ACE-2 HILLCLOUD. An overview of the ACE-2 ground-based cloud experiment

Abstract

The ACE‐2 HILLCLOUD experiment was carried out on the island of Tenerife in June–July 1997 to investigate the interaction of the boundary layer aerosol with a hill cap cloud forming over a ridge to the north‐east of the island. The cloud was used as a natural flow through reactor to investigate the dependence of the cloud microphysics and chemistry on the characteristics of the aerosols and trace gases entering cloud, and to simultaneously study the influence of the physical and chemical processes occurring within the cloud on the size distribution, chemical and hygroscopic properties of the aerosol exiting cloud. 5 major ground base sites were used, measuring trace gases and aerosols upwind and downwind of the cloud, and cloud microphysics and chemistry and interstitial aerosol and gases within the cloud on the hill. 8 intensive measurement periods or runs were undertaken during cloud events, (nocturnally for seven of the eight runs) and were carried out in a wide range of airmass conditions from clean maritime to polluted continental. Polluted air was characterised by higher than average concentrations of ozone (>50 ppbv), fine and accumulation mode aerosols (>3000 and >1500 cm−3, respectively) and higher aerosol mass loadings. Cloud droplet number concentrations N, increased from 50 cm−3 in background maritime air to >2500 cm−3 in aged polluted continental air, a concentration much higher than had previously been detected. Surprisingly, N was seen to vary almost linearly with aerosol number across this range. The droplet aerosol analyser (DAA) measured higher droplet numbers than the corrected forward scattering spectrometer probe (FSSP) in the most polluted air, but at other times there was good agreement (FSSP=0.95 DAA with an r2=0.89 for N<1200 cm−3). Background ammonia gas concentrations were around 0.3 ppbv even in air originating over the ocean, another unexpected but important result for the region. NO2 was present in background concentrations of typically 15 pptv to 100 pptv and NO˙3 (the nitrate radical) was observed at night throughout. Calculations suggest NO˙3 losses were mainly by reaction with DMS to produce nitric acid. Low concentrations of SO2(∼30 pptv), HNO3 and HCl were always present. HNO3 concentrations were higher in polluted episodes and calculations implied that these exceeded those which could be accounted for by NO2 oxidation. It is presumed that nitric and hydrochloric acids were present as a result of outgassing from aerosol, the HNO3 from nitrate rich aerosol transported into the region from upwind of Tenerife, and HCl from sea salt aerosol newly formed at the sea surface. The oxidants hydrogen peroxide and ozone were abundant (i.e., were well in excess over SO2 throughout the experiment). Occasions of significant aerosol growth following cloud processing were observed, particularly in cleaner cases. Observations and modelling suggested this was due mainly to the take up of nitric acid, hydrochloric acid and ammonia by the smallest activated aerosol particles. On a few occasions a ...

A GCM study of organic matter in marine aerosol and its potential contribution to cloud drop activation

“…These are 90±27 cm −3 for the remote North Atlantic and up to 180 cm −3 for the eastern North Atlantic (Bennartz, 2007). In situ observations of CDNC in the North Atlantic marine atmosphere are reported by Leaitch et al (1996), Borys et al (1998) and Bower et al (2000). Measured CDNC ranges between 50 and 400 cm −3 for relatively clean conditions.…”

A simple relationship between cloud drop number concentration and precursor aerosol concentration for the regions of Earth's large marine stratocumulus decks

“…However, the precise location will be a complex function of numerous variables such as the temperature at the lifting condensation level, updraft velocity, the shape and composition of the aerosol size spectrum, etc (cf., Chuang et al, 2000;Leaitch et al, 1996). For example, we note that Bowers et al (2000) found no evidence of a "role off" for aerosol concentrations until the concentrations (which encompassed both clean marine and continental pollution cases) exceeded 4000 cm −3 . On the other hand, Lu et al (2008), looking at the CDNC-AMNC relationship in polluted air in the Gulf of Mexico, reported a decrease in activation efficiency in the 400-800 cm −3 AMNC range, about the same as the "role-off" point in Martin et al Similarly, Leaitch et al (1986) found a "role-off" point for continental clouds of ∼700-800 cm −3 with a slope below this point quite similar to that derived from our data.…”

Influence of cloud processing on CCN activation behaviour in the Thuringian Forest, Germany during HCCT-2010

“…Bower et al, 1999;Herrmann et al, 2005). The use of hill cap clouds as a natural flow-through reactor was for example realised successfully on Great Dun Fell (UK) (Bower et al, 1999;Choularton et al, 1997;, on the German mountain Kleiner Feldberg (Wobrock et al, 1994), during the cap cloud experiment of the 2nd Aerosol Characterisation Experiment (ACE-2 HILLCLOUD) at Teneriffe, Spain (Bower et al, 2000) and during FEBUKO (Field Investigations of Budgets and Conversions of Particle Phase Organics in Tropospheric Cloud Processes) at Mt. Schmücke .…”