The planetary boundary layer has many dynamic features, and one of them is its assimilative capacity to endure the pollution load. The present study deals with the boundary layer's load-bearing capacity changes over the past two decades and its future projection with a prelude to PM_2.5 exposure risk assessment. The surface atmospheric Ventilation parameter (SAVP) is considered to assess boundary-layer assimilative capacity. Twenty years (2000–2019), micro-meteorological parameters from radiosonde & radio wind (RS/RW) data are used to compute surface atmospheric ventilation parameter at a geopotential height of 850 hPa (approx.1.5 km). The atmospheric aerosol load is considered and extracted from the satellite data. The AirQ+ model designed by the World Health Organization (WHO) is exercised for quantifying adult mortality from chronic obstructive pulmonary disease (COPD) attributed to long-term exposure to ambient PM_2.5. Boundary-Layer Assimilative Potential (BLAP) is formulated in amalgamation with SAVP and PM_2.5 concentration for two different seasons (summer and winter) during the past 20 years (2000–2019). The prognostic analysis using the Holt-Winters method is performed based on which coming decades scenario is forecasted. By assessing the atmosphere's assimilative potential, the study aims to project health risk exposure and mortality and support in building sustainable urban planning.