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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.

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This study examines hilly terrain's effect on black carbon (BC) dispersion. The apportionment of distinct sources obtained by a two-component mixing model (Aethalometer: AE-33) using improved radiative transfer equations showed the dominance of tra c-derived black carbon (BCFF) emissions in the study region. The AERMOD was used to model BC emissions from moving tra c as line source and parking lot as area source using observational and WRF-processed meteorology for the winter (Jan to Mar 2020). The model results showed that the BC levels substantially vary with local meteorological conditions, tra c volume, and composition. The hilly terrain obstructs the winds and develops a negative pressure loading to a vacuum on the other side of the hills, which promotes the accumulation of emissions, causing high BC concentrations. The pockets of higher concentration were seen at the locations where steep slopes were associated with low winds (<1 m-s−1) and hill fogs. The AERMOD model, after statistical evaluation against the observational datasets, has been applied to study the reduction in BCFF concentrations due to the implementation of Indian emission norms as mitigation measures, i.e., BS-IV (equivalent to Euro 4) and BS-VI (equivalent to Euro 6). It was found that the BCFF concentrations for BS-IV and BS-VI reduced by 35% and 75%, respectively. The model was also used to study the contribution of the different vehicular categories to BC concentration. HighlightsFirst time, the AERMOD model was employed to study the dispersion of fossil-fuel-originated black carbon into hilly terrain.Low wind with hill fog facilitated the rise of BC concentration in the complex terrain.On-site meteorological data produce more reliable dispersion calculations than WRF modeled data.Application of the model to study the impacts showed that tra c-derived BC for BS-IV decreased by 35% and 75% for BS-VI emission norms.

This study examines hilly terrain’s effect on black carbon (BC) dispersion. The apportionment of distinct sources obtained by a two-component mixing model (Aethalometer: AE-33) using improved radiative transfer equations showed the dominance of traffic-derived black carbon (BCFF) emissions in the study region. The AERMOD was used to model BC emissions from moving traffic as line source and parking lot as area source using observational and WRF-processed meteorology for the winter (Jan to Mar 2020). The model results showed that the BC levels substantially vary with local meteorological conditions, traffic volume, and composition. The hilly terrain obstructs the winds and develops a negative pressure loading to a vacuum on the other side of the hills, which promotes the accumulation of emissions, causing high BC concentrations. The pockets of higher concentration were seen at the locations where steep slopes were associated with low winds (<1 m-s−1) and hill fogs. The AERMOD model, after statistical evaluation against the observational datasets, has been applied to study the reduction in BCFF concentrations due to the implementation of Indian emission norms as mitigation measures, i.e., BS-IV (equivalent to Euro 4) and BS-VI (equivalent to Euro 6). It was found that the BCFF concentrations for BS-IV and BS-VI reduced by 35% and 75%, respectively. The model was also used to study the contribution of the different vehicular categories to BC concentration.