Source Apportionment of Cloud Nuclei at 260 m and Ground Level in Urban Beijing

Cloud condensation nuclei (CCN) is a critical parameter for simulating aerosol-cloud interaction and aerosol indirect effects. Here we characterize the vertical differences of CCN number concentration (N-CCN) between ground level and 260 m based on field measurements on a meteorological tower in Beijing. The observed ground aerosol number concentration was about 16% higher than 260 m, unexpectedly, the ground N-CCN was on average 20% lower than 260 m within the atmospheric typical supersaturation (S) range of 0.1%-0.4%. Using positive matrix factorization analysis, we clarified that the higher N-CCN at 260 m was primarily due to increased contributions from both regional transport and secondary processes. The aerosol particles from the regional transport contributed 20%-50% and 31%-57% of CCN respectively at ground and 260 m within typical S, with higher proportions at smaller S. The secondary processes contributed generally about the equal amount CCN between ground (42%-55%) and 260 m (37%-52%). The total contribution of regional transport and secondary processes, as the main sources, contributed 72%-94% CCN during the campaign. While, nucleation events and local primary sources accounted for the smallest proportion of N-CCN at both heights. Under polluted conditions, the vertical difference was further enlarged. The results suggest that surface observations under clean conditions may well represent cloud nuclei aloft. However, the N-CCN measured at ground may be unexpectedly much lower than the tower height in polluted conditions, likely leading to an underestimation on cloud nuclei aloft during the field campaign.