Atmospheric particulate and semi-volatile substances are classified by use of specific vapour pressure (p_s). Airborne particulate matter consists of many different species which all have individual mass size distributions. To all substances of p_s < 1 Pa a Mass Median Diameter (MMD) may be assigned. Substances of p_s < 10~(-11) Pa totally are bound in particulate phase. Substances of p_s between 10~0 and 10~(-11) Pa, which comprises almost all organic compounds, partly are in gaseous phase and partly in particulate phase. Airborne substances of p_s > 1 Pa entirely are in gaseous phase. Substances of p_s between 10~0 and 10~(-24) Pa belong to the fine particle mode (particle diameter below 2,5 μm). Substances of p_s < 10~(-24) Pa belong to the coarse particle mode (particle diameter above 2,5 μm). By knowledge of p_s of a substance it's MMD can be calculated. Thus all airborne particulate substances can be classified in respect to particle size. In addition the amount of the gaseous and particulate fraction of a semi-volatile substance may be related to p_s or alternatively to MMD. Semi-volatile substances are bound in the particle diameter range between 0,1 and 1 μm, where the maximum of the fine particle mode is located. By knowledge of the MMD the calculation of the tropospheric residence time of particulate and semi-volatile substances becomes simplified.
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