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 f 44 , 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 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 > 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 (f 44 < 11 %); the underestimation decreased to 32 and 23 % at the corresponding supersaturations, however, if the particles were with more hygroscopic secondary organics (f 44 > 15 %). The N CCN at the supersaturation of 0.76 % was underestimated by 11 and 4 %, respectively, at f 44 < 11 and f 44 > 15 %. However, for the particles composed of low organics (e.g., x org < 40 %), 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 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.

Abstract. A multi-axis differential optical absorption spectroscopy (MAX-DOAS) instrument was deployed in May and June 2016 at a monitoring station (37.18∘ N, 114.36∘ E) in the suburban area of Xingtai, which is one of the most polluted cities in the North China Plain (NCP), during the Atmosphere-Aerosol-Boundary Layer-Cloud (A2BC) experiment and Air chemistry Research In Asia (ARIAs) joint experiments to derive tropospheric vertical profiles of NO2, SO2, HONO, HCHO, CHOCHO and aerosols. Aerosol optical depths derived from MAX-DOAS were found to be consistent with collocated sun-photometer measurements. Also the derived near-surface aerosol extinction and HCHO mixing ratio agree well with the coincident visibility meter and in situ HCHO measurements, with mean HCHO near-surface mixing ratios of ∼3.5 ppb. Underestimations of MAX-DOAS results compared to in situ measurements of NO2 (∼60 %) and SO2 (∼20 %) are found expectedly due to vertical and horizontal inhomogeneity of trace gases. Vertical profiles of aerosols and NO2 and SO2 are reasonably consistent with those measured by a collocated Raman lidar and aircraft spirals over the station. The deviations can be attributed to differences in sensitivity as a function of altitude and substantial horizontal gradients of pollutants. Aerosols, HCHO and CHOCHO profiles typically extended to higher altitudes (with 75 % integrated column located below ∼1.4 km) than NO2, SO2 and HONO did (with 75 % integrated column below ∼0.5 km) under polluted conditions. Lifted layers were systematically observed for all species (except HONO), indicating accumulation, secondary formation or long-range transport of the pollutants at higher altitudes. Maximum values routinely occurred in the morning for NO2, SO2 and HONO but occurred at around noon for aerosols, HCHO and CHOCHO, mainly dominated by photochemistry, characteristic upslope–downslope circulation and planetary boundary layer (PBL) dynamics. Significant day-to-day variations are found for all species due to the effect of regional transport and changes in synoptic pattern analysed with the backward propagation approach based on HYSPLIT trajectories. Low pollution was often observed for air masses from the north-west (behind cold fronts), and high pollution was observed from the southern areas such as industrialized Wu'an. The contribution of regional transport for the pollutants measured at the site during the observation period was estimated to be about 20 % to 30 % for trace gases and about 50 % for aerosols. In addition, agricultural burning events impacted the day-to-day variations in HCHO, CHOCHO and aerosols. It needs to be noted that although several MAX-DOAS measurements of trace gases and aerosols in the NCP area have been reported in previous studies, this study is the first work to derive a comprehensive set of vertical profiles of NO2, SO2, HONO, HCHO, CHOCHO and aerosols from measurements of one MAX-DOAS instrument. Also, so far, the validation of MAX-DOAS profile results by comparison with various surface in situ measurements as well as profile measurements from lidar and aircraft is scarce. Moreover, the backward propagation approach for characterizing the contributions of regional transport of pollutants from different regions was applied to the MAX-DOAS results of trace gases and aerosols for the first time.

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