Reply on RC2

Abstract

To better understand the characteristics of aerosol activation ability and optical properties, a comprehensive airborne campaign was implemented over the North China Plain (NCP) from May 8 to June 11, 2016. Vertical profiles of cloud condensation nuclei (CCN) number concentration (NCCN) and aerosol optical properties were measured simultaneously. Seventy-two-hour air mass back trajectories show that during the campaign the measurement region is mainly influenced by air masses in northwest and southeast. Air mass sources, temperature structure, anthropogenic emissions, and terrain distribution are factors influencing NCCN profiles. CCN spectra suggest that the ability of aerosol activation into CCN is stronger in southeast air masses than in northwest air masses and stronger in the free atmosphere than near the surface. Vertical distributions of aerosol scattering Ångström exponent (SAE) indicate that aerosols near the surface mainly originate from primary emissions consisting of more fine particles. The combined effect of aerosol lifting aloft and long-distance transport increase SAE and make it vary more in the free troposphere than near the surface. For parameterizing NCCN, the equation NCCN = 10β ∙ σγ is used to fit the relationship between NCCN and the aerosol scattering coefficient (σ) at 450 nm. The fitting parameters β and γ have linear relationships with the SAE. Empirical estimates of NCCN at 0.7% water vapor supersaturation (ss) from aerosol optical properties are thus retrieved for the two air masses: NCCN = 10-0.22∙SAE+2.39 ∙ σ0.30∙SAE+0.29 for northwest air masses and NCCN =10-0.07∙SAE+2.29 ∙ σ0.14∙SAE+0.28 for southeast air masses. The estimated NCCN 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 NCCN from aerosol optical properties.