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

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 f(44), the fraction of ma center dot zaEuro-44 in aerosol organic material) are examined. A strong dependence of CCN number concentration (N-CCN) on the x(org) and f(44) 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)aEuro-> aEuro-60aEuro-%), the N-CCN at the supersaturation of 0.075 and 0.13aEuro-% was underestimated by 46 and 44aEuro-%, respectively, if aerosol particles were freshly emitted with primary organics (f(44)aEuro-< aEuro-11aEuro-%); the underestimation decreased to 32 and 23aEuro-% at the corresponding supersaturations, however, if the particles were with more hygroscopic secondary organics (f(44)aEuro-> aEuro-15aEuro-%). The N-CCN at the supersaturation of 0.76aEuro-% was underestimated by 11 and 4aEuro-%, respectively, at f(44)aEuro-< aEuro-11 and f(44)aEuro-> aEuro-15aEuro-%. However, for the particles composed of low organics (e.g., x(org)aEuro-< aEuro-40aEuro-%), the effect caused by the f(44) 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 400aEuro-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 > aEuro-0.2aEuro-% (overestimation of 2-4aEuro-%). However, N-CCN was overestimated by aEuro-20aEuro-% at supersaturation levels of < aEuro-0.1aEuro-%. This suggests that the overestimation is mainly due to the smaller proportion of aged and oxidized organic aerosols present at Xianghe compared to Xinzhou.