Smoke shade as a historic proxy for elemental carbon

The blackness of aerosol filter samples, meant as a proxy for particulate mass concentration, is the earliest parameter for which
monitoring data exist. The smoke shade method (developed in the late 1910s) was the standardised approach to collect samples.
The blackness was visually appraised by a comparison with a set of reference sheets covered with an increasing number of layers
(shades) of a suspension of carbon black (ink). Samples were assigned a shade number (SN) according to the number of shades of
ink on the reference sheet with a corresponding blackness. Automated hourly sampling started in 1921. After World War II, the
blackness was measured with a reflectometer and light absorption was translated to the parameter British Smoke (BrS). One unit
of SN was equivalent to a specific loading of BrS of 8 μg cm−2
. BrS in turn is a proxy for elemental carbon (EC) as we showed in
an earlier publication, where we found that a value of BrS of 8 μg cm−2 corresponded to an EC load of 1.4 μg cm−2
. SNs can thereby be translated to historic EC loadings/concentrations. In an evaluation of SN-data, we noticed that average pre-war values
were mostly at the lower detection limit (one SN) and overestimated because reference sheets faded over time. Data for smog
periods with their elevated SNs, however, are quite reliable and can be used for exposure estimates. After World War II, daily
sampling resulted in higher filter loadings and reliable average values. Wintertime concentrations corresponding to up to
100 μg m−3 EC were reached. On the other hand, the increased loadings due to the long sampling times resulted in a decrease
of the upper limit of detection to an extent that it was exceeded at most of the measuring stations during the “Great Smog” of
London in 1952. In this study, we analysed SN data from this episode in depth. At one measuring site, precautions were taken to
minimise the loading by using a large filter, but even then the maximum SN was at the upper limit of detection. From a one-to-one
relation of BrS and gravimetric mass, established during later smog periods, we deduced that the maximum 2-day mass
concentration of EC must have been around 1000 μg m−3
. This value is twenty times the new EU-limit for workplace EC
exposure (EU 2019) and the 24-h ambient PM10 mass concentration.