Direct measurement of the growth of small particles in ambient air using captive aerosol chambers

Formation and growth of particles influence the solar radiation budget and the microphysics and properties of clouds. Quantifying the rate at which atmospheric particles grow and understanding the meteorological and chemical controls of that growth are challenging because observations at a fixed location represent a convolution of changes resulting from atmospheric processing and those reflecting the time-varying origin and history of the sampled air. The dual-chamber Captive Aerosol Growth and Evolution (CAGE) chamber system was developed to study particle growth in different environments. Inside the chambers, controlled populations of particles are exposed to an environment in which the air composition and solar intensity track those just outside. Here we present results from the use of the CAGE chamber system at the DOE Atmospheric Radiation Measurement (ARM) Program's Southern Great Plains (SGP) site over two months in the fall of 2021. Both chambers were operated continuously, with monodisperse seed particles injected every several hours and then intermittently measured by a scanning mobility particle sizer. The time dependence of the growth rate measured throughout the study is quantified. The sensitivity of particle growth to the liquid water content of injected seed particles and to the addition of precursor gases was studied by using one chamber as a reference or control and the other as a perturbation chamber.