Long term black carbon measurements in the southwestern Iberia Peninsula

Black carbon mass concentration ( BC PM 10 ) measurements carried out at Évora, Portugal, between 2007 and 2009, were analyzed and interpreted at different timescales. Additional measurements of aerosol mass concentration ( M PM 10 ) were included and the black carbon to total mass fraction (BC mass fraction) was derived when measurements of both quantities were coincident. BC PM 10 values were found to vary between 0.3 and 5 μg m−3, mainly in the range of 0.5–2 μg m−3. A clear and consistent seasonal behavior was found; an increase by a factor of two in the average BC PM 10 values was observed from summer (0.9 μg m−3) to winter (1.8 μg m−3) which is reflected in the BC mass fraction, amplified from about 4 to 10%. Comparison of the average BC PM 10 mass concentrations on week days and week ends indicate that local traffic emissions strongly influence the observed average diurnal patterns. Other factors, such as wood burning for heating, lower boundary layer height and more frequent winter-time temperature inversions, also likely influence the observed BC PM 10 but were not directly studied here. When different air mass types are considered then black carbon levels show a much lower variation than PM10 mass concentrations, stressing the relevance of local emissions in the BC PM 10 levels. BC PM 10 values under continental or maritime influence only differ by a factor of approximately 1.4 only, much lower than the factor of 10 reported for a coastal rural site in Portugal.