From hygroscopic aerosols to cloud droplets: The HygrA-CD campaign in the Athens basin - An overview.

The international experimental campaign Hygroscopic Aerosols to Cloud Droplets (HygrA-CD), organized in the Greater Athens Area (GAA), Greece from 15 May to 22 June 2014, aimed to study the physico-chemical properties of aerosols and their impact on the formation of clouds in the convective Planetary Boundary Layer (PBL). We found that under continental (W-NW-N) and Etesian (NE) synoptic wind flow and with a deep moist PBL (~2-2.5km height), mixed hygroscopic (anthropogenic, biomass burning and marine) particles arrive over the GAA, and contribute to the formation of convective non-precipitating PBL clouds (of ~16-20µm mean diameter) with vertical extent up to 500m. Under these conditions, high updraft velocities (1-2ms-1) and cloud condensation nuclei (CCN) concentrations (~2000cm-3 at 1% supersaturation), generated clouds with an estimated cloud droplet number of ~600cm-3. Under Saharan wind flow conditions (S-SW) a shallow PBL (<1-1.2km height) develops, leading to much higher CCN concentrations (~3500-5000cm-3 at 1% supersaturation) near the ground; updraft velocities, however, were significantly lower, with an estimated maximum cloud droplet number of ~200cm-3 and without observed significant PBL cloud formation. The largest contribution to cloud droplet number variance is attributed to the updraft velocity variability, followed by variances in aerosol number concentration.

Correcting global solar ultraviolet spectra recorded by a brewer spectroradiometer for its angular response error.

We present a methodology for correcting the global UV spectral measurements of a Brewer MKIII spectroradiometer for the error introduced by the deviation of the angular response of the instrument from the ideal response. This methodology is applicable also to other Brewer spectroradiometers that are currently in operation. The various stages of the methodology are described in detail, together with the uncertainties involved in each stage. Finally global spectral UV measurements with and without the application of the correction are compared with collocated measurements of another spectroradiometer and with model calculations, demonstrating the efficiency of the method. Depending on wavelength and on the aerosol loading, the cosine correction factors range from 2% to 7%. The uncertainties involved in the calculation of these correction factors were found to be relatively small, ranging from ~0.2% to ~2%.