AbstractAirborne measurements of aerosols were conducted over the western Pacific in the spring of 2009 during the Aerosol Radiative Forcing in East Asia (A‐FORCE) aircraft campaign. The A‐FORCE flights intensively covered an important vertical‐latitudinal range in the outflow region of East Asia (0–9 km altitude; 27°N–38°N). This paper presents the variability of aerosol particle number concentrations obtained by condensation particle counters and a Single‐Particle Soot Photometer (SP2), with the focus on those in the free troposphere. The number concentration data include total condensation nuclei with particle diameters (dp) larger than 10 nm (total CN10), PM0.17‐CN10 (dp ~10–130 nm), and SP2 black carbon (NBC; dp ~75–850 nm). Large increases in total CN10 that were not associated with NBC were observed in the free troposphere, suggesting influences from new particle formation (NPF). Statistical characteristics of total CN10, PM0.17‐CN10, and NBC in the lower troposphere (LT; 0–3 km), middle troposphere (MT; 3–6 km), and upper troposphere (UT; 6–9 km) are investigated. The correlation between total CN10 and NBC, along with the ratio of PM0.17 to total CN10 and carbon monoxide mixing ratio (CO), is used to interpret the observed variability. The median concentrations of total CN10 and PM0.17‐CN10 in the UT were higher than those in the MT by a factor of ~1.4 and ~1.6, respectively. We attribute the enhancements of CN10 in the UT to NPF. Possible mechanisms affecting NPF in the free troposphere are discussed.