Vertical profiles of cloud condensation nuclei number concentration and its empirical estimate from aerosol optical properties over the North China Plain

To better understand the characteristics of aerosol activation ability and optical properties, a comprehensive airborne campaign was conducted over the North China Plain (NCP) from 8 May to 11 June 2016. Vertical profiles of cloud condensation nuclei (CCN) number concentration (N-CCN) and aerosol optical properties were measured simultaneously. Seventy-two-hour air mass back trajectories show that during the campaign, the measurement region was mainly influenced by air masses from the northwest and southeast. Air mass sources, temperature structure, anthropogenic emissions, and terrain distribution are factors influencing N-CCN profiles. Cloud condensation nuclei spectra suggest that the ability of aerosol to activate into CCN is stronger in southeasterly air masses than in northwesterly air masses and stronger in the free atmosphere than near the surface. Vertical distributions of the aerosol scattering Angstrom exponent (SAE) indicate that aerosols near the surface mainly originate from primary emissions consisting of more fine particles. The long-distance transport decreases SAE and makes it vary more in the free troposphere than near the surface. To parameterize N-CCN, the equation N-CCN = 10(beta) . sigma(gamma) is used to fit the relationship between N-CCN and the aerosol scattering coefficient (sigma) at 450 nm. The fitting parameters beta and gamma have linear relationships with the SAE. Empirical estimates of N-CCN at 0.7 % water vapor supersaturation (SS) from aerosol optical properties are thus retrieved for the two air masses: N-CCN = 10(-0.22.SAE+2.39) . sigma(0.30.SAE+0.29) for northwesterly air masses and N-CCN = 10(-0.07.SAE+2.29) . sigma(0.14.SAE+0.28) for southeasterly air masses. The estimated N-CCN at 0.7 % SS agrees with that measured, although the performance differs between low and high concentrations in the two air masses. The results highlight the important impact of aerosol sources on the empirical estimate of N-CCN from aerosol optical properties.

Automated summary

This study investigates the characteristics of aerosol activation ability and optical properties over the North China Plain. The results show that the aerosol activation ability is influenced by air mass sources, temperature structure, anthropogenic emissions, and terrain distribution. Aerosols have a stronger activation ability in southeasterly air masses and in the free atmosphere. The aerosol scattering properties are mainly influenced by primary emissions near the surface and long-distance transport in the free troposphere. A parameterization method based on aerosol optical properties can effectively estimate the aerosol activation number concentration.