Effect of COVID-19 epidemic-led lockdowns on aerosol black carbon concentration, sources and its radiation effect in northeast India

ABSTRACT: The COVID-19 epidemic-led lockdown (LD) from March 25 to May 31, 2020, had a different level of impact on air quality in the ecologically sensitive region of northeast India, even though the restriction on main anthropogenic activities was expected to reduce particulate matter concentration. The daily average black carbon concentration measured at 880 nm (BC(880)) was 1.5–15.6 μg m(−3) (mean: 5.75±4.24 μg m(−3)) during the measurement period. It was 9.29±4.11 μg m(−3) during pre-LD (February 12–March 21), 4.70±0.95 μg m(−3) during LD1 (March 25–April 14), 3.41±0.56 μg m(−3) during LD2 (April 15–May 3), 3.69±1.50 μg m(−3) during LD3 (May 4–17), 2.94±0.93 μg m(−3) during LD4 (May 18–31), and 6.56±5.35 μg m(−3) during the Post-LD (June 6–July 3) of 2020. It decreased up to 68% during the lockdowns. The source apportionment based on an improved method showed a significant improvement in the contribution of BC(880) sources. The radiation effect determined by Angstrom Absorption Exponent showed that brown carbon accounted for 25% of the aerosol light absorption at 370 nm during the lockdown period. Relative humidity correlates substantially with BC(880), while rainfall, temperature, and solar radiation were negatively correlated. The bivariate analysis showed the dominance of local emissions in the BC(880) concentrations. RESEARCH HIGHLIGHTS: Black carbon concentration decreased up to 68% during the different phases of lockdown. BC associated with fossil fuel was 51–78%, and biomass burning was 22–49%. The fraction of fossil fuel and biomass burning in whole BC fallen to 0.73 and 0.65 during the lockdowns. Air quality improved by about 47–58% on the 4th and 7th day of lockdown. Brown carbon and meteorological parameters significantly impacted aerosol light absorption in this region. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s12040-022-01883-4.