Formation of anthropogenic secondary organic aerosol (SOA) and its influence on biogenic SOA properties

摘要

Secondary organic aerosol (SOA) formation from mixed anthropogenic andbiogenic precursors has been studied exposing reaction mixtures to naturalsunlight in the SAPHIR chamber in Jülich, Germany. In this studyaromatic compounds served as examples of anthropogenic volatile organiccompound (VOC) and a mixture of α-pinene and limonene as an examplefor biogenic VOC. Several experiments with exclusively aromatic precursorswere performed to establish a relationship between yield and organic aerosolmass loading for the atmospheric relevant range of aerosol loads of 0.01 to10 μg m. The yields (0.5 to 9%) were comparable to previousdata and further used for the detailed evaluation of the mixed biogenic andanthropogenic experiments. For the mixed experiments a number of differentoxidation schemes were addressed. The reactivity, the sequence of addition,and the amount of the precursors influenced the SOA properties. Monoterpeneoxidation products, including carboxylic acids and dimer esters wereidentified in the aged aerosol at levels comparable to ambient air. OHradicals were measured by Laser Induced Fluorescence, which allowed forestablishing relations of aerosol properties and composition to theexperimental OH dose. Furthermore, the OH measurements in combination withthe derived yields for aromatic SOA enabled application of a simplifiedmodel to calculate the chemical turnover of the aromatic precursor andcorresponding anthropogenic contribution to the mixed aerosol. The estimatedanthropogenic contributions were ranging from small (≈8%) up tosignificant fraction (>50%) providing a suitable range tostudy the effect of aerosol composition on the aerosol volatility (volumefraction remaining (VFR) at 343 K: 0.86–0.94). The aromatic aerosol hadhigher oxygen to carbon ratio O/C and was less volatile than the biogenicfraction. However, in order to produce significant amount of aromatic SOAthe reaction mixtures needed a higher OH dose that also increased O/C andprovided a less volatile aerosol. The SOA yields, O/C, and (the massfraction of CO ions in the mass spectra which can be consideredas a measure of carboxylic groups) in the mixed photo-chemical experimentscould be described as linear combinations of the corresponding properties ofthe pure systems. For VFR there was in addition an enhancement effect,making the mixed aerosol significantly less volatile than what could bepredicted from the pure systems. A strong positive correlation was foundbetween changes in volatility and O/C with the exception during dark hourswhere the SOA volatility decreased while O/C did not change significantly.Thus, this change in volatility under dark conditions as well as theanthropogenic enhancement is due to chemical or morphological changes notaffecting O/C.