Size distribution, mixing state and source apportionment of black carbon aerosol in London during wintertime

Abstract. Black carbon aerosols (BC) at a London urban site were characterised in both winter- and summertime 2012 during the Clean Air for London (ClearfLo) project. Positive matrix factorisation (PMF) factors of organic aerosol mass spectra measured by a high-resolution aerosol mass spectrometer (HR-AMS) showed traffic-dominant sources in summer but in winter the influence of additional non-traffic sources became more important, mainly from solid fuel sources (SF). Measurements using a single particle soot photometer (SP2, DMT), showed the traffic-dominant BC exhibited an almost uniform BC core size (D_c) distribution with very thin coating thickness throughout the detectable range of D_c. However, the size distribution of D_c (project average mass median D_c = 149 ± 22 nm in winter, and 120 ± 6 nm in summer) and BC coating thickness varied significantly in winter. A novel methodology was developed to attribute the BC number concentrations and mass abundances from traffic (BC_tr) and from SF (BC_sf), by using a 2-D histogram of the particle optical properties as a function of BC core size, as measured by the SP2. The BC_tr and BC_sf showed distinctly different D_c distributions and coating thicknesses, with BC_sf displaying larger D_c and larger coating thickness compared to BC_tr. BC particles from different sources were also apportioned by applying a multiple linear regression between the total BC mass and each AMS-PMF factor (BC–AMS–PMF method), and also attributed by applying the absorption spectral dependence of carbonaceous aerosols to 7-wavelength Aethalometer measurements (Aethalometer method).

Air masses that originated from westerly (W), southeasterly (SE), and easterly (E) sectors showed BC_sf fractions that ranged from low to high, and whose mass median D_c values were 137 ± 10 nm, 143 ± 11 nm and 169 ± 29 nm, respectively. The corresponding bulk relative coating thickness of BC (coated particle size/BC core – D_p/D_c) for these same sectors was 1.28 ± 0.07, 1.45 ± 0.16 and 1.65 ± 0.19. For W, SE and E air masses, the number fraction of BC_sf ranged from 6 ± 2% to 11 ± 5% to 18 ± 10%, respectively, but importantly the larger BC core sizes lead to an increased fraction of BC_sf in terms of mass than number (for W, SE and E air masses, the BC_sf mass fractions ranged from 16 ± 6%, 24 ± 10% and 39 ± 14%, respectively). An increased fraction of non-BC particles (particles that did not contain a BC core) was also observed when SF sources were more significant. The BC mass attribution by the SP2 method agreed well with the BC–AMS–PMF multiple linear regression method (BC–AMS–PMF : SP2 ratio = 1.05, r² = 0.80) over the entire experimental period. Good agreement was found between BC_sf attributed with the Aethalometer model and the SP2. However, the assumed absorption Ångström exponent (α_wb) had to be changed according to the different air mass sectors to yield the best comparison with the SP2. This could be due to influences of fuel type or burn phase.