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atmospheric aerosols; secondary aerosol formation; chemical composition; trajectories; variability
Abstract:
A novel method is, presented to reveal the significance and contribution of source types and characteristic formation times for individual aerosol constituents: Backward trajectory analyses are used to allocate time-resolved information about residence time of air masses over different types of ground surfaces. The correlations between the residence time of air mass over individual ground surface types and aerosol constituent concentrations (or particulate matter mass fractions) are investigated by a time-weighting method. The correlation coefficients between the concentrations of individual aerosol constituents and the residence times of air masses over certain types of ground surfaces at a certain time difference to arrival time were used to compose time profiles'. These are suggested to reflect the time-resolved ground emissions' influence on aerosol composition, which is particularly relevant for secondary aerosol constituents. The method has been applied to aerosol chemical composition data from various seasons and from rural and urban sites in Germany. For various ground types, we obtain correlations between weighted (and normalized) residence times ('source loadings') on one hand and the abundances of trace constituents known as markers for marine (Na, Cl), continental-rural (e.g. mineral dust components) and industrial sources (e.g., organic and elemental C, As, Pb) on the other hand. The occurrence of super-pm particulate NO3 in central Europe is found to originate largely in the marginal seas. The time profiles indicate that the characteristic formation time of the secondary aerosol is 48-72 h, while the coarse mode particulate matter including some heavy metals was determined by emissions < 36 h back. The occurrence of particulate elemental carbon was temporally bimodal with regard to the, elapsed time since emission (maxima at At 60 h and At = 12-24 h), which indicates the presence of two types undergoing a selection process during aging. The factors which explained most of the variability of the aerosol chemical composition were the season and the type of ground surface in contact with the air mass during its transport. More immediate influences on the samples, such as the weather conditions during sampling and the type of site (rural or urban) were distinctly less significant. (C) 2002 Elsevier Science Ltd. All rights reserved.
