Elsevier

Atmospheric Environment

Volume 244, 1 January 2021, 117857
Atmospheric Environment

Short communication
On the equivalency of Black Smoke / British Smoke and historic Elemental Carbon

https://doi.org/10.1016/j.atmosenv.2020.117857Get rights and content

Highlights

  • The Black Smoke method yields similar values for EC in coal soot and diesel exhaust.

  • The light-absorption is a generic function of aerosol loading.

  • Proportional sampling is crucial in standardisation of an aerosol measuring method.

Abstract

Elemental Carbon (EC) is a good indicator for aerosol air quality and a main component in global warming; therefore data on its historic concentration would be highly valuable for an assessment of historic levels. There were already major sources of EC in Europe in the early 20th century, but the component has only been measured since the late 1980s when analysis procedures were developed and even until recently data are scarce. However, historic EC-concentrations may be deducible from the parameter British Smoke (BrS), respectively the equivalent parameter Black Smoke (BS; in continental Europe), monitored on a daily basis since the late 1940s. Values of the parameter were based on the light-absorptivity of aerosol filter samples and these should be a measure for EC, because it is/was the dominant light-absorbing species in the countries in which the method was used. We show here that BrS was a linear proxy for EC in the standardisation studies performed in the early 1960s, based on the approach of “proportional” sampling. In this procedure filter samples were collected in parallel sampling lines with increasing filter loadings of the same aerosol. The light-absorption as a function of mass loading, of a variety of aerosol types, followed a generic curve. This graph was normalised in the “BrS-curve” by setting BrS equal to the average mass loading of the urban aerosol in the tests. Because of the proportional sampling the mass-loading of the component EC followed, by necessity, the BrS-curve as a function of light-absorption. The proportionality factor between BrS and EC is not known because the latter parameter was not analysed then. When EC became measured, the major source of urban EC was diesel exhaust. We show that filter samples with urban aerosols containing EC deriving from diesel combustion and from domestic coal use give similar light-absorption signals when probed with the BrS/BS-method. This implies that the equivalency factor between EC and BS, as assessed in recent years, must also have applied in the past. In the intercomparison studies that we found in literature the correlation factor (R2) between EC and BS was on average 0.85, but the absolute EC values were quite different because of the different analysis methods used for EC. We converted the EC-data to values comparable to the new EU-standard “EUSAAR2-TOT”. We then arrive at an equivalency factor of 0.15 (±25%) respectively 0.18 (±25%) to translate historic BS or BrS data to concentrations of EC.

Introduction

The parameter British Smoke (BrS) was monitored in the UK since the late 1940s until the year 2014. It served as a cheap proxy for aerosol mass and was measured at hundreds of urban sites in the latter half of the 20th century. Its value was based on the standardised measurement of the light-absorptivity of dedicated filter samples. The measuring method was taken up and used in a number of countries in continental Europe starting in the mid-1950s in France; the equivalent parameter was known as Black Smoke (BS) measured at a maximum number of measuring sites of around 600 in the EU in the 1980s (Zierock et al., 1985). The parameter served at that time as an official proxy for aerosol mass (concentration) in the EU.

BS/BrS is a poor proxy for aerosol mass because the method only probes the light-absorbing species in the aerosol. Such components were of a carbonaceous nature as already mentioned in the report on the standardisation of the measuring method for BrS/BS (OECD, 1964). In the earliest years of the application and during standardisation of the BrS-method in the early sixties smoke from domestic coal combustion was the dominant source of carbonaceous material (Craxford et al., 1971; Mansfield, 1989).

The light-absorbing carbon was Elemental Carbon (EC) in the countries in which the BrS/BS method was used. This was concluded from tests with a two-step combustion procedure for EC in which possible light-absorbing organic compounds were oxidised at a low temperature (Cachier et al., 1989). The blackness of the filters did not significantly decrease after the first step, but was completely gone after the second step that defined “EC”; this applied for all aerosol samples tested. Yet, it should be mentioned that EC is not a unique chemical compound but an aerosol constituent of which the mass is operationally defined according to the analysis method used, as further discussed in section 3. In recent years a standard protocol was defined in the EU: the EUSAAR2-TOT protocol (Brown et al., 2017; Cavalli et al., 2010). EC-data obtained via other methods can be converted to standard values via the results of intercomparison studies as done in this study in section 4.

A conversion BrS/BS to EC data is of interest both because of the climatic effects of aerosols and their health effects. EC has been found to be a better measure to gauge health effects of aerosols than PM10 (Janssen et al., 2011). EC data are only available starting in the late 1980s in Europe and have been so scarce until recently that they do not allow assessing long-term trends. A recovery of historic EC-concentrations and trends would therefore be helpful in assessing exposure histories and trends of combustion-derived air pollution.

In this respect the approach and data gathered in Germany are of prime relevance. The relation between EC and BS was addressed in a continuous decade long project started thirty years ago (BLUME, 2003). Its aim was to see whether BrS could serve as a cheap proxy for EC in compliance measurements for the “Ruß Gesetz” (“Soot Law”). This law set a maximum to the concentration of “soot” from diesel exhaust at road sites of 8 μg m−3. This value was extrapolated from work place limits for inhalation risks. Soot was to be represented by EC that in turn had to be measured then with an analysis procedure prescribed by the VDI (Verein Deutsche Ingenieure, Association of German Engineers; see discussion in section 4).

Apart from an application in exposure trends, historic EC concentrations are of interest because of the role played by the light-absorbing properties of EC in global warming. It was the main warming component around the middle of the previous century, according to estimates of its levels by Bond et al., (2007), that were based on estimates of fuel use and percentage of fuel converted to EC (e.g., Novakov and Hansen, 2004). Actual concentration data derived from BrS/BS measurements could validate those estimates.

In the present study we first examine whether the measuring procedure forms a basis to assess the relation between BrS/BS and EC. We start with a re-examination of the standardisation of the measuring method and the definition of BrS/BS as undertaken in the 1960s, i.e., in the years in which domestic coal combustion was the dominant light-absorbing carbon source. When EC became measured in the late 1980s the dominant EC source was traffic and the relation of EC and BrS/BS might not apply for coal smoke. We therefore first examine the comparability of light-absorption, as probed via the BS-method, of aerosol with EC from diesel soot versus that with EC from domestic coal smoke.

This is followed by an evaluation of the decade-long monitoring study in Berlin (BLUME, 2003) of the relation between EC and BS. The relation between EC and BS was also addressed in the UK and the Netherlands in longer term campaigns and we translated the results to EC-values according to the new EU-reference method EUSAAR2. Studies by Quincey (2007), Quincey et al., (2011) and Heal and Quincey (2012) in UK cities focused on assessing the relation between BS and Black Carbon (BC) in order to provide continuity because of a change in measurement of BrS in the national measuring network to that of BC.

It must be mentioned that BC is an optical proxy for EC of which the equivalency factor of the manufacturer was used that is based on EC as determined in the 1980s (Gundel et al., 1984). We used new results from the research group of Quincey in which they translate BC into EC as measured via the EU-standard method EUSAAR2. Via this primary standard for EC the results can be compared with those in the other historic intercomparison studies.

Section snippets

Measuring method

The measurement approach for BrS/BS is fully standardised. It follows a protocol developed shortly after World War 2 that remained virtually unaltered and is summarised here. Aerosol is collected through an inverted funnel as inlet of the sampling line with an upper size cut-off of 5 μm aerodynamic equivalent diameter (McFarland et al., 1982). A Whatman-1 paper filter is used as the collection substrate. The diameter of the filter and the flow rate are fixed as well as the sample period of

Light-absorption by EC from vehicle/diesel exhaust versus that by EC in coal smoke

In the earliest years of the application, and specifically during standardisation of the BrS-method in the early sixties (section 2), the carbonaceous material was primarily smoke from domestic coal combustion (Craxford et al., 1971; Mansfield, 1989) specifically at the urban background locations at which the standardisation measurements were made. The light-absorption in Paris, though, must already have been dominated by vehicle emissions (Williams, 1961). “Vehicle”-EC and “coal”-EC in an

Equivalency factor between BS and EC

After the development of measuring approaches for EC in the late 1980s the parameter was simultaneously measured with BrS in a number of studies discussed below. Before we proceed, we have to mention that the parameter Black Smoke (BS) was assessed in these studies. However, by convention, BS is fully equivalent with British Smoke (OECD, 1964), with a scaling factor of 1.17, as discussed in section 2.3. In the studies below the mass concentration of BS is used. This is based on a prescribed

Available historic data

British Smoke has been measured since the late 1940s at a number of sites that increased to over a hundred stations in the 1950s and over 600 in the 1980s with a decrease in numbers towards the end of the 20th century, with two stations still operational in 2014 (Butterfield et al., 2015). The earliest data that are publicly accessible are for 1954 (DSIR, 1957). It should be mentioned that in the first decade and a half of its use, the BrS-method was known as “daily smoke filter”. Evaluation of

Conclusions

The parameter British Smoke, an outdated proxy for aerosol mass concentration, had a sound scientific basis via the standardisation tests. The key procedure was the application of proportional sampling, by which filters with different loadings of the same aerosol were obtained. The curve relating loading and light-absorption had a generic shape. This implies that this was also the curve between the loading of EC and light-absorption. The curve was normalised by defining BrS as the loading of

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors are very grateful for the data provided by Dr. Andreas Petzold (Forschungszentrum Juelich, Germany).

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