Sources and formation mechanisms of carbonaceous aerosol at a regional background site in the Netherlands: insights from a year-long radiocarbon study

摘要

We measured the radioactive carbon isotope C(radiocarbon) in various fractions of the carbonaceous aerosol sampledbetween February 2011 and March 2012 at the Cesar Observatory in theNetherlands. Based on the radiocarbon content in total carbon (TC), organiccarbon (OC), water-insoluble organic carbon (WIOC), and elemental carbon(EC), we estimated the contribution of major sources to the carbonaceousaerosol. The main source categories were fossil fuel combustion, biomassburning, and other contemporary carbon, which is mainly biogenic secondaryorganic aerosol material (SOA).A clear seasonal variation is seen in EC from biomass burning (EC),with lowest values in summer and highest values in winter, but EC isa minor fraction of EC in all seasons. WIOC from contemporary sources ishighly correlated with EC, indicating that biomass burning is adominant source of contemporary WIOC. This suggests that most biogenic SOAis water soluble and that water-insoluble carbon stems mainly from primarysources. Seasonal variations in other carbon fractions are less clear andhardly distinguishable from variations related to air mass history.Air masses originating from the ocean sector presumably contain littlecarbonaceous aerosol from outside the Netherlands, and during theseconditions measured carbon concentrations reflect regional sources. In thesesituations absolute TC concentrations are usually rather low, around 1.5 µg m, andEC is always very low ( ∼  0.05 µg m), even in winter, indicating that biomass burning is not astrong source of carbonaceous aerosol in the Netherlands. In continental airmasses, which usually arrive from the east or south and have spent severaldays over land, TC concentrations are on average by a factor of 3.5 higher.EC increases more strongly than TC to 0.2 µg m. Fossil ECand fossil WIOC, which are indicative of primary emissions, show a moremoderate increase by a factor of 2.5 on average.An interesting case is fossil water-soluble organic carbon (WSOC, calculatedas OC-WIOC), which can be regarded as a proxy for SOA from fossilprecursors. Fossil WSOC has low concentrations when regional sources aresampled and increases by more than a factor of 5 in continental air masses.A longer residence time of air masses over land seems to result in increasedSOA concentrations from fossil origin.