Planetary Boundary Layer Height Modulates Aerosol—Water Vapor Interactions During Winter in the Megacity of Delhi

Abstract

The megacity of Delhi is located within the Indo-Gangetic Plain (IGP) and is one of the major sources of anthropogenic air pollution. It is a continental metropolitan area in a large valley south of the Himalayas, which causes the air masses to be constrained within the IGP (Figure 1). The air masses are vulnerable to high levels of particulate matter emissions from the megacity all year round (Bhandari et al., 2020), since it is a fast-growing urban agglomeration (Jain et al., 2016;Paul et al., 2021). During winter, a strong radiative thermal inversion causes the megacity to be enveloped by a shallow planetary boundary layer (PBL) in the nighttime, resulting in high relative humidity (RH) and aerosol mass burden (Arun et al., 2018;Murthy et al., 2020). The cold, humid and polluted conditions coupled with low wind speeds make the landlocked atmosphere conducive to fog and haze formation (Dhangar et al., 2021;Dumka et al., 2019;Ojha et al., 2020). Moreover, the aerosols in Delhi have enhanced water uptake ability as reported in the companion study by Gunthe et al. (2021), which can facilitate multiphase processes for formation of aerosols and thereby cause drastic visibility deterioration.

Global organic and inorganic aerosol hygroscopicity and its effect on radiative forcing

“…1 # campaign / site name & atm. conditions lat, long time Reference 1 Amazon Tall Tower Observatory (ATTO), BRA, remote tropical rain forest 2.143° S, 59.000° W Mar 2014–Feb 2015 9 , 19 , 52 2 PRIDE-PRD2006, Guangzhou, CHN, rural site near megacity 23.548° N, 113.066° E Jul 2006 58 , 59 3 CAREBeijing-2006, Beijing, CHN, suburban site near megacity 39.515° N, 116.305° E Aug & Sep 2006 60 , 61 4 Delhi-2018 campaign, IND, strongly polluted Indo-Gangetic plain 28.588° N, 77.221° E Feb & Mar 2018 62 – 64 5 Barbados Atmospheric Chemistry Observatory (BACO), BRB, marine background with episodes of African long-range transport 13.165° N, 59.432° W Jan & Feb 2020 66 6 Puerto Rico Aerosol Cloud Interaction Study (PRACS), PRI, clean marine air with some pollution 18.390° N, 65.619° W Dec 2004 22 7 Feldberg Aerosol Characterization Experiment (FACE-2005), Taunus Observatory, DEU, rural background 50.223° N, 8.447° E Jun–Jul 2005 24 8 Cloud and Aerosol Characterization Experiment (CLACE-06), Jungfraujoch, CHE, high alpine site probing free troposphere background 46.550° N, 7.983° E Mar 2007 23 9 Amazonian Aerosol Characterization Experiment (AMAZE-08), ZF2 site, BRA, tropical rain forest 2.594° S, 60.209° W Mar 2008 16 , 67 10 European Integrated project on Aerosol Cl...…”

“…Delhi, India: The Delhi-2018 campaign was performed to characterize the aerosol hygroscopicity in a strongly polluted region in India, comprehensive aerosol measurements during the Delhi-2018 campaign in the Indo-Gangetic plain were conducted 62 , 63 . The campaign site was on the campus of the India Meteorological Department (IMD, Delhi, India), which is located in the midst of heavy traffic and small industries.…”

Black carbon aerosol reductions during COVID-19 confinement quantified by aircraft measurements over Europe

“…This is due to (I) removal processes driven by the hydrological circle, dominated by cloud processing (Ohata et al, 2016). (II) The extent of the PBL, determining the concentration and vertical extent of bulk BC and other aerosol species (S. Raj et al, 2021). (III) Seasonal variations in emission types and strengths.…”