secondary aerosol formation process secondary atmospheric aerosol formation event rapid formation of liquid or solid particles from gaseous material in an atmosphere envoAtmo Note that the detection of such events, and thus the thresholds used to define them in the field, are operational and dependent on the technology and observation strategy used. The event must be intense enough, occur in a large enough area, and persist for enough time to allow the formed particle population to be observed. There are no exact or universally agreed upon thresholds for the intensity, area and duration of such events. If elevated new particle formation rate is observed in the atmosphere (away from direct sources of the related gas-phase molecules) for an operationally defined period of time, practitioners often declare that such an event has taken place. A secondary aerosol formation process which occurs in an atmosphere at an increased rate relative to background processes of the same kind. atmospheric new particle formation event