The effect of anthropogenic aerosols on cloud droplet concentrations and radiative properties is the source of one of the largest uncertainties in the radiative forcing of climate over the industrial period. This uncertainty affects our ability to estimate how sensitive the climate is to greenhouse gas emissions. Here we perform a sensitivity analysis on a global model to quantify the uncertainty in cloud radiative forcing over the industrial period caused by uncertainties in aerosol emissions and processes. Our results show that 45 per cent of the variance of aerosol forcing since about 1750 arises from uncertainties in natural emissions of volcanic sulphur dioxide, marine dimethylsulphide, biogenic volatile organic carbon, biomass burning and sea spray. Only 34 per cent of the variance is associated with anthropogenic emissions. The results point to the importance of understanding pristine pre-industrial-like environments, with natural aerosols only, and suggest that improved measurements and evaluation of simulated aerosols in polluted present-day conditions will not necessarily result in commensurate reductions in the uncertainty of forcing estimates.

Fundamental 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. N ucleation of particles occurs throughout Earth's atmosphere by condensation of trace vapors (1-3). Around 40 to 70% of global cloud condensation nuclei (CCN) (4-6) are thought to originate as nucleated particles, so the process has a major influence on the microphysical properties of clouds and the radiative balance of the global climate system. However, laboratory measurements are needed to disentangle and quantify the processes that contribute to particle formation, and very few laboratory measurements exist under atmospheric conditions (7)(8)(9)(10). This leaves open fundamental questions concerning the origin of particles on a global scale. First, it is not known whether nucleation is predominantly a neutral process, as assumed in most models (11-13), or whether atmospheric ions are important (6,(14)(15)(16). This relates to the question of whether solar-modulated galactic cosmic rays (GCRs) affect aerosols, clouds, and climate (17-21). Second, the lack of measurements of nucleation rates at low temperatures means that the origin of new particles in the vast regions of the cold free troposphere has not yet been experimentally established. Third, whereas it has been shown that nucleation of sulfuric acid (H 2 SO 4 )-water particles in the boundary layer requires stabilizing molecules such as ammonia (NH 3 ), amines, or oxidized organic compounds (7,8,(22)(23)(24), it is not yet known from existing experimental data over how much of the troposphere these molecules are important for nucleation. Robust atmospheric models to answer these questions need to be founded on direct measurements of nucleation rates. At present, to simulate nucleation over a very wide range of atmospheric conditions, global models must use theoretical nucleation models (25, 26), which can require adjustments to the nucleation rates of several orders of magnitude to obtain reasonable agreement with ambient observations (27,28).The lack of an experimentally based model of global particle nucleation is in stark contrast to global models of atmos...

Atmospheric black carbon makes an important but poorly quantified contribution to the warming of the global atmosphere. Laboratory and modelling studies have shown that the addition of non-black carbon materials to black carbon particles may enhance the particles' light absorption by 50 to 60% by refracting and reflecting light. Real world experimental evidence for this 'lensing' effect is scant and conflicting, showing that absorption enhancements can be less than 5% or as large as 140%. Here we present simultaneous quantifications of the composition and optical properties of individual atmospheric black carbon particles. We show that particles with a mass ratio of non-black carbon to black carbon of less than 1.5, which is typical of fresh traffic sources, are best represented as having no absorption enhancement. In contrast, black carbon particles with a ratio greater than 3, which is typical of biomass burning emissions, are best described assuming optical lensing leading to an absorption enhancement. We introduce a generalised hybrid model approach for estimating scattering and absorption enhancements based on laboratory and atmospheric observations. We conclude that the occurrence of the absorption enhancement of black carbon particles is determined by the particles' mass ratio of non-black carbon to black carbon.Atmospheric black carbon (BC) makes the second largest single contribution after CO 2 to climate forcing in the present-day atmosphere 1 . Previous detailed modelling and laboratory studies have shown that weakly absorbing non-BC materials contained within the same particles as BC can significantly enhance the absorption per unit mass of the latter through refraction and internal reflections, sometimes referred to as the 'lensing effect' 2,3 . A "coreshell" description 4 has often been applied to describe this effect when coatings envelop the central BC core, but this oversimplifies the complex particle morphologies 5 . The non-BC components may not be evenly distributed and the BC core is not necessarily completely enclosed, and as such the absorption enhancement predicted using the core-shell approach could greatly overestimate the real value 3 . Microscopy 5,6 can examine BC microphysical properties but has limited quantitative capability and may evaporate semi-volatile materials.By detecting the remaining non-BC fragment after laser induced incandescence with a single particle soot photometer (SP2 7 , DMT inc.), Sedlacek et al. 8 and Moteki et al. 9 reported the non-core-shell structure of some BC particles, however they did not provide an appropriate model approach to estimate optical properties. Measurement of single BC particle mass ratioIn this study, for the first time we quantify the mixing state of individual BC particles using morphology-independent measurements of the total particle mass (M p ) and the mass of the refractory black carbon, rBC (M rBC ) from a variety of laboratory and field experiments. We determined the mass ratio, M R (= M non-BC /M rBC ), where M non-BC is the mas...

Abstract. We synthesised observations of total particle number (CN) concentration from 36 sites around the world. We found that annual mean CN concentrations are typically 300-2000 cm −3 in the marine boundary layer and free troposphere (FT) and 1000-10 000 cm −3 in the continental boundary layer (BL). Many sites exhibit pronounced seasonality with summer time concentrations a factor of 2-10 greater than wintertime concentrations. We used these CN obserCorrespondence to: D. V. Spracklen (dominick@env.leeds.ac.uk) vations to evaluate primary and secondary sources of particle number in a global aerosol microphysics model. We found that emissions of primary particles can reasonably reproduce the spatial pattern of observed CN concentration (R 2 =0.46) but fail to explain the observed seasonal cycle (R 2 =0.1). The modeled CN concentration in the FT was biased low (normalised mean bias, NMB=−88%) unless a secondary source of particles was included, for example from binary homogeneous nucleation of sulfuric acid and water (NMB=−25%). Simulated CN concentrations in the continental BL were also biased low (NMB=−74%) unless the number emission of anthropogenic primary particles was increased or a Published by Copernicus Publications on behalf of the European Geosciences Union. 4776 D. V. Spracklen et al.: Explaining global aerosol number concentrations mechanism that results in particle formation in the BL was included. We ran a number of simulations where we included an empirical BL nucleation mechanism either using the activation-type mechanism (nucleation rate, J , proportional to gas-phase sulfuric acid concentration to the power one) or kinetic-type mechanism (J proportional to sulfuric acid to the power two) with a range of nucleation coefficients. We found that the seasonal CN cycle observed at continental BL sites was better simulated by BL particle formation (R 2 =0.3) than by increasing the number emission from primary anthropogenic sources (R 2 =0.18). The nucleation constants that resulted in best overall match between model and observed CN concentrations were consistent with values derived in previous studies from detailed case studies at individual sites. In our model, kinetic and activation-type nucleation parameterizations gave similar agreement with observed monthly mean CN concentrations.

Aerosol–cloud interaction effects are a major source of uncertainty in climate models so it is important to quantify the sources of uncertainty and thereby direct research efforts. However, the computational expense of global aerosol models has prevented a full statistical analysis of their outputs. Here we perform a variance-based analysis of a global 3-D aerosol microphysics model to quantify the magnitude and leading causes of parametric uncertainty in model-estimated present-day concentrations of cloud condensation nuclei (CCN). Twenty-eight model parameters covering essentially all important aerosol processes, emissions and representation of aerosol size distributions were defined based on expert elicitation. An uncertainty analysis was then performed based on a Monte Carlo-type sampling of an emulator built for each model grid cell. The standard deviation around the mean CCN varies globally between about ±30% over some marine regions to ±40–100% over most land areas and high latitudes, implying that aerosol processes and emissions are likely to be a significant source of uncertainty in model simulations of aerosol–cloud effects on climate. Among the most important contributors to CCN uncertainty are the sizes of emitted primary particles, including carbonaceous combustion particles from wildfires, biomass burning and fossil fuel use, as well as sulfate particles formed on sub-grid scales. Emissions of carbonaceous combustion particles affect CCN uncertainty more than sulfur emissions. Aerosol emission-related parameters dominate the uncertainty close to sources, while uncertainty in aerosol microphysical processes becomes increasingly important in remote regions, being dominated by deposition and aerosol sulfate formation during cloud-processing. The results lead to several recommendations for research that would result in improved modelling of cloud–active aerosol on a global scale