Characterisation of Light-Absorbing Particles in the Brussels Sub-urban Atmosphere and Implications for the Emission Scheme of a Regional Chemical Transport Model

Abstract

The Royal Meteorological Institute of Belgium (RMI) gathered ambient aerosol data in Brussels with a 7-wavelengths aethalometer and a 3 wavelengths integrating nephelometer. The amount of light-absorbing particles showed a clear daily and weekly cycle, with a sharp peak in the morning rush hour time and a broader peak in the evening. During weekends, the rush hour peak diminished. The spectral dependency of the absorption coefficient revealed peak contributions of traffic emissions to the amount of light-absorbing particles of up to 90%. Other sources (like wood burning from households) showed peak contributions of up to 35%. These percentages showed in addition a clear daily, weekly and seasonal cycle, with higher contributions of these other sources during night time, weekends and summer.

Questions and Answers

Questioner 1: F. Lenartz

Question 1: Can the aethalometer provide you directly with an information about the particle size? How do you retrieve it?

Answer 1: It is not the aethalometer which can provide information on particle size. It is the nephelometer. As explained in the last paragraph of page 2, the Scattering Angstrøm Exponent gives an indication of particle size.

Questioner 2: Oriol Jorba

Question 2: Have you detected episodes Where AAE > 1.5–2 and if this correlates well with expected emissions of wood combustion?

Answer 2: Yes, periods with an AAE > 1.5–2 were detected. They occur preferentially during winter months and during night hours. Daily means of AAE reached values of 1.8 ± 0.3 during November and December 2018, for example. However, we have not yet performed a detailed analysis on the exact timing of these events. We also have no direct proof that it is wood combustion. For this, chemical analyses would be necessary. But the distinctly higher AAE > 1.5–2 indicates that a distinct part of the measured particles were not from traffic emissions but from heating sources.