Surfaces, silica and semivolatile organics-limonene uptake and desorption indoors and outdoors

Adsorption of organics on surfaces is important in both outdoor and indoor environments. Surfaces can serve as sinks for gas-phase species, act as reservoirs by emitting previously partitioned organics back into the gas phase, and can facilitate heterogeneous chemistry. We report here studies of the uptake and desorption energetics of gas-phase limonene, a volatile and widely-distributed monoterpene, on solid silica nanoparticles using a unique apparatus that allows for temperature programmed desorption (TPD) measurements of surface binding energies as well as Knudsen cell gas uptake measurements. A multiphase kinetic model was applied to these data to provide additional molecular-level understanding of the processes involved. TPD experiments yielded an average desorption energy of 47.5 ± 8.2 kJ mol^-1 (±1s, sample standard deviation), the first direct experimental measurement of this parameter over a broad temperature range (150-320 K). Initial net uptake coefficients (γ_0,net) range from (1.7 ± 0.3) × 10^-3 (±1s) at 210 K to (2.3 ± 0.4) × 10^-4 (±1s) at 250 K, reflecting increased rates of desorption with an increase in temperature combined with increased rates of diffusion and re-adsorption within the pores between adjacent silica nanoparticles. Effective Langmuir constants, which also reflect the effects of pore diffusion and re-adsorption, were determined from the uptake data and vary from (1.8 - 0.3) × 10^-13 cm³ per molecule over the same temperature range. These results are in excellent agreement with previous studies around room temperature and with theoretical calculations of the energetics of the limonene-silica interaction. The strong attraction between limonene and the polar silica surface shows the importance of including such interactions in models of the atmospheric fates of terpenes both indoors and outdoors.