Impacts of organic aerosols and its oxidation level on CCN activity from measurement at a suburban site in China

Abstract. This study is concerned with the impacts of organic aerosols on cloud condensation nuclei (CCN) activity based on field measurements made at a suburban site in Northern China. The sensitivity of the estimated CCN number concentration (N_CCN) to both volume fraction of organic material (x_org) and aerosol oxidation level (using f₄₄, the fraction of m∕z 44 in aerosol organic material) are examined. A strong dependence of CCN number concentration (N_CCN) on the x_org and f₄₄ was noted. The sensitivity of N_CCN to volume fraction of organics increased with increasing x_org. The impacts of the aerosol particles oxidization or aging level on estimating N_CCN were also very significant. When the particles were mostly composed of organics (x_org > 60 %), the N_CCN at the supersaturation of 0.075 and 0.13 % was underestimated by 46 and 44 %, respectively, if aerosol particles were freshly emitted with primary organics (f₄₄ < 11 %); the underestimation decreased to 32 and 23 % at the corresponding supersaturations, however, if the particles were with more hygroscopic secondary organics (f₄₄ > 15 %). The N_CCN at the supersaturation of 0.76 % was underestimated by 11 and 4 %, respectively, at f₄₄ < 11 and f₄₄ > 15 %. However, for the particles composed of low organics (e.g., x_org < 40 %), the effect caused by the f₄₄ was quite insignificant both at high and low supersaturations. This is because the overall hygroscopicity of the particles is dominated by inorganics such as sulfate and nitrate, which are more hygroscopic than organic compounds. Our results indicated that it would decrease the uncertainties in estimating N_CCN and lead to a more accurate estimation of N_CCN to increase the proportion of secondary organics, especially when the composition of the aerosols is dominated by organics. The applicability of the CCN activation spectrum obtained at Xinzhou to the Xianghe site, about 400 km to the northeast of Xinzhou, was investigated, with the aim of further examining the sensitivity of N_CCN to aerosol type. Overall, the mean CCN efficiency spectrum derived from Xinzhou performs well at Xianghe when the supersaturation levels are > 0.2 % (overestimation of 2–4 %). However, N_CCN was overestimated by  ∼  20 % at supersaturation levels of < 0.1 %. This suggests that the overestimation is mainly due to the smaller proportion of aged and oxidized organic aerosols present at Xianghe compared to Xinzhou.