Anthropogenic Aerosol Contribution to Urban Environments. Aerosol Optical Depth Analysis During COVID Pandemic and Post-Pandemic Periods

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Abstract
This study presents a multi-instrument remote sensing approach to characterize the optical properties and vertical distribution of aerosols within the planetary boundary layer (PBL) over New York City during the COVID-19 pandemic and post-pandemic periods. The analysis focuses on changes in aerosol loading associated with variations in anthropogenic activity and examines the relationship between column-integrated aerosol optical properties, vertically resolved aerosol distributions, and near-surface fine particulate matter (PM2.5). Continuous ceilometer observations are used to characterize profiles of attenuated backscatter and to investigate aerosol vertical structure and variability within the PBL. These measurements are complemented by sun-photometer observations of column-integrated aerosol optical properties, radiosonde profiles of atmospheric thermodynamic structure, and ground-based PM2.5 measurements. The synergistic use of these datasets provides a more comprehensive characterization of aerosol optical and physical properties under varying meteorological and boundary-layer conditions than can be obtained from individual instruments alone. In addition, air-mass back trajectories calculated using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model are employed to investigate the potential influence of local, regional, and long-range aerosol transport on observed aerosol variability over New York City. Comparisons between the pandemic and post-pandemic periods reveal substantial differences in aerosol loading and pollution levels, highlighting the combined influence of changes in anthropogenic emissions, PBL dynamics, meteorological conditions, and transported aerosol on the urban aerosol environment. The results demonstrate the value of integrating vertically resolved active remote sensing with columnar and surface observations and atmospheric transport modeling for assessing temporal changes in urban aerosol distributions and their controlling processes.

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