On The Importance Of Nitrate For The Droplet Concentration In Stratocumulus In The North-Sea Region

Anthropogenic aerosol particles serve as extra Cloud Condensation Nuclei (CCN), increasing the cloud droplet number concentration and modifying the cloud properties. The influence of manmade aerosol is the highest in a marine environment due to a limited number of natural CCN. Marine stratocumulus-like clouds (MSC) are the most frequent cloud-type in the North Sea region. The general assumption is that sulphate is the dominant compound of the CCN. However, high levels of manmade nitrate in the area suggest a significant role for this component. We made a first assessment of how many CCN with a marine origin were dominated by nitrate via two intensive campaigns in a large flow-through cloud chamber at the coast in the Netherlands wherein the formation of stratocumulus is simulated. It was consistently observed that the CCN were virtually all in the submicron range. In marine air masses from the NW-quadrant submicron nitrate was negligible and sulphate was the dominant CCN-component; this was also the case in air masses that passed over the narrow corridor of the English Channel. In air masses from the SW quadrant, climatologically the most frequent air mass with stratuslike overcast, submicron-nitrate was the dominant compound apparently produced from sources located over southern UK and W-France/Belgium. The CCN were mostly (98%) in the size range of 100?450 nm. During the summer campaign (2007), nitrate was virtually absent in the smaller and more numerous CCN in this size range. During a, shorter, study in the first half of April (2009), all CCN were dominated by nitrate. This possible seasonal difference in the role of nitrate in SW-air was the main subject evaluated in a monitoring effort in 2008 with a total of 7 months of valid data. The mass concentration ratio of nitrate to sulphate in the ?CCN?-range (the range in which 85% of the CCN number concentration is present) was used to identify the periods when nitrate was the prominent compound. In winter/early spring CCN-nitrate was the dominant compound in the CCN, while during summer the ratio of CCN-nitrate to CCN-sulphate was 0.2 and close to zero in the CCN with a diameter smaller than 150 nm. This seasonal difference can be explained by the low stability of the semi-volatile ammonium nitrate at the elevated temperatures in summer, which specifically applies for the smallest CCN due to their small mass amounts. Further data are required to obtain a reliable climatology. Note: sea salt contributed negligibly to the CCN-number; organic aerosol, measured in the April campaign, was present in significant concentrations but of minor importance because of its low hygroscopicity.