PM₂.₅ black carbon aerosols in ambient air of Raipur, India, were monitored for the first time during the peak winter season 1⁻³1 December 2014. The monthly (December) average of BC mass concentration was 23.03±7.50 µgm⁻³ and the hourly average varied from 2.55 to 81.81 µgm⁻³. Daily average maximum was observed on 23 December (35.25 µgm⁻³) and minimum (5.76 µgm⁻³) on 31 December. Daytime average was 17.40 µgm⁻³ and nighttime average was 26.99 µgm⁻³, with 1.55 being the monthly average ratio for nighttime and daytime concentrations. Monthly average diurnal variation showed two distinct peaks, at morning with 39.84 µgm⁻³ (7:00-8:00 IST (Indian Standard Time)), and at night with 35.55 µgm⁻³ (00:00 -1:00 IST); and two distinct valleys, with lowest BC concentration (7.11 µgm⁻³) during late afternoon (15:30-16:30 IST), and during early morning (3:30-4:30 IST) with BC mass concentration of 27.04 µgm⁻³, consistent with the nighttime average. A clear negative correlation between BC mass concentration and wind speed (r = -0.75), relative humidity (r = -0.30) and temperature (r = -0.19) was observed. The back-trajectory at surface level (50 m and 500 m) mostly originated from northeastern India, and for higher altitude (5,000 m), from Middle-Eastern and Arabian countries, except during the last week of December (Indian Ocean). The ratio and difference between BC mass measured at 370 nm and 880 nm showed biomass burning being the dominant cause at night.
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