Size distribution, mixing state and source apportionment of black carbon aerosol in London during wintertime
Size distribution, mixing state and source apportionment of black carbon aerosol in London during wintertime
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DOI：
10.5194/acp-14-10061-2014
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发表时间：
2014-01-01
影响因子：
6.3
通讯作者：
Zotter, P.
中科院分区：
文献类型：
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作者：
Liu, D.;Allan, J. D.;Zotter, P.
在伦敦清洁空气（ClearfLo）项目期间，在2012年冬季和夏季对伦敦城市站点的黑碳气溶胶（BC）进行了表征。正矩阵因子（PMF）的有机气溶胶质谱测量的高分辨率气溶胶质谱仪（HR-AMS）的因素表明，交通占主导地位的来源在夏季，但在冬季的影响，额外的非交通来源变得更加重要，主要是从固体燃料源（SF）。使用单颗粒烟尘光度计（SP2，DMT）的测量显示，交通主导的BC表现出几乎均匀的BC核尺寸（D-c）分布，在D-c的可检测范围内具有非常薄的涂层厚度。然而，D-c的粒度分布（冬季项目平均质量中值D-c = 149 +/- 22 nm，夏季为120 +/- 6 nm）和BC涂层厚度在冬季变化显著。开发了一种新的方法属性的BC数浓度和质量丰度从交通（BCtr）和SF（BCsf），通过使用2-D直方图的颗粒光学特性作为BC核心大小的函数，测量的SP2。BCtr和BCsf显示出明显不同的D-c分布和涂层厚度，与BCtr相比，BCsf显示出更大的D-c和更大的涂层厚度。来自不同来源的BC颗粒也通过应用BC总质量和每个AMS-PMF因子之间的多元线性回归（BC-AMS-PMF方法）进行了分配，并通过将碳质气溶胶的吸收光谱依赖性应用于7波长Aethalometer测量（Aethalometer方法）进行了归因。来自西风（W），东南风（SE）和东风（E）扇区的空气质量显示BCsf分数从低到高，其质量中值D-c值分别为137 +/- 10 nm，143 +/- 11 nm和169 +/- 29 nm。对于这些相同的区段，BC的相应的整体相对涂层厚度（涂层颗粒尺寸/BC芯-Dp/D-c）为1.28 +/- 0.07、1.45 +/- 0.16和1.65 +/- 0.19。对于W、SE和E气团，BCsf的数量分数范围分别为6 +/- 2%至11 +/- 5%至18 +/-10%，但重要的是，较大的BC核心尺寸导致BCsf的质量分数比数量分数增加（对于W、SE和E气团，BCsf质量分数分别为16 +/-6%、24 +/- 10%和39 +/-14%）。当SF源更显著时，还观察到非BC颗粒（不含BC核的颗粒）的增加的分数。在整个实验期间，通过SP2方法的BC质量归属与BC-AMS-PMF多元线性回归方法（BC-AMS-PMF：SP2比率= 1.05，r（2）= 0.80）一致。BCsf与Aethalometer模型和SP2之间存在良好的一致性。然而，假设的吸收埃指数（α（α））必须根据不同的空气质量扇区进行改变，以产生与SP2的最佳比较。这可能是由于燃料类型或燃烧阶段的影响。
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 (BCtr) and from SF (BCsf), by using a 2-D histogram of the particle optical properties as a function of BC core size, as measured by the SP2. The BCtr and BCsf showed distinctly different D-c distributions and coating thicknesses, with BCsf displaying larger D-c and larger coating thickness compared to BCtr. 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 BCsf 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 -Dp/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 BCsf ranged from 6 +/- 2% to 11 +/- 5% to 18 +/- 10 %, respectively, but importantly the larger BC core sizes lead to an increased fraction of BCsf in terms of mass than number (for W, SE and E air masses, the BCsf 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 BCsf attributed with the Aethalometer model and the SP2. However, the assumed absorption Angstrom exponent (alpha(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.