Model of tropospheric ion composition: a first attempt

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 5km altitude. Above 7km, 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 13km 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 6km, whereas the second type dominates between 6 and 10km. NO₃^--core ions are the most abundant ions above 10km. 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 10km altitude.