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A global model perturbed parameter ensemble of 60 simulations was used to explore how combinations of six parameters related to secondary organic aerosol (SOA) formation affect particle number concentrations and organic aerosol mass. The parameters represent the formation of organic compounds with different volatilities from biogenic and anthropogenic sources. The most plausible parameter combinations were determined by comparing the simulations against observations of the number concentration of particles larger than 3 nm diameter (N3), the number concentration of particles larger than 50 nm diameter (N50), and the organic aerosol (OA) mass concentration. The simulations expose a high degree of model equifinality in which the skill of widely different parameter combinations cannot be distinguished against observations. We therefore conclude that, based on the observations we have used, a six-parameter SOA scheme is under-determined. Nevertheless, the model skill in simulating N3 and N50 is clearly determined by the low-volatility and extremely low-volatility compounds that affect new particle formation and growth, and the skill in simulating OA mass is determined by the low-volatility and semi-volatile compounds. The biogenic low-volatility class of compounds that grow nucleated clusters and condense on all particles is found to have the strongest effect on the model skill in simulating N3, N50, and OA. The simulations also expose potential structural deficiencies in the model: we find that parameter combinations that are best for N3 and N50 are worst for OA mass, and the ensemble exaggerates the observed seasonal cycle of particle concentrations – a deficiency that we conclude requires an additional anthropogenic source of either primary or secondary particles.

Understanding the cloud vertical structure and its variation in space and time is important to reduce the uncertainty in climate forcing. Here, we present the cloud climatology over the oceanic regions (Arabian Sea, Bay of Bengal, and South Indian Ocean) adjacent to the Indian subcontinent using data from the Multiangle Imaging Spectroradiometer (MISR), Moderate Resolution Imaging Spectroradiometer (MODIS), GCM-Oriented CALIPSO Cloud Product (GOCCP), and International Satellite Cloud Climatology Project (ISCCP). Fractional cloud cover (fc) shows stronger seasonal variations over the Arabian Sea (mean annual fc lies in the range 0.5–0.61) and Bay of Bengal (mean annual fc lies in the range 0.69–0.75) relative to the South Indian Ocean (mean annual fc lies in the range 0.64–0.71). Inter-comparison of statistics from passive (MISR, MODIS and ISCCP) and active (GOCCP) sensors reveals the challenges in interpreting satellite data for climate implications. While MISR detects more low clouds because of its stere...

[1] Structural evolution of monsoon clouds in the core monsoon region of India has been examined using multisensor data. Invigoration of warm clouds above 4.5 km (occurring in only 15.4% days of the last 11 monsoon seasons) is associated with a transition from negative to positive normalized rainfall anomaly. Cloud top pressure reduces with an increase in aerosol optical depth at a higher rate of invigoration in drier condition (characterized by large fraction of absorbing aerosols) than wet condition. Cloud effective radius for warm clouds does not show any significant change with an increase in aerosol concentration in the presence of high liquid water path, probably due to strong buffering role of meteorology. The structural evolution of monsoon clouds is influenced by both dynamic and microphysical processes that prolong the cloud lifetime, resulting in infrequent rainfall. Our results call for improved representation of aerosol and cloud vertical structures in the climate models to resolve this issue.

Atmospheric aerosols and their effect on clouds are thought to be important for anthropogenic radiative forcing of the climate, yet remain poorly understood1. Globally, around half of cloud condensation nuclei originate from nucleation of atmospheric vapours2. It is thought that sulfuric acid is essential to initiate most particle formation in the atmosphere3,4, and that ions have a relatively minor role5. Some laboratory studies, however, have reported organic particle formation without the intentional addition of sulfuric acid, although contamination could not be excluded6,7. Here we present evidence for the formation of aerosol particles from highly oxidized biogenic vapours in the absence of sulfuric acid in a large chamber under atmospheric conditions. The highly oxygenated molecules (HOMs) are produced by ozonolysis of α-pinene. We find that ions from Galactic cosmic rays increase the nucleation rate by one to two orders of magnitude compared with neutral nucleation. Our experimental findings are supported by quantum chemical calculations of the cluster binding energies of representative HOMs. Ion-induced nucleation of pure organic particles constitutes a potentially widespread source of aerosol particles in terrestrial environments with low sulfuric acid pollution.

New particle formation in the atmosphere produces around half of the cloud condensation nuclei that seed cloud droplets. Such particles have a pivotal role in determining the properties of clouds and the global radiation balance. Dunne et al. used the CLOUD (Cosmics Leaving Outdoor Droplets) chamber at CERN to construct a model of aerosol formation based on laboratory measured nucleation rates. They found that nearly all nucleation involves either ammonia or biogenic organic compounds. Furthermore in the present day atmosphere cosmic ray intensity cannot meaningfully affect climate via nucleation.Science this issue p. 1119Fundamental questions remain about the origin of newly formed atmospheric aerosol particles because data from laboratory measurements have been insufficient to build global models. In contrast gas phase chemistry models have been based on laboratory kinetics measurements for decades. We built a global model of aerosol formation by using extensive laboratory measurements of rates of nucleation involving sulfuric acid ammonia ions and organic compounds conducted in the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber. The simulations and a comparison with atmospheric observations show that nearly all nucleation throughout the present day atmosphere involves ammonia or biogenic organic compounds in addition to sulfuric acid. A considerable fraction of nucleation involves ions but the relatively weak dependence on ion concentrations indicates that for the processes studied variations in cosmic ray intensity do not appreciably affect climate through nucleation in the present day atmosphere.U http://science.sciencemag.org/content/sci/354/6316/1119.full.pdf