The trends of the few-second-aged small positive air ion spectra caused by enhanced concentrations of certain trace gases (HCl, ammonia, HNO3, certain pyridine derivatives, (CH3)(2)SO, (C4H9)(3)N, C6H5CHO, CH3CN) have been studied. The measurements have been accompanied by the simulations of the chemical composition of the ions. Experiments, simulations, and chemical considerations confirmed that nitrogen containing organic molecules such as pyridine, picolines, lutidines are the relevant compounds of the ions. However, as discussed, the origin of the pyridinated compounds within the natural ions remains problematic, in certain cases the observed abundance of natural pyridine compound ions can be overestimated. Therefore, H3O+(H2O)(n) and NH4+ . X . Y cycles are specially discussed, and certain problematic ions can belong to these groups. (C) 2003 Elsevier Ltd. All rights reserved.