A study of wood burning and traffic aerosols in an Alpine valley using a multi-wavelength Aethalometer

We present a study of aerosol light absorption using a multi-wavelength Aethalometer (λ=370–950 nm) in an Alpine valley where the major local emissions of aerosols in winter are from domestic wood burning and traffic. The measurements were done in winter and summer periods in 2004 and 2005. Much stronger diurnal trends in CO, NOx and aerosol light absorption parameters were observed in winter than in summer. The average (±1 S.D.) PM10 concentrations measured at this site were 31.5±21.7 μg m−3 in winter and 15.8±10.0 μg m−3 in summer. The highest PM10 concentrations were observed between 18:00 and 22:00 h CET in both campaigns, with 45.4±21.0 μg m−3 for winter and 21.0±9.5 μg m−3 for summer. The average (±1 S.D.) power law exponents of the absorption coefficients (also called absorption exponent) with λ=370–950 nm, α370–950 nm were 1.6±0.25 in winter and 1.1±0.05 in summer. The calculation of α separately for lower and higher wavelengths (i.e., α370–520 nm and α660–950 nm) provided a better description of the wavelength dependence from the UV- to the near-IR region. The highest mean values of α370–520 nm and α660–950 nm were observed between 22:00 and 02:00 h CET in winter with 2.7±0.4 and 1.3±0.1, respectively. Comparison of α370–520 nm with CO and NOx data indicated that the relative contribution of wood burning versus traffic was responsible for the seasonal and diurnal variability of α. The seasonal and diurnal trends of α were not attributed to changes in the particle size since the aerosol volume size distributions (dV/d log D) were found to be similar in both campaigns.