Abstract
Recent atmospheric ion composition measurements in the troposphere have
revealed the presence of several new families of ions below the
tropopause, which had not been observed above this level. In a chemical
model of tropospheric positive ions, several new channels are proposed
to explain this observation, and the presence of very heavy clustered
aerosol ions (charged ultrafine particles) are considered. Parent
neutral compounds, that are responsible for the formation of positive
cluster ions in the troposphere, include ammonia, pyridine, picoline,
lutidine, acetone, etc. Model results show that the clustered aerosol
ions are dominant near the surface, above which pyridinated cluster ions
are most abundant up to about 5 km altitude. Above 7 km, ions having
acetone as parent neutral species are in majority. Ammonia and methyl
cyanide cluster ions are found to be less abundant as compared to the
above. Above 13 km the relative abundance of methyl cyanide cluster ions
is rapidly increasing, which suggests that the present model results
conform with stratospheric ion models. Experimental data suggest,
however, that the concentration of pyridinated compounds is highly
variable from one location to another and that the relative abundance of
ammonia cluster ions could be high in some remote environments.
Similarly, our model shows that NO3-. HNO3 (H2O)(n), HSO4-, and
NO3--core families of ions are the most abundant negative ions in the
troposphere during nighttime. The first family dominates below 6 km,
whereas the second type dominates between 6 and 10 km. NO3--core ions
are the most abundant ions above 10 km. However, some observational data
suggest a dramatic increase in the concentration of sulfuric acid vapor,
malonic acid, and methane sulfonic acid during daytime with a related
change in the negative ion composition. Our model suggests that under
these conditions HSO4--core ions are the dominant ions below 10 km
altitude.
