Vergelijkende PM10-metingen in Nederland. deel A: laboratoriumexperimenten

Automatic FAG 62I-N beta-dust monitors have been used in the National Air Quality Monitoring Network to measure PM10 (particles smaller than 10 um aerodynamic diameter) in the Netherlands. In comparing the 5-method with the European reference method (EN12341) for PM10, this automatic beta-method was found to underestimate the PM10 levels observed using the reference method by about 25%. Laboratory research was then carried out to explore the causes. These laboratory experiments focussed on the influence of moisture on the response of the monitor, and on the influence of the inlet heating on both the volatile ammonium nitrate aerosol and the less volatile ammonium sulphate aerosol. Humidity will only slightly increase the concentrations measured by the beta-dust monitor. At very high humidities (RH over 80%), positive offsetting (overestimation) of the dust concentration of up to about 3-4 ug/m3 was observed when compared to the reference method. Thus humidity can not explain the aforementioned difference of 25% at commonly encountered PM10 levels. The inlet heating of the beta-dust monitor definitively does evaporate volatile ammonium nitrate aerosol. At the maximum set temperature (80 degrees Celcius) of the inlet heating element of the FAG 62I-N beta-dust monitor, ammonium nitrate will be completely evaporated, and so no ammonium nitrate is measured. At the standard setting of 50 degrees Celcius the ammonium nitrate will partly evaporate, i.e. only part of the ammonium nitrate is measured. In contrast, the TEOM (Tapered Element Oscillation Microbalance) evaporates all the ammonium nitrate. The heating efficiencies of the monitors are clearly different. For this reason they can not be compared when volatile aerosol is present. The less volatile ammonium sulphate aerosol does not experience any influence of inlet heating, so the methods of measurement are comparable in this case.