This field evaluation study was conducted to assess new technology designed to measure number concentrations of strongly acidic ultrafine particles. Interest in these particles derives from their potential to cause adverse health effects. Current methods for counting and sizing airborne ultrafine particles cannot isolate those particles that are acidic. We hypothesized that the size-resolved number concentration of such particles to which people are exposed could be measured by newly developed iron nanofilm detectors on which sulfuric acid (H2SO4*) droplets produce distinctive ringed reaction sites visible by atomic force microscopy (AFM). We carried out field measurements using an array of samplers, with and without the iron nanofilm detectors, that allowed indirect comparison of particle number concentrations and size-resolved measures of acidity. The iron nanofilm detectors are silicon chips (5 mm x 5 mm x 0.6 mm) that are coated with iron by vapor deposition. The iron layer was 21.5 or 26 nm thick for the two batches used in these experiments. After exposure the detector surface was scanned topographically by AFM to view and enumerate the ringed acid reaction sites and deposited nonacidic particles. The number of reaction sites and particles per scan can be counted directly on the image displayed by AFM. Sizes can also be measured, but for this research we did not size particles collected in the field. The integrity of the surface of iron nanofilm detectors was monitored by laboratory analysis and by deploying blank detectors and detectors that had previously been exposed to H2SO4 calibration aerosols. The work established that the detectors could be used with confidence in temperate climates. Under extreme high humidity and high temperature, the surface film was liable to detach from the support, but remaining portions of the film still produced reliable data. Exposure to ambient gases in a filtered air canister during the field tests did not affect the film quality. Sampling sessions to obtain particle measurements were scheduled for two 1-week periods in each of the four seasons at a rural site in Tuxedo, New York. This schedule was selected to test outdoor performance of the iron nanofilm detectors under a variety of weather conditions. To seek possible artifacts caused by local source differences, we also sampled outdoors for two 1-week sessions during the winter in New York City. Indoor tests were conducted in the cafeteria at the Nelson Institute of Environmental Medicine (NIEM) in Tuxedo and in a residence in Newburgh, New York. For the outdoor tests we simultaneously deployed several particle samplers to obtain several measures: --the number concentration of acidic and total particles that penetrated the 100-nm cut size of a microorifice impactor (MOI) and were electrically precipitated in an electrostatic aerosol sampler (EAS) onto the iron nanofilm detectors; --the number concentrations of acidic and total particles estimated from detectors placed in a simple ultrafine diffusion monitor (UDM); --the size-fractionated mass concentration of strong acids in samples from the submicrometer collection stages of the MOI and from a polycarbonate filter, parallel to the EAS, that also collected particles penetrating the MOI's 100-nm cut size; and --the number concentration of all ambient particles with diameters of 300 nm or smaller, determined using a scanning mobility particle sizer (SMPS). In the results from these samplers, the mean number concentration of acidic particles ranged from about 100 to 1800/cm3, representing 10% to 88% of all ambient ultrafine particles for the different seasons and sites. The number concentration did not correlate with the acidic mass (hydrogen ion, or H+, content) for particles smaller than 100 nm in diameter. This was not surprising because a single 100-nm particle may contain the same acid volume as many smaller particles if they are pure acid droplets. The ambient concentrations of H+, sulfate (SO4(2-)), and ammonium (NH4+), collected on polycarbonate filters and measured as a function of particle size, were highest for particles with diameters between 280 and 530 nm, but the size distributions also suggested that a small peak of these ions existed in the particle size range below 88 nm. The H+ / SO4(2-) ratio was somewhat higher for particles below 88 nm, suggesting greater excess acidity for these small particles. Our continuous monitoring showed that airborne concentrations of ultrafine particles varied substantially with time. The iron nanofilm detectors provided a time-integrated number concentration over several days or weeks. The counts on the detectors were relatively low for some of the sampling sessions, resulting in high statistical errors in calculations. Nonetheless, agreement of the mean values was remarkably good for some of the measurements. In future tests, longer collection times and new technologies, such as improved particle-charging methods for electrical precipitation samplers, could provide more efficient collection of particles onto the detectors, higher counts, and lower count-associated uncertainties. In general, concentrations of ultrafine particles determined by AFM analysis of the detectors in the MOI-EAS and UDM appeared to underestimate the total number concentration as determined by comparison samplers. The ability to monitor airborne acidic particles provided by these iron nanofilm detectors enlarges the array of air quality variables that can be measured. This may help to resolve some of the outstanding questions related to causal relations between demonstrated health effects of ambient particles and particulate matter (PM) components.