Measured particle water uptake enhanced by co-condensing vapours
Abstract. Co-condensation of inorganic or organic vapours on growing droplets could significantly enhance both cloud condensation nucleus (CCN) and cloud droplet number concentration, thereby influencing cloud albedo and climate. Until now, there has been very few direct observational evidence of this process. We have measured the growth of inorganic salt particles exposed to both water and organic vapours at 291.15 K in the laboratory, showing that co-condensation of the organic vapours significantly enhances water uptake of aerosols. After exposure to water and propylene glycol vapours, ammonium sulfate particles grew much more than any previously measured particles, inorganic or organic, at the same relative humidity (RH). The maximum equivalent hygroscopicity parameter, κ, was observed to reach up to 2.64, very much higher than values (0.1 < κ < 0.9) measured for atmospheric particulate matter using conventional instrumentation, which may be blind to this effect. Under a continuously replenishing organic vapour field, the particles never reached equilibrium owing to the presence of the involatile solute and were observed to continuously grow with increasing exposure time, in agreement with model simulations. Co-condensation of butylene glycol (which has similar volatility but, at aw = 0.9, a higher Sorg than propylene glycol in our system) and tri-ethylene glycol (which has lower volatility and, at aw = 0.9, lower Sorg than propylene glycol in our system) vapours was additionally measured in this study. The maximum equivalent hygroscopicity parameter, κ, reached as high as 8.48 for ammonium sulfate particles exposed to water and tri-ethylene glycol vapours at 90 % RH. This enhancement of particle water uptake through co-condensation of vapours constitutes the direct measurement of this process, which may substantially influence cloud droplet formation in the atmosphere. In addition, the model simulations for exposure to co-condensing butylene glycol and tri-ethylene glycol vapours with water show that there are factors other than Sorg which influence the co-condensation of semi-volatile organic compounds (SVOCs) that are as yet not understood.