Secondary Aerosol Formation and Growth Analysis Using the mCAGE Environmental Chambers

Measurements of secondary aerosol formation and growth rates were made using the mobile Captive Aerosol Growth and Evolution (mCAGE) environmental chambers during the summer of 2015 at a site in the WG Jones State Forest just outside of the Houston, TX metropolitan area. The mCAGE system was developed to monitor and study secondary aerosol production under both ambient and controlled conditions. Initially monodisperse aerosols injected into the chambers are sampled over periods of up to a day as they are altered due to exposure to UV intensity and trace gas concentrations that mimic those just outside. During this study smaller particles (70 nm) were injected and their size tracked to quantify time-dependent growth rate and larger particles (300 nm) were injected to maintain a surface area concentration in the chambers that was comparable to that outside. Initial periods of growth attributed to UV-driven photochemistry during the hours of peak solar intensity were recorded daily. A second distinct period of growth occurred in the hours following sunset. This secondary growth period is thought to be largely due to reaction of biogenic volatile organic compounds (BVOC) such as isoprene and monoterpenes with the nitrate radical (NO3), which is relatively unimportant during the day when it is rapidly photolyzed. Combining nocturnal growth rates with available oxidant and trace gas concentrations provides insight into the controls and significance of the nighttime NO3 chemistry at this site.