Model of tropospheric ion composition: A first attempt

Recent atmospheric ion composition measurements in the troposphere have rev ealed the presence of several new families of ions below the tropopause, wh ich had not been observed above this level. In a chemical model of troposph eric positive ions, several new channels are proposed to explain this obser vation, and the presence of very heavy clustered aerosol ions (charged ultr afine particles) are considered. Parent neutral compounds, that are respons ible for the formation of positive cluster ions in the troposphere, include ammonia, pyridine, picoline, lutidine, acetone, etc. Model results show th at the clustered aerosol ions are dominant near the surface, above which py ridinated 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 c onform with stratospheric ion models. Experimental data suggest, however, t hat 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 ab undant 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 obse rvational 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 und er these conditions HSO4--core ions are the dominant ions below 10 km altit ude.