Monitoring of the ratio of nitrate to sulphate in size-segregated submicron aerosol in the Netherlands

We followed the concentration of nitrate and sulphate in size-segregated aerosol. In the evaluation we focused on the data in the smallest aerosol, that is, in particles with an (aerodynamic) diameter smaller than 0.18 μm. The average ratio of nitrate to sulphate in this range was 0.6. The nitrate was present in the form of the semi-volatile ammonium nitrate, which was representatively measured with a prototype “MARGA-sizer”. This instrument works as follows. Aerosol is grown to droplets that are easily collected; these then drain to on-line wet-chemical analysis systems. The size-classification of the aerosol, prior to collection, is accomplished with impactors. There is also a parallel line in which the aerosol is removed with a filter. Data stemming from this line serve as the in-situ field blanks. The instrument is fully automated and, with a measuring cycle of 2 h, provides a semi-continuous data set. nitoring campaign occurred at the CESAR-site of Cabauw, at the highest observation level of the meteo-tower (200 m). The campaign took place in the summer and a diurnal variation of the semi-volatile ammonium nitrate had been expected, with lower concentrations during the warm daytime hours, especially for the smallest aerosol fraction. Such a diurnal cycle was not observed. A first explanation is that the ammonium nitrate is stabilised by the high levels of ammonia-gas, even at the elevated temperatures occurring during the campaign. The issue, however, is more complex because of horizontal and vertical transport, which calls for analysis with detailed atmospheric chemical modelling.