Source apportionment and the role of meteorological conditions in the assessment of air pollution exposure due to urban emissions
As particulate matter (PM) impacts human health, knowledge about its composition, exposure and source apportionment is required. A study of the urban atmosphere in the case of Augsburg, Germany, during winter (31 January-12 March 2010) is thus presented here. Investigations were performed on the basis of aerosol mass spectrometry and further air pollutants and meteorological measurements, including mixing layer height. Organic matter was separated by source apportionment of PM1 with positive matrix factorization (PMF) in three factors: OOA - oxygenated organic aerosol (secondary organic factor), HOA - hydrocarbon-like organic aerosol (traffic factor or primary organic factor) and WCOA - wood combustion organic aerosol (wood combustion factor), which extend the information from black carbon (BC) measurements. PMF was also applied to the particle size distribution (PSD) data of PM2.5 to determine different source profiles and we assigned them to the particle sources: nucleation aerosol, fresh traffic aerosol, aged traffic aerosol, stationary combustion aerosol and secondary aerosol. Ten different temporal phases were identified on the basis of weather characteristics and aerosol composition and used for correlations of all air pollutants and meteorological parameters.
While source apportionment from both organic PM composition and PSD agree and show that the main emission sources of PM exposure are road traffic as well as stationary and wood combustion, secondary aerosol factor concentrations are very often the highest ones. The hierarchical clustering analysis with the Ward method of cross-correlations of each air pollutant and PM component and of the correlations of each pollutant with all meteorological parameters provided two clusters: "secondary pollutants of PM1 and fine particles" and "primary pollutants (including CO and benzene) and accumulation mode particles". The dominant meteorological influences on pollutant concentrations are wind speed and mixing layer height which are coupled with a certain wind direction. The compounds of the cluster "secondary pollutants and fine particles" show a negative correlation with absolute humidity, i.e., low concentrations during high absolute humidity and vice versa. The PM10 limit value exceedances originated not only from the emissions but also in combination with specific meteorological conditions. NC3-10 (number concentration of nucleation mode particles) and NC10-30 (Aitken mode particles), i.e., ultrafine particles and the fresh traffic aerosol, are only weakly dependent on meteorological parameters and thus are driven by emissions. The results of this case study provide information about chemical composition and causes of PM exposure during winter time in urban air pollution.