A recent study investigating traffic emissions during 2018 at a kerbside station in Malta has shown that vehicle exhaust is not, in fact, the main contributor to the levels of airborne particulate matter (PM10) derived from traffic, and that the bulk of such particulate pollution results from wear on brakes and tyres and from resuspended dust from the road surface.
The study focused on the contribution of traffic to atmospheric aerosols, namely PM10, and also succeeded, for the first time locally, in separating this contribution into an exhaust and a non-exhaust fraction. The use of a novel methodology allowed chemical speciation data to be used in a source apportionment exercise in which the output of the positive matrix factorization (PMF) model was further refined using the lesser-known, constraint weighted non-negative matrix factorization (CW – NMF) model.
Samples were collected on a daily basis at the Msida sampling site between January and December 2018, and results show that PM10 levels exceeded one of the daily limit values set by the European Union on 41 separate occasions during this period. This is of concern since research has consistently linked traffic-derived PM10 to hospitalizations and mortalities due to cardiac failure, strokes and other cardiovascular illnesses, and to an increased lung cancer risk, among other health problems.
Traffic was estimated to contribute just over 38% of the total PM10 particulate matter measured at the sampling site. The results further show that just 3.4% of total PM10 levels are derived from vehicle exhaust, while tyre and brake-wear contributes 17% and an additional 18% comes from resuspended road dust.
This major contribution to total traffic-derived PM10 of non-exhaust emissions is important in terms of policy-making, since any future measures aimed at reducing the population’s exposure to PM10 will not be effective if they focus on just reducing tailpipe emissions. The study therefore makes a number of recommendations for short to medium term measures such as congestion charging zones which reduce the amount of vehicle km driven, and carefully planned speed reductions in urban areas, which have been shown to reduce both wear and resuspended emissions.
The study was carried out by Dr Mark M. Scerri from the Institute of Earth Systems together with a number of researchers from Germany, France and Italy, and with the collaboration of Malta’s Environment and Resources Authority. The full paper, titled ‘Exhaust and non-exhaust contributions from road transport to PM10 at a Southern European traffic site’ was published in the journal Environmental Pollution, and is available online.
