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Lifetime of nanocluster aerosol in urban environment
Nanocluster aerosol particles have longer than anticipated lifetime in urban air; and there are means to reduce their emissions!
Nanocluster aerosol (NCA) particles emitted by traffic were detected still as far as 250 m from a highway, a ring road in Helsinki. Busses were proven to be remarkable NCA emitters—when good engine after treatment solutions were not used. Two scientific publications that address the importance of NCA in urban air, particles as small as 1.3–4 nm, have been published by Finnish scientists, Kangasniemi et al. and Järvinen et al. Scary results, but also hopeful: we do have the means to improve the situation.
In their paper addressing dispersion of (ultra)fine particles and NCA, Kangasniemi et al. show that by the roadside, the concentration of the nanocluster aerosol particles decreased faster than that of larger particles, with distance from traffic. But contrary to some estimates, the NCA did not vanish completely.
Rather the scientists concluded that the number of NCA particles decreased due to condensational growth into larger particle sizes, and for example coagulation onto the pre-existing particles was not significant. Thus, the NCA contributes both to health and climate effects of vehicle emissions. Total roadside NCA fraction of sub-micron particles was 36% in the morning and 48% in the afternoon; in the urban background the scientist estimated it to be 29% and 29% respectively.
Järvinen et al. showed that chase measurements are capable of distinguishing differences between the engine after treatment (EAT) methods used in busses. They noticed that after treatment methods may be efficient with respect to NOx and soot particles, but in some cases not against NCA.
Their study further suggested that NCA and larger nano particles are formed in the same processes, but different to that of larger soot particles. The scientists also observed that the fraction of NCA emissions of the total particle number was lowest, 0-50%, for the newest EAT technologies (Euro VI busses and EEV busses retrofitted with the SCR and DPF).
To study health and climate impact of aerosol particles, also the smallest particles should be measured at the emission sources—omitting them will cause underestimation of total particle numbers. Engine after treatment systems do influence the vehicle emissions—meaning we can make a change!
To learn more about their findings, read the papers by Kangasniemi et al. and Järvinen et al.:
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