Summertime measurements of the atmospheric concentrations and aerodynamic size distributions of NH4+ and NO3- and other major aerosol species were made between 1991 and 1996 at a rural site in central Massachusetts to examine the nature of aerosol chemistry at the Harvard Forest. The bulk aerosol can be described as a mixture of submicron ammonium (bi)sulfate aerosols with smaller amounts of soil-derived particles. Approximately one third of the samples had an anion surplus of greater than 20%, which was assumed to result from unmeasured H+ ions. Aerosols in air masses from the southwest were rarely neutralized, especially when SO42- concentrations were greater than ≈75 nmol m-3. This result suggests that twice this level (i.e., 150 nmol m-3) may describe an upper limit to the amount of gaseous NH3 encountered enroute to this sampling site during the typical summer transport observed from this region. Aerosol NO3- was observed at concentrations 4--8 times lower than NH4+, and while occasionally found in the fine mode, the majority of the NO3- was associated with the water-soluble fraction of supermicron soil-derived Ca2+. The similarity of the ammonium and oxalate particle size distributions suggests that (NH4)2(COO)2 aerosols may result from the reaction of gaseous precursors. ¿ 2001 American Geophysical Union