Secondary Aerosol Formation and Growth Due to Nitrate Radical Chemistry

During the summer of 2016, measurements of secondary aerosol formation and resulting particle growth rates were made using the Captive Aerosol Growth and Evolution (CAGE) environmental chambers. The chambers were employed at a site in the WG Jones States Forest north of Houston, TX, where reaction of biogenic volatile organic compounds (BVOC) such as isoprene and the monoterpenes is enhanced through interaction with anthropogenic primary and secondary pollutants from the Houston area. The CAGE chambers are designed to study secondary aerosol production under ambient conditions. Monodisperse particles are intermittently injected into the chambers and their growth monitored to quantify time- and day-dependent aerosol production rate. Daytime particle growth can be attributed to photochemistry driven by UV radiation during peak solar intensity hours. After sunset, a distinct second period of growth occurs. This growth period is largely due to reaction of the nitrate radical (NO₃*) with isoprene and the monoterpenes. The nitrate radical is rapidly photolyzed during the day and does not become important in secondary aerosol production until nighttime. Oxidant and trace gas concentration measurements combined with nocturnal growth rates shows that nighttime NO₃* chemistry is significant for secondary aerosol formation at this site.