A. Mehlmann et P. Warneck, ATMOSPHERIC GASEOUS HNO3 PARTICULATE NITRATE, AND AEROSOL-SIZE DISTRIBUTIONS OF MAJOR IONIC SPECIES AT A RURAL SITE IN WESTERN GERMANY, Atmospheric environment, 29(17), 1995, pp. 2359-2373
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.