Negative ion species were identified and studied at three separate field sites. The first study was conducted under an operating high-voltage direct current (HVDC) transmission line, The second was near a HVDC test line, and measurements were performed with the line in both an energized and de-energized state. The third was far from any anthropogenic ionization souces. During clear, sunny days, HSO4-⋅H2SO4 was observed to be the dominant ion in the natural ion spectru. At night, however, HSO4- core ions virtually disappeared and were replaced by NO3- and its first nitric acid cluster. Both spectra also typically contained lesser concentrations of the malonate ion and its nitric acid cluster. In contrast, the transmission line spectra were typically dominated by the NO3- core ions during both day and night, presumably because the much shorter lifetime of these ions did not permit sufficient time for reactions with H2SO4. The C3H3O4- and HSO4- ions were also observed in the HVDC line daytime spectra, but their relative concentrations dropped dramatically at night. It appeared that the primary difference between the ion species observed near the transmission line, as compared to the natural environment, was attributable to the shortening of the ion's reaction time, caused by it being driven to the ground or other conductor by strong transmission line electric fields. Additionally, the daytime gas phase concentration of H2SO4 was estimated to be ~1 ppt from both transmission line and natural ion spectral measurements. ¿ American Geophysical Union 1989