Accurate estimation of black carbon (BC) from the widely used optical attenuation technique is important for the reliable assessment of their climatic impact. The optical instruments use Mass Absorption Cross- (MAC) for converting light attenuation records to BC mass concentrations and Aethalometer is a widely used optical instrument for BC estimation. Several studies have shown large variability in MAC values. It is thus necessary to examine the accuracy and consistency of MAC values obtained using Aethalometer over distinct geographic locations and seasons. In the present study, MAC values are derived using simultaneous observations (2014-2017) from an EC-OC analyzer and an Aethalometer (AE-42) over a high altitude central Himalayan site at Nainital (29.4(o)N, 79.5(o)E, 1958 a.m.s.l). The observations reveal that the annual mean value of MAC (5.03 +/- 0.03 m(2)g(- 1) at 880nm) is significantly lower than the constant value used by the manufacturer (16.6 m(2)g(- 1) at 880nm). The estimated MAC values also showed significant seasonal variation, spanning over a range from 3.7 to 6.6 m(2)g(- 1). It is found that the seasonal variability of elemental carbon (EC), air mass variation and meteorological parameters play an important role in the changes in MAC values over this region. Multi-wavelength determination of MAC shows the contribution of absorption by species other than EC at shorter wavelengths. MAC does not show a clear diurnal variation, unlike EC and absorption coefficient. The slope of EC vs. corrected equivalent black carbon (eBC) showed a significant improvement during all seasons when compared with uncorrected eBC. This lends credibility to the fact that the use of site-specific MAC leads to more reliable estimates of eBC over the central Himalayan region. It is found that, instead of using the site specific MAC value, had we used the one supplied by the instrument, we would have underestimated the radiative forcing by about 7.8Wm(- 2) which amounts to a reduction by 24 %.

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.

Equivalent black carbon (EBC) was measured with a seven-wavelength Aethalometer (AE-31) in the Urumqi River Valley, eastern Tien Shan, China. This is the first high-resolution, online measurement of EBC conducted in the eastern Tien Shan allowing analysis of the seasonal and hourly variations of the light absorption properties of EBC. Results showed that the highest concentrations of EBC were in autumn, followed by those in summer. The hourly variations of EBC showed two plateaus during 8:00-9:00 h local time (LT) and 16:00-19:00 h LT, respectively. The contribution of biomass burning to EBC in winter and spring was higher than in summer and autumn. The planetary boundary layer height (PBLH) showed an inverse relationship with EBC concentrations, suggesting that the reduction of the PBLH leads to enhanced EBC. The aerosol optical depths (AOD) over the Urumqi River Valley, derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) data and back trajectory analysis, showed that the pollution from Central Asia was more likely to affect the atmosphere of Tien Shan in summer and autumn. This suggests that long-distance transported pollutants from Central Asia could also be potential contributors to EBC concentrations in the Urumqi River Valley, the same as local anthropogenic activities.

In this study, real-time absorption coefficients of carbonaceous species in PM2.5 was observed using a dual-spot 7-wavelength Aethalometer between November 1, 2016 and December 31, 2017 at an urban site of Gwangju. In addition, 24-hr integrated PM2.5 samples were simultaneously collected at the same site and analyzed for organic carbon and elemental carbon (OC and EC) using the thermal-optical transmittance protocol. A main objective of this study was to estimate mass absorption cross (MAC) values of black carbon (BC) particles at the study site using the linear regression between aethalometer-based absorption coefficient and filter-based EC concentration. BC particles observed at 880 nm is mainly emitted from combustion of fossil fuels, and their concentration is typically reported as equivalent BC concentration (eBC). eBC concentration calculated using MAC value of 7.77 m(2)/g at wavelength of 880 nm, which was proposed by a manufacturer, ranged from 0.3 to 7.4 mu g/m(3) with an average value of 1.9 +/- 1.2 mu g/m(3), accounting for 7.3% (1.5 similar to 20.9%) of PM2.5. The relationship between aerosol absorption coefficients at 880 nm and EC concentrations provided BC MAC value of 15.2 m(2)/g, ranging from 11.4 to 16.2 m(2)/g. The eBC concentrations calculated using the estimated MAC of 15.2 m(2)/g were significantly lower than those reported originally from aethalometer, and ranged from 0.2 to 3.8 mu g/m(3), with an average of 1.0 +/- 0.6 mu g/m(3), accounting for 3.7% of PM2.5 (0.8 similar to 10.7%). Result from this study suggests that if the MAC value recommended by the manufacturer is applied to calculate the equivalent BC concentration and radiative forcing due to BC absorption, they would result in significant errors, implying investigation of an unique MAC value of BC particles at a study site.