17AEROSOL New particle formation from isoprene under upper-tropospheric conditions

Aircraft observations have identified high concentrations of freshly nucleated particles in the upper free troposphere above the Amazon rainforest. Monoterpene oxidation products from tropical rainforests have traditionally been proposed as a potential source for upper tropospheric particle formation, given their rapid nucleation abilities. Yet, monoterpene concentrations sufficient to explain the observed aerosol levels in the upper troposphere have not been documented. In contrast, recent satellite observations have detected elevated nighttime isoprene concentrations—reaching up to 1 ppbv—in the upper troposphere over tropical regions such as South America, central Africa, and Southeast Asia. Isoprene persists during nighttime due to its long atmospheric lifetime against ozone oxidation, but it is quickly depleted during the day by the hydroxyl radical, with a lifetime of around one hour. Isoprene has traditionally been thought to inhibit biogenic new particle formation by reducing the yield of low-volatility oxygenated compounds from monoterpenes. However, previous experimental studies on isoprene-driven particle formation were conducted at warmer temperatures. In this study, we present results from experiments conducted in the CERN CLOUD chamber which explored new particle formation from isoprene under upper tropospheric conditions at temperatures of -30°C and -50°C. Atmospheric concentrations of isoprene, HO_x, and NO_x were directly measured, along with all gas-phase and particle-phase compounds within the CLOUD chamber.