A method to estimate the distribution of various fractions of PM10 in ambient air in the Netherlands

Eight rules based on measurements, model calculations, additional deductions and expert judgement have been devised for use in estimating the distribution and sources of various fractions of PM10 in the Netherlands. As some of the underlying assumptions to these rules are debatable, they can be best characterized as 'rules of thumb'. A brief rationale is given for each rule. Using these rules it is concluded that a previous expert judgement of a contribution of 20% from Dutch sources to PM10 levels in the Netherlands is probably too low. Different values for the national contribution to the ambient PM10 levels in the Netherlands have been evaluated. A contribution of 40-45% to PM10 levels from Dutch sources is shown to meet two decisive criteria: (1) a reasonable ratio of PM10 originating from national and foreign sources and (2) a reasonable ratio of the fine and coarse fractions of PM10. Such a contribution also agrees with available data. This study shows that a higher national contribution to the ambient PM10 levels implies a larger share of local emissions of coarse particles > 2.5 μm than previously estimated. It appears that the original concept of a 'blanket' of homogeneous PM10 concentrations covering the Netherlands, a useful concept to describe the occurrence of shallow concentration gradients on geographical scales smaller than the country as a whole, has turned out to be more of a 'patchwork quilt'. Although the measured concentration gradients are low (< 20%), probably because the distribution of emission sources in a densely populated country like the Netherlands is fairly homogeneous, the chemical composition of PM10 may differ spatially due to the contribution of local sources emitting aerosols in the coarser fraction of PM10.