Climate and health in the pristine Himalayan region are largely impacted by the transport of carbonaceous aerosols from the polluted regions of Asia and Europe. Yet, there is a scarcity of source apportionment studies that can explain diurnal scale phenomena concerning various emission sources and radiative forcing. Here, we report the first simultaneous high-resolution delineation of primary organic carbon (POC) and secondary organic carbon (SOC) content and quantify the contributions of fossil fuel combustion and biomass burning over the Central Himalayas using four-year (2014-2017) online observations. Four different methods are employed to deconvolute organic carbon (OC) into POC and SOC. Unlike SOC, POC exhibits significant unimodal diurnal variations with higher values during daytime in all four methods. These methods show intra-annual variations in POC (56-80%) and SOC (20-44%) concentrations but they agree that overall POC (4.7-8 & mu;g m-3) dominates over SOC (2.4-3.9 & mu;g m-3). The role of crop residue burning in northern India and forest fires is shown to be dominant in spring while local heatingpurpose emissions dominate in winter. Further, we show that the contribution of fossil fuel combustion (eBCff) is 3.5 times greater than that of biomass burning (eBCbb). Monthly variations in mean diurnal amplitudes of eBCff and eBCbb reveal that the differences in their amplitudes (9- 32%) is smallest during April-May, depicting the relative importance of biomass emissions at the diurnal scale during spring. The estimated daily radiative forcing shows that eBCff contributes more (16.4%) atmospheric forcing than eBCbb. Atmospheric forcing from both eBCff and eBCbb are higher (19.8 and 13.0 W m-2, respectively) in the afternoon than morning. These findings underscore the need for high-resolution data when researching aerosol-radiation interaction over the Himalayan area and are vital for developing aerosol mitigation plans.

A nationwide lockdown was imposed in India due to the Coronavirus Disease 2019 (COVID-19) pandemic which significantly reduced the anthropogenic emissions. We examined the characteristics of equivalent black carbon (eBC) mass concentration and its source apportionment using a multiwavelength aethalometer over an urban site (Ahmedabad) in India during the pandemic induced lockdown period of year 2020. For the first time, we estimate the changes in BC, its contribution from fossil (eBC(ff)) and wood (eBC(wf)) fuels during lockdown (LD) and unlock (UL) periods in 2020 with respect to 2017 to 2019 (normal period). The eBC mass concentration continuously decreased throughout lockdown periods (LD1 to LD4) due to enforced and stringent restrictions which substantially reduced the anthropogenic emissions. The eBC mass concentration increased gradually during unlock phases (UL1 to UL7) due to the phase wise relaxations after lockdown. During lockdown period eBC mass concentration decreased by 35%, whereas during the unlock period eBC decreased by 30% as compared to normal period. The eBC(wf) concentrations were higher by 40% during lockdown period than normal period due to significant increase in the biomass burning emissions from the several community kitchens which were operational in the city during the lockdown period. The average contributions of eBC(ff) and eBC(wf) to total eBC mass concentrations were 70% and 30% respectively during lockdown (LD1 to LD4) period, whereas these values were 87% and 13% respectively during the normal period. The reductions in BC concentrations were commensurate with the reductions in emissions from transportation and industrial activities. The aerosol radiative forcing reduced significantly due to the reduction in anthropogenic emissions associated with COVID-19 pandemic induced lockdown leading to a cooling of the atmosphere. The findings in the present study on eBC obtained during the unprecedented COVID-19 induced lockdown can provide a comprehensive understanding of the BC sources and current emission control strategies, and thus can serve as baseline anthropogenic emissions scenario for future emission control strategies aimed to improve air quality and climate.

In the present study, we focused on the impact of lockdown on black carbon (eBC) mass concentrations and their associated radiative implications from 01st March to 30th June 2020, over a semi-arid station, i.e., in the district of Anantapur in Southern India. The mean eBC mass concentration was observed before lockdown (01st-24th March 2020) and during the lockdown (25th March-30th June 2020) period and was about 1.74 +/- 0.36 and 1.11 +/- 0.14 mu g m(-3), respectively. The sharp decrease (similar to 35%) of eBC mass concentration observed during the lockdown (LD) period as compared with before lockdown (BLD) period, was mainly due to the reduction of anthropogenic activities and meteorology. Furthermore, during the entire LD period, the net composite forcing at the top of the atmosphere (TOA) and at the surface (SUR) varied from -4.52 to -6.19 Wm(-2) and -22.91 to -29.35 Wm(-2), respectively, whereas the net forcing in the atmosphere (ATM) varied from 17.27 to 23.16 Wm(-2). Interestingly, the amount of energy trapped in the atmosphere due to eBC is 11.19 Wm(-2) before LD and 8.56 Wm(-2) during LD. It is concluded that eBC contributes almost 43-50% to the composite forcing. As a result, the eBC atmospheric heating rate decreased significantly (25%) when compared to before lockdown days to lockdown days.

Understanding of carbonaceous aerosols from different combustion sources and their optical properties are important to better understand atmospheric aerosol sources and estimate their radiative forcing. In this study, eight organic carbon (OC) and elemental carbon (EC) sub-fractions and light absorption properties of EC are investigated using thermal/optical method and compared among six typical solid and liquid fossil fuel combustion sources (e.g., coal combustion, industry, power plant, diesel and gasoline vehicle, and ship emissions) and within each source type, with consideration of different fuel types and combustion conditions. The results indicate that OC and EC sub-fraction distributions and mass absorption efficiency of EC (MAE(EC)) are sensitive and specific to sources, fuels, combustion and operating conditions. The differences in carbon fractions and AE(EC) between solid and fossil fuel source emissions are statistically significant (p < 0.05). The average MAE(EC) from liquid fossil fuel sources (7.9 +/- 3.5 m(2)/g) are around1.5-fold higher than those from solid fossil fuels (5.3 +/- 4.0 m(2)/g). Correlation analysis indicates that light attenuation of EC positively correlates with EC1 and EC2 fractions with correlation coefficients (r) around 0.6, while negatively correlates with the percentages of OC2 and OC3 in total carbon. Inter-comparisons of distributions of carbon sub-fractions and MAE(EC) from different coal samples indicate the tested new stoves and honeycomb-like shape may contribute to lower EC emission factors but with stronger light absorptivity of EC, suggesting curbing short-lived pollutants (e.g., EC) with improvement of coal stoves and clean coal at current stage might not always result in co-benefits of air quality and climate.

Temporal and seasonal variabilities in black carbon (BC) mass concentrations, equivalent BC from fossil fuel (BCff) and wood burning (BCwb) are investigated using multiwavelength aethalometer measurements made over urban (Ahmedabad) and high altitude remote (Gurushikhar) sites in western India during 2015-2016. BC, BCff and BCwb mass concentrations exhibit strong diurnal variation over Ahmedabad compared to Gurushikhar. Annual mean contribution of BCff to total BC mass concentration is estimated to be 80 and 72% respectively over Ahmedabad and Gurushikhar, which indicates the dominance of fossil fuel emissions. To delineate the impact of BC aerosols on the Earth atmosphere radiation budget aerosol radiative forcing due to composite aerosols, and BC aerosols alone is estimated. Maximum atmospheric forcing due to BC is observed during December (15 Wm(-2)) and November (8 Wm(-2)) over Ahmedabad and Gurushikhar respectively, because BC mass concentration is highest in the respective months. Surface composite forcing is higher during postmonsoon (-27 Wm(-2)) and premonsoon (-16 Wm(-2)) over Ahmedabad and Gurushikhar respectively due to high aerosol optical depths. BC aerosols contribute similar or equal to 60% to the shortwave atmospheric forcing. The present study shows that the large spatial and temporal variation in BC mass concentrations over an urban and high altitude remote site can produce significant regional variabilities in aerosol radiative effects.