Aerosol hygroscopicity and activity observation at two suburban sites in North China

Abstract:

Aerosol hygroscopicity and cloud condensation nuclei (CCN) activity under background conditions and during polluted events (e.g. biomass burning) are investigated based on field measurements made at two distinct locations (Xianghe, Hebei and Xinzhou, Shanxi) in China. Our study is concerned with the impacts of aerosol physicochemical properties on its hygroscopicity and the efficiency as CCN. For particles in accumulated mode, the hygroscopicity parameter kappa (κ) under background conditions is ~20% higher than κ derived under polluted conditions; the situation is more complex especially for biomass burning influenced particles in nucleation or Aitken mode. Accordingly, the CCN nucleation efficiency of aerosols produced by local biomass burning was very low. This is because the particles were freshly emitted with low oxidation level organics and thus are less hygroscopic. The CCN activation efficiency was enhanced significantly when the site was under the influence of air transported from far away, during which aerosol properties changed with more hygroscopic secondary organic and inorganic components.

While the particle number size distribution is found to play a dominant role in predicting (N_CCN), a strong dependence of N_CCN on the mass fraction of organics (x_org) was also noted. N_CCN was underestimated by 52% and 13% at supersaturation levels of 0.13% and 0.76%, respectively, when x_org = 66%. N_CCN was slightly overestimated, or in good agreement, with observations when x_org was reduced to 35% (R²=0.94). 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 goal of further examining the sensitivity of 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 with Xinzhou.