Characterisation of light-absorbing atmospheric particles in the Brussels sub-urban atmosphere

The particle composition is important for air quality studies in the urban atmosphere. In particular, particles smaller than 1 μm are increasingly in the focus with respect to human health because of they are inhalable deeply into the human lung. A relevant part of such (ultra-)fine particles are light-absorbing particles. Important sources for such particles in the cities and residential areas are emissions from traffic (mainly soot), but also emissions from wood-burning stoves in private households. The relative contributions of these sources to the atmospheric particle load are important to know in order to be able to reduce hazardous emissions by specific measures. The Royal Meteorological Institute of Belgium (RMI) gathers ambient aerosol data in Brussels with a 7-wavelengths aethalometer (mass concentration and absorption coefficient of light-absorbing aerosol). The measurement site is located in the sub-urban, rather residential southern part of Brussels and measurements are representative for the urban background. At the same site, the boundary layer height and atmospheric stability parameter are derived from a co-located ceilometer and eddy-covariance system. Further, RMI uses the multi-scale chemical transport model CHIMERE, coupled to the high resolution regional numerical weather prediction limited area model ALARO. In this context, CHIMERE will be used to evaluate emission scenarios. The wavelength dependency of the measured aerosol parameters revealed distinct variations of aerosol composition, both on a daily, weekly, and seasonal scale. The Absorption Angstrom Exponent (AAE; spectral dependency of the absorption coefficient) revealed values around 1.3 during winter and around 1.0 during summer months. As fresh soot has a flat spectrum, (i.e., AAE around 1.0), higher AAE values during winter meant that other light-absorbing compounds, which absorb stronger in the UV, increased in importance (e.g., wood burning aerosol). The multi-wavelengths data is exploited to derive the relative contribution of fresh soot (thus traffic emissions) and other sources to the amount of light-absorbing aerosol. The influence of boundary layer height and atmospheric stability on the aerosol data will be presented.