Abstract: Systems and devices to measure atmospheric gaseous reactive gases that may form atmospheric aerosol particles may generally includes a reactor in fluid communication with a particle counter. The reactor may include a first inlet configured to receive a continuous flow of a first fluid and a second inlet configured to receive a continuous flow of a second fluid. The reactor may include a first outlet in fluid communication with the particle counter to detect a reaction product of the first fluid and second fluid. The reaction product may include 1-3 nanometer particles. The reactor may include a second outlet to exhaust any remaining portion of the reaction product flow from the reactor. Methods of making and using the systems and devices to measure atmospheric gaseous reactive gases that may form atmospheric aerosol particles are also described.