ATMOSPHERIC GASEOUS HNO3 PARTICULATE NITRATE, AND AEROSOL-SIZE DISTRIBUTIONS OF MAJOR IONIC SPECIES AT A RURAL SITE IN WESTERN GERMANY

Nitric acid and particulate nitrate in addition to other trace species were measured in the air at Deuselbach, a rural site in western Germa ny, in June and July 1985 under background atmospheric conditions. Hig h-volume open face triple filter packs and cascade impactors were used together with ion-chromatographic analyses. Laboratory tests showed g ood correspondence between gaseous nitric acid and nitrate deposited o n nylon back-up filters for low ambient aerosol concentrations as obse rved in the field. High aerosol loadings typically found in Mainz caus ed part of nitric acid to be retained together with particulate nitrat e on the teflon front filter. The concentration of nitric acid observe d in the field went through a maximum during the day and a minimum at night with a clear anti-correlation with relative humidity. For r.h. l ess than or equal to 60% the average fraction of gaseous to total nitr ate was 39+/-8%. The average fraction from all data was 22%. The molar fraction of total nitrate to nitrogen dioxide was 24%. It is shown th at the diurnal variation of HNO3 is partly due to absorption by liquid water associated with the aerosol, which increases with rising relati ve humidity (at night). The absorption is significant only because sol ution pH is buffered by the presence of sulfate and the formation of b isulfate. Most of the field data showed particulate nitrate to occur p rimarily in the coarse size range (greater than or equal to 2 mu m dia meter) with sodium providing the main cation. Sea salt was identified as the principal source of sodium. Ammonium nitrate occurred only spor adically in the fine particle mode (greater than or equal to 2 mu m di ameter). Ammonium nitrate was largely absent because the product of th e concentrations of nitric acid (observed) and ammonia (inferred) was below the minimum required for equilibrium concentrations of particula te N4H(N)O(3) to form. In addition, there often was insufficient ammon (i)um (and other measurable cations) present in fine particles to bala nce the amount of sulfate.