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Semi-volatile compounds (organics, nitrate, chloride) are ubiquitous in atmospheric aerosols and usually contribute over 50 % to particulate matter worldwide (Jimenez et al., 2009). Co-condensation of semi-volatiles and water vapour can enhance aerosol particle growth and facilitate their activation to cloud droplets, affecting cloud properties and thus the Earth's radiation balance.Yet, the effect of co-condensation on aerosol hygroscopic growth is not well constrained as the loss of semi-volatiles during drying and heating in traditional aerosol sampling devices (e.g., by HTDMA, CCN counter) is poorly understood. Here, we developed a novel method to derive aerosol hygroscopic growth by considering the co-condensation effect, from open-access data including visibility, PM2.5 mass concentration and meteorological parameters (Wang and Chen, 2019). By applying our visibility method and thermodynamic modelling in Delhi (India), we found that the co-condensation of HCl with water vapour can largely enhance aerosol hygroscopicity by doubling the light extinction coefficient of wetted particles and halving the critical supersaturation needed for cloud droplet activation (as shown in Fig. 1 a-b, Gunthe et al., 2021). Our recent results showed that the co-condensation effect in Chinese megacities (Beijing, Guangzhou, and Shanghai) is as significant as in Delhi, but acts via co-condensation of HNO3.The next question is how significant the co-condensation effect is globally and how much it can alter clouds and the radiation balance. Here, we combine novel field observation, a cloud parcel model, and an aerosol-climate model to disentangle this question. The particle and gas composition, particle size distribution, and air parcel cooling rate are essential factors for the co-condensation effect in a rising air parcel.Our preliminary sensitivity study (doubling hygroscopicity) in a climate model showed that the co-condensation effect plays only a minor role in clouds formation globally, but significantly increases cloud droplet number concentration and liquid cloud cover in regions with large anthropogenic emissions, e.g. East/Southeast Asia, India, Europe, East US. Consistently, parcel model calculations confirm that for a given cooling rate, co-condensation increases the number of activated cloud nuclei. Our study will help to develop a parameterization for aerosol-climate models to include the co-condensation effect on cloud formation.ReferencesGunthe, S. S., et al. (2021), Enhanced aerosol particle growth sustained by high continental chlorine emission in India, Nature Geoscience.Jimenez, J. L., et al. (2009), Evolution of Organic Aerosols in the Atmosphere, Science, 326(5959), 1525-1529.Wang, Y., and Y. Chen (2019), Significant Climate Impact of Highly Hygroscopic Atmospheric Aerosols in Delhi, India, Geophysical Research Letters, 46(10), 5535-5545. |
