Estimation of Aerosol Activation Ratio and Water Vapor Supersaturation at Cloud Base Using Aircraft Measurement

Abstract

The vertical evolution of aerosol physiochemical properties significantly influence the capacity of particle water uptake at different atmospheric levels, which are important for the estimation of direct and indirect radiative impacts of aerosol. However, as one of the most important environmental parameters during the formation of cloud or fog, water vapor supersaturation cannot be directly measured. An aircraft observation was carried out on 13 November 2016 with stable stratiform clouds covering Beijing area. The results of aircraft in-situ measured aerosol particle size distribution, chemical composition, cloud droplet spectrum, and vertical distribution of aerosol physicochemical properties and activation ability near the cloud base are analyzed. The results show that Beijing area is under polluted conditions during the flight detection with the surface aerosol concentration (0.11-3 μm) of 4600 cm-3. The height range of cloud layer is 800-1200 m. The number concentration of aerosols at the cloud base is greatly lower compared with the surface, and the effective diameter significantly increases from 0.3 μm to 0.6 μm. Aerosol composition varies drastically with altitudes. The hydrophobic primary organic aerosol (POA) has a significant contribution at surface, but sharply decreases at the cloud base. Meanwhile, the fraction of inorganic species and secondary organic aerosol (SOA) increase significantly from surface to cloud base, resulting in hygroscopic parameter (κ) increasing from 0.25 (ground) to 0.32 (cloud base). The number concentration size spectrum of cloud droplets and aerosols in the cloud can be well connected, which could be deemed as activated and un-activated particles respectively. Meanwhile, the sum of their number concentrations is approximately equal to the total aerosol concentration below the cloud, indicating aerosol particles below the cloud base has a dominant contribution to the cloud droplet formation near the cloud base by activation. Thereby, the actual activation ratio of aerosol particles serves as cloud condensation nuclei (CCN) in the measured cloud can be obtained. Combining with in-situ measured aerosol size distribution, chemical composition, and calculated hygroscopic parameter below the cloud base, the aerosol activation ratio under different supersaturation ratio can be derived. Estimated through the comparison of calculated activation rate and the measurement, the mean supersaturation near this stratiform cloud base is about 0.048%. It implies that the aerosol characteristics at surface may not represent that at upper levels, where the evolution in vertical direction should be considered in evaluating the contribution of surface emissions to cloud particle nucleation and their atmospheric lifetime. This supersaturation estimating method is mainly based on conventional measurement of aerosol and droplets, which has a potential value for further application on cloud analysis.

Keywords

• water vapor supersaturation
• aircraft measurement
• aerosol physicochemical properties
• cloud droplet spectrum