Hygroscopicity and cloud nucleating properties of aerosols measured at two islands in the Yellow Sea

East Asia, due to rapid economic growth within the region, is regarded as one of the largest anthropogenic source of aerosols. Yellow Sea lies at the downwind region of Chinese eastern coast where many industrial facilities are located. Therefore, the eastern side of the Yellow Sea is in such a great geographical setting for monitoring Asian continental outflow.

During the summer 2006, spring 2007 and summer-autumn 2008, field campaigns were conducted at Gosan, Jeju Island, located south of the Korean Peninsula. Gosan is about 500 km away from Shanghai. Measured properties are total aerosol number concentration (N_CN), submicron aerosol size distribution, CCN number concentration (N_CCN) and hygroscopic growth factor (GF) of aerosols smaller than 250 nm. Similar measurements were made during the summer 2009 campaign at Baengyeong Island.

Critical supersaturation (S_c) of ambient aerosols was additionally measured for 2008 and 2009 campaigns. Monodisperse aerosols were classified by a DMA and fed into both DMT CCN Counter (0.07~1.3% S range) and TSI CPC 3010. Such instrumentation setting required about 70 min of sampling to obtain one S_c measurement for two selected sizes near 100 nm. Such setting does not require the assumption that the aerosol should have homogenous chemical composition throughout the whole submicron size distribution. Since the local disturbance was minimal, required sampling time was short enough to obtain reliable S_c for most of the samples.

The GF measurements by the H-TDMA system and the S_c measurements by the DMA-CCN Counter system were converted to hygroscopicity parameter κ (Petters and Kreidenweis, 2007). The average values of κ derived from GF measurements were 0.43 ± 0.23, 0.25 ± 0.06, 0.16 ± 0.05, 0.36 ± 0.08 for the summer 2006, spring 2007, summer-autumn 2008 and summer 2009 campaigns, respectively, for particles of 100 nm diameter. The κ values derived from S_c measurements were 0.25 ± 0.06 and 0.39 ± 0.11 for summer-autumn 2008 and summer 2009, respectively. Overall κ values from S_c measurements showed a good agreement with the ones from GF measurements. For the summer-autumn 2008 campaign, κ values from S_c were larger than those from GF by about 0.1 for the whole period. For most of the 2009 campaign, the absolute value agreed well when the values were smaller than 0.5. N_CCN/N_CN ratios for each campaign were 0.65 ± 0.15, 0.52 ± 0.14, 0.45 ± 0.14 and 0.70 ± 0.30 during the summer 2006, spring 2007, summer-autumn 2008 and summer 2009, respectively. This is consistent with the order of κ values from GF measurements except that the largest κ is in 2006 while N_CCN/N_CN ratio is the largest in 2009.

It was found that κ values from larger particles (~200 nm) had negative correlation with relative humidity (RH) for 2008 campaign, while for 2009 campaign κ value showed no RH dependency for all sizes of particles. Moreover κ and N_CCN/N_CN showed large variation from day to day. Further analysis will be presented at the conference.