandey &Vinoj 2021, Sharma et al. 2020, Singh &Chauhan 2020, Singh et al. 2020b). These studies showed a remarkable decline in the levels of various gas-phase and particle-phase trace species across the Indian region. Tropical megacity and India’s capital - Delhi, observed a counter-intuitive enhancement in fine particulate matter, which is attributed to a remarkable interplay between meteorology and baseline emissions(Dhaka et al. 2020). This, along with few other studies(Li et al. 2020, Su et al. 2020, Wang &Zhang 2020), highlight the need to investigate other distinct geographical locations with their own peculiar meteorology in order to assess the range of effects such interactions can offer. Tropical coastal setup offers one such distinct atmosphere where the effects of coastal meteorology need to be delineated in the observed aerosol dynamics. Studies on the impacts of anthropogenic and biomass burning sources on BC with segregated continental and marine air masses have still been lacking.
In the discussed scenario, we have performed continuous measurements of aerosol spectral absorption and meteorological parameters at a tropical urban coastal location, Chennai (12°59'26.5"N, 80°13'51.8"E, 6 m above mean sea level) in India (Fig. 1a,) from 21-January-2020 to 30-June-2020. The Chennai megacity region experiences typical mesoscale coastal dynamics, comprising land and sea breezes, transporting continental and marine air to the sampling site (see, section - Methods for additional details). The BC measurements have been segregated into BC in the marine air mass and BC in the continental air mass by combining the air mass history and the onset of sea-breeze and land-breeze(Hari Prasad et al. 2019, Raj et al. 2002, Reddy et al. 2020). Analysis has been performed for both pre-lockdown (21st January – 23rd March 2020) and lockdown (24th March − 30th June 2020) periods, separately. Figure 1a shows a representative schematic of the continental and marine air masses arriving at the measurement site. The continental air masses travel and meander over more than 600 km of the landmass to reach the measurement site and therefore are well exposed to the regional emissions of peninsular India before reaching the measurement site. In contrast, the marine air masses reside entirely over the Bay of Bengal and reach the measurement site without any considerable and strong anthropogenic influences.