Impact of aerosol composition on cloud condensation nuclei activity
The impact of aerosol composition on cloud condensation nuclei (CCN) activity were analyzed in this study based on field experiments carried out at downtown Tianjin, China in September 2010. In the experiments, the CCN measurements were performed at supersaturation (SS) of 0.1%, 0.2% and 0.4% using a thermal-gradient diffusion chamber (DMT CCNC), whereas the aerosol size distribution and composition were simultaneously measured with a TSI SMPS and an Aerodyne Aerosol Mass Spectrometer (AMS), respectively. The results show that the influence of aerosol composition on CCN activity is notable under low SS (0.1%), and their influence decreased with increasing SS. For example, under SS of 0.1%, the CCN activity increases from 4.5 +/- 2.6% to 12.8 +/- 6.1% when organics fraction decrease from 30-40% to 10-20%. The rate of increase reached up to 184%. While under SS of 0.4%, the CCN activity increases only from 35.7 +/- 19.0% to 46.5 +/- 12.3% correspondingly. The calculated N-CCN based on the size-resolved activation ratio and aerosol number size distribution correlated well with observed N-CCN at high SS (0.4%), but this consistence decreased with the falling of SS. The slopes of linear fitted lines between calculated and observed N-CCN are 0.708, 0.947, and 0.995 at SS of 0.1%, 0.2% and 0.4% respectively. Moreover, the stand deviation (SD) of calculated N-CCN increased with the decreasing of SS. A case study of CCN closure analyses indicated that the calculated error of N-CCN could reach up to 34% at SS of 0.1% if aerosol composition were not included, and the calculated error decreased with the raising of SS. It is decreased to 9% at SS of 0.2%, and further decreased to 4% at SS of 0.4%.