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

Abstract

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, r 2 = 0.80) over the entire experimental period. Good agreement was found between BC sf attributed with the Aethalometer model and the Published by Copernicus Publications on behalf of the European Geosciences Union. D. Liu et al.: Size distribution, mixing state and source apportionment of BC in LondonSP2. However, the assumed absorption Ångström exponent (α wb ) had to be changed according to the d...

Downwind evolution of the volatility and mixing state of near-road aerosols near a US interstate highway

“…25 The substantial presence of thinly-coated (fresh) rBC suggested by the SP2 data (Figure 4) is consistent with our independently measured V-TDMA observations of externally-mixed characteristics for LV mode particles (Figure 3, Figure S9). These observations are also in agreement with several recent studies that examined the mixing state of rBC from traffic emissions using a range of techniques (China et al, 2014;Kuwata et al, 2009;Liu et al, 2014;McMeeking et al, 2011b;Willis et al, 2016). Willis et al (2016) reported approximately 90% of rBC mass resides in rBC-rich particles using Soot 30…”

“…CC BY 4.0 License. reported to be uncoated or very thinly coated by Laborde et al (2013) and Liu et al (2014) using SP2 measurements in urban environments. Figure 5 compares similar V-TDMA measurements of 100 nm particles at different distances from the highway to examine the evolution of mixing state of particles with downwind transport.…”

“…assessment models. Although a number of laboratory and field studies have investigated the volatility (Kuwayama et al, 2015;May et al, 2013a, 2013b, Li et al, 2016, Biswas et al, 2007Grieshop et al, 2009) and mixing state (Tiitta et al, 2010, Liu et al, 2014, China et al, 2014, Willis et al, 2016 of traffic-emitted particles using various techniques, they have largely focused on measurements of sources or at fixed ambient location. To the best of our knowledge, no studies have been conducted to systematically explore the evolution of volatility and mixing state of near-road aerosols at different distances from the 5 roadway under diverse environmental conditions.…”

Black-carbon absorption enhancement in the atmosphere determined by particle mixing state

“…The LEO methodology is detailed in Liu et al 22 : briefly, the scattering signals for the last 200 non-BC scattering-only particles are used to determine the laser profile; the number of data points used for LEO is optimised to represent the scattering signal before the onset of particle volatilisation, and is automatically obtained by comparing the measured signal and laser profile, with uncertainty <12% (from the parametrization of the LEO fitting). Only single particles with successful LEO fitting are included in the calculation of optical properties.…”

Downwind evolution of the volatility and mixing state of near-road aerosols near a US interstate highway

“…Although a number of laboratory and field studies have investigated the volatility (Kuwayama et al, 2015;May et al, 2013a, b;Li et al, 2016;Biswas et al, 2007;Grieshop et al, 2009) and mixing state (Tiitta et al, 2010;Liu et al, 2014;China et al, 2014;Willis et al, 2016) of traffic-emitted particles using various techniques, they have largely focused on source characterization or measurements at a fixed ambient location. To the best of our knowledge, no studies have been conducted to systematically explore the evolution of volatility and mixing state of near-road aerosols at different distances from the roadway under diverse environmental conditions.…”