Ultrafine Particle Formation and Growth in the Upper Troposphere during the Deep Convective Clouds and Chemistry (DC3) Campaign

Ultrafine particles impact climate and weather as a main source of cloud condensation nuclei. The vertical transport of boundary layer pollutants to the upper troposphere during convection drives new particle formation. During midlatitude summers, atmospheric convection occurs frequently. However, the extent to which convection influences upper tropospheric particle formation remains unclear. Herein, we report measurements of ultrafine particles in the upper troposphere during the Deep Convective Clouds and Chemistry (DC3) campaign. Across 21 flights tracking convective outflow, several flights exhibited elevated concentrations of particles with diameters of less than 100 nm (N_100 nm), often exceeding 1000 cm^–3. One particular flight followed and intersected a plume downwind several times showing an ultrafine particle growth rate of 1.4 ± 0.1 nm h^–1 due to photochemical aging. Condensation of sulfuric acid can only explain a small fraction of this growth. Gas-phase measurements indicate that oxidized organic vapors likely contribute to particle growth in this region. Future measurements are needed to constrain these processes in atmospheric models, particularly of the molecular speciation of gas-phase precursors. Upper tropospheric ultrafine particles in this region may become more important as climate models project increasing frequency of atmospheric convection in the future.