Standard Black Smoke data in developing countries as (a first) proxy for "Elemental Carbon"

Black Smoke (BS) served and still serves in several countries as official measure for Particulate Matter (PM) because of its easy determination via the blackness of filter samples. However, BS is a poor proxy for total PM but a rather good equivalency quantity for light absorbing "Elemental Carbon" deriving from diesel emissions (ten Brink et al., 2021). The latest WHO Report on Air Quality (WHO, 2021) recommends monitoring of Elemental Carbon (EC), but does not prescribe a specific measurement method, so using the BS method to obtain EC concentrations remains an inexpensive option in countries where BS is still measured and expensive direct EC measurements are not easily available. Yet there is a caveat: national regulations demand the use of the measurement protocol given in ISO-9835 (ISO, 1993), which prescribes optical reflectance measurements on filter samples. We analysed this report and found a series of critical issues. It only mentions a dimensionless "Black Smoke Index" (BSI), whereas in official legislation BS must be provided as mass concentration (in μg m^-3). In addition ISO-9835 gives no reference for the conversion of the measured light absorbance to BSI; in fact the report does not even provide a proper definition of BS. Therefore we strongly argue to use the official EU-manual by Christolis et al. (1992) for BS determination when and where ever BS is still measured. This manual prescribes the conversion of light absorption as measured with the standard EEL 43 Smoke Stain Reflectometer for samples on the standard substrate of Whatman-1 cellulose fibre filters in monitoring networks. The BS concentrations obtained following this manual can then be provisionally converted to EC with the factor of 0.15 ± 25 % found by ten Brink et al. (2021) to obtain EC values as if measured according to the European reference EUSAAR2-TOT protocol (EN 16909:2017) in the countries still measuring BS until local equivalency is established with proper protocols.