Description:
<jats:title>Abstract</jats:title><jats:p>A total of 118 stratiform water clouds were observed by ground‐based remote sensing instruments at the Mace Head Atmospheric Research Station on the west coast of Ireland from 2009 to 2015. Microphysical and optical characteristics of these clouds were studied as well as the impact of aerosols on these properties. Microphysical and optical cloud properties were derived using the algorithm SYRSOC (SYnergistic Remote Sensing Of Clouds). Ground‐based in situ measurements of aerosol concentrations and the transport path of air masses at cloud level were investigated as well. The cloud properties were studied in dependence of the prevailing air mass at cloud level and season. We found higher cloud droplet number concentrations (CDNC) and smaller effective radii (<jats:italic>r</jats:italic><jats:sub>eff</jats:sub>) with greater pollution. Median CDNC ranged from 60 cm<jats:sup>−3</jats:sup> in marine air masses to 160 cm<jats:sup>−3</jats:sup> in continental air. Median <jats:italic>r</jats:italic><jats:sub>eff</jats:sub> ranged from 8 μm in polluted conditions to 10 μm in marine air. Effective droplet size distributions were broader in marine than in continental cases. Cloud optical thickness (COT) and albedo were lower in cleaner air masses and higher in more polluted conditions, with medians ranging from 2.1 to 4.9 and 0.22 to 0.39, respectively. However, calculation of COT and albedo was strongly affected by liquid water path (LWP) and departure from adiabatic conditions. A comparison of SYRSOC results with MODIS (Moderate‐Resolution Imaging Spectroradiometer) observations showed large differences for LWP and COT but good agreement for <jats:italic>r</jats:italic><jats:sub>eff</jats:sub> with a linear fit with slope near 1 and offset of −1 μm.</jats:p>