Characteristics of equivalent black carbon aerosols over Doon Valley in NW Indian Himalaya during COVID-19 lockdown 2020

Abstract

Recently, black carbon (BC) has been identified as a potential transmitter for COVID-19 besides being responsible for climate change and serious health hazards. To mitigate the dreaded consequences of COVID-19 pandemic, the Government of India declared a nationwide lockdown on March 24, 2020. Accordingly, observations on equivalent black carbon (EBC) aerosols using AE 51 Aethalometer were performed during different lockdowns in Doon Valley. During April, May, June, and July, the monthly average EBC mass concentration recorded 2.12 ± 1.14 μg m^-3, 2.58 ± 1.46 μg m^-3, 2.74 ± 1.49 μg m^-3, and 2.12 ± 1.32 μg m^-3, respectively. A comparison of diurnal variation patterns with earlier studies indicates a significant reduction in EBC mass concentration levels. Bipolar NWR analysis for April and May depicts that relatively high EBC concentration was experienced with prominent south-easterly winds. The EBC concentration level during daytime was high compared to nighttime hours. Preliminary visualization of scanning electron micrographs indicates the variable morphology of aerosols. The bulk particle EDX spectral analysis indicates C, O, Na, F, Al, Si, K, Ca, and Ti elements with a dominance of C and O. Windblown dust seems to be the major contributor to the ambient aerosols. Furthermore, MODIS recorded the fire anomaly (attributed to the wheat stubble burning) starting from mid of April to early-June along the Indo-Gangetic Basin. Heavy loading of polluted aerosols was visible in CALIPSO data imageries. HYSPLIT cluster trajectories indicate that the study region is strongly influenced by the air mass transporting from the Gangetic Plain, Iran, Pakistan, Afghanistan, and Gulf region.

Keywords: Air pollution; Black carbon; CALIPSO; COVID-19 lockdown; HYSPLIT; Himalaya; MODIS.

Fig. 1

(a) Elevation map indicating the topographical setting of Dehra Doon Valley. Lesser Himalaya towards North, outer Shivalik Hills, and Indo-Gangetic basin towards South. (b) Satellite Image indicating growth of Dehradun city along with inset image pointing the site in southern Asia. (c) Cityscape, where the study site is indicated by a red arrow

Fig. 2

Diurnal variation box plots of EBC for the months of April, May, June, and July, respectively. The upper and lower boundaries of boxes indicate the 75th and 25th percentiles; the line within the box marks the median; the whiskers above and below boxes indicate the maxima and minima respectively; the red dot represent the means; and the blue line is connecting the mean value

Fig. 3

Biopolar non-parametric wind regression (NWR) plot of EBC for the months of April and May of complete lockdown period. (a) Bipolar-NWR plots of EBC mass concentration for April and May; (b) bipolar-NWR plots for daylight hours (i.e., 06:00–18:00 IST) for April and May; (c) bipolar-NWR plot for night-time hours (i.e., 18:00–06:00 IST) for April and May

Fig. 4

Identification map of scanning electron micrograph. Different elements are shown in different colors

Fig. 5

(a, b) Two examples of a randomly selected unique section of scanning electron micrograph at different resolutions along with the corresponding bulk EDX-spectrum of aerosols at the study site

Fig. 6

MODIS detected active fire events (red, purple, and blue dots representing confidence level greater than 80%, between 30 and 80% and less than 30%, respectively) plotted over elevation map

Fig. 7

The 4-cluster solutions to monthly resolved HYSPLIT backward trajectories arriving above Dehradun at 500 m agl, for the months of April, May, June, and July 2020, respectively

Fig. 8

CALIPSO lidar curtains on 17 May 2020 during 21:02:17.5–21:15:46.2 UTC over north-western Indo-Gangetic Basin in close vicinity of Dehradun valley region. Latitude and longitudes of locations along the CALIPSO ground track are given below the image and altitude (km) is indicated as the vertical axis. (a) 532 nm attenuated backscatter (km⁻¹ sr⁻¹). (b) Depolarization ratio and inset image indicate the satellite track. (c) Vertical feature mask. (d) Loading of different types of aerosols. The white circle indicates the IGB region closely associated with the study site