The radiative effect of ion-induced inorganic nucleation in the free troposphere

To determine the effect of cosmic rays on the Earth's climate via ion-induced nucleation, a parametrisation of inorganic nucleation was formulated based on experiments at the CERN CLOUD experiment. The parametrisation was implemented in the GLOMAP aerosol microphysics model and used to estimate the radiative effect of the change in ionisation experienced over an 11-year solar cycle

Ternary H_2SO_4-H_2O-NH_3 Neutral and Charged Nucleation Rates for a Wide Range of Atmospheric Conditions

The formation of new particles for the ternary system involving sulfuric acid, water vapor and ammonia has been studied in detail. The nucleation rates were obtained from experiments at the CERN CLOUD chamber which allows the measurement of new particle formation under very well defined conditions. Some of its key features are the suppression of contaminants at the technological limit and a very precise control of a wide range of temperatures, trace gas concentrations and nucleation rates. The effect of ionizing radiation on the ternary nucleation rates was investigated by using the CERN proton synchrotron beam (beam conditions), natural galactic cosmic rays (gcr conditions) as well as the high voltage clearing field inside the chamber to suppress the effect of charges (neutral conditions). The dependence of the nucleation rate on ion concentration, sulfuric acid and ammonia concentration as well as temperature was studied extensively. This way, an unprecedented set of data was collected giving insight into the role of neutral and charged ternary NH_3 nucleation and the relative importance of the different parameters

Causes and importance of new particle formation in the present-day and preindustrial atmospheres

New particle formation has been estimated to produce around half of cloud-forming particles in the present-day atmosphere, via gas-to-particle conversion. Here we assess the importance of new particle formation (NPF) for both the present-day and the preindustrial atmospheres. We use a global aerosol model with parametrizations of NPF from previously published CLOUD chamber experiments involving sulfuric acid, ammonia, organic molecules, and ions. We find that NPF produces around 67% of cloud condensation nuclei at 0.2% supersaturation (CCN0.2%) at the level of low clouds in the preindustrial atmosphere (estimated uncertainty range 45-84%) and 54% in the present day (estimated uncertainty range 38-66%). Concerning causes, we find that the importance of biogenic volatile organic compounds (BVOCs) in NPF and CCN formation is greater than previously thought. Removing BVOCs and hence all secondary organic aerosol from our model reduces low-cloud-level CCN concentrations at 0.2% supersaturation by 26% in the present-day atmosphere and 41% in the preindustrial. Around three quarters of this reduction is due to the tiny fraction of the oxidation products of BVOCs that have sufficiently low volatility to be involved in NPF and early growth. Furthermore, we estimate that 40% of preindustrial CCN0.2% are formed via ion-induced NPF, compared with 27% in the present day, although we caution that the ion-induced fraction of NPF involving BVOCs is poorly measured at present. Our model suggests that the effect of changes in cosmic ray intensity on CCN is small and unlikely to be comparable to the effect of large variations in natural primary aerosol emissions. Plain Language Summary New particle formation in the atmosphere is the process by which gas molecules collide and stick together to form atmospheric aerosol particles. Aerosols act as seeds for cloud droplets, so the concentration of aerosols in the atmosphere affects the properties of clouds. It is important to understand how aerosols affect clouds because they reflect a lot of incoming solar radiation away from Earth's surface, so changes in cloud properties can affect the climate. Before the Industrial Revolution, aerosol concentrations were significantly lower than they are today. In this article, we show using global model simulations that new particle formation was a more important mechanism for aerosol production than it is now. We also study the importance of gases emitted by vegetation, and of atmospheric ions made by radon gas or cosmic rays, in preindustrial aerosol formation. We find that the contribution of ions and vegetation to new particle formation was also greater in the preindustrial period than it is today. However, the effect on particle formation of variations in ion concentration due to changes in the intensity of cosmic rays reaching Earth was small.Peer reviewe

Experimental particle formation rates spanning tropospheric sulfuric acid and ammonia abundances, ion production rates, and temperatures

Binary nucleation of sulfuric acid and water as well as ternary nucleation involving ammonia are thought to be the dominant processes responsible for new particle formation (NPF) in the cold temperatures of the middle and upper troposphere. Ions are also thought to be important for particle nucleation in these regions. However, global models presently lack experimentally measured NPF rates under controlled laboratory conditions and so at present must rely on theoretical or empirical parameterizations. Here with data obtained in the European Organization for Nuclear Research CLOUD (Cosmics Leaving OUtdoor Droplets) chamber, we present the first experimental survey of NPF rates spanning free tropospheric conditions. The conditions during nucleation cover a temperature range from 208 to 298K, sulfuric acid concentrations between 5x10(5) and 1x10(9)cm(-3), and ammonia mixing ratios from zero added ammonia, i.e., nominally pure binary, to a maximum of -1400 parts per trillion by volume (pptv). We performed nucleation studies under pure neutral conditions with zero ions being present in the chamber and at ionization rates of up to 75ion pairs cm(-3)s(-1) to study neutral and ion-induced nucleation. We found that the contribution from ion-induced nucleation is small at temperatures between 208 and 248K when ammonia is present at several pptv or higher. However, the presence of charges significantly enhances the nucleation rates, especially at 248K with zero added ammonia, and for higher temperatures independent of NH3 levels. We compare these experimental data with calculated cluster formation rates from the Atmospheric Cluster Dynamics Code with cluster evaporation rates obtained from quantum chemistry.Peer reviewe

Comparison of the SAWNUC model with CLOUD measurements of sulphuric acid-water nucleation

Binary nucleation of sulphuric acid-water particles is expected to be an important process in the free troposphere at low temperatures. SAWNUC (Sulphuric Acid Water Nucleation) is a model of binary nucleation that is based on laboratory measurements of the binding energies of sulphuric acid and water in charged and neutral clusters. Predictions of SAWNUC are compared for the first time comprehensively with experimental binary nucleation data from the CLOUD chamber at European Organization for Nuclear Research. The experimental measurements span a temperature range of 208-292K, sulphuric acid concentrations from 110(6) to 110(9)cm(-3), and distinguish between ion-induced and neutral nucleation. Good agreement, within a factor of 5, is found between the experimental and modeled formation rates for ion-induced nucleation at 278K and below and for neutral nucleation at 208 and 223K. Differences at warm temperatures are attributed to ammonia contamination which was indicated by the presence of ammonia-sulphuric acid clusters, detected by an Atmospheric Pressure Interface Time of Flight (APi-TOF) mass spectrometer. APi-TOF measurements of the sulphuric acid ion cluster distributions (H2SO4)HSO4 with i = 0, 1, ..., 10) show qualitative agreement with the SAWNUC ion cluster distributions. Remaining differences between the measured and modeled distributions are most likely due to fragmentation in the APi-TOF. The CLOUD results are in good agreement with previously measured cluster binding energies and show the SAWNUC model to be a good representation of ion-induced and neutral binary nucleation of sulphuric acid-water clusters in the middle and upper troposphere.Peer reviewe

Evolution of particle composition in CLOUD nucleation experiments

Peer reviewe

Reduced anthropogenic aerosol radiative forcing caused by biogenic new particle formation

The magnitude of aerosol radiative forcing caused by anthropogenic emissions depends on the baseline state of the atmosphere under pristine preindustrial conditions. Measurements show that particle formation in atmospheric conditions can occur solely from biogenic vapors. Here, we evaluate the potential effect of this source of particles on preindustrial cloud condensation nuclei (CCN) concentrations and aerosol-cloud radiative forcing over the industrial period. Model simulations show that the pure biogenic particle formation mechanism has a much larger relative effect on CCN concentrations in the preindustrial atmosphere than in the present atmosphere because of the lower aerosol concentrations. Consequently, preindustrial cloud albedo is increased more than under present day conditions, and therefore the cooling forcing of anthropogenic aerosols is reduced. The mechanism increases CCN concentrations by 20-100% over a large fraction of the preindustrial lower atmosphere, and the magnitude of annual global mean radiative forcing caused by changes of cloud albedo since 1750 is reduced by 0.22 W m-2 (27%) to -0.60 W m-2. Model uncertainties, relatively slow formation rates, and limited available ambient measurements make it difficult to establish the significance of a mechanism that has its dominant effect under preindustrial conditions. Our simulations predict more particle formation in the Amazon than is observed. However, the first observation of pure organic nucleation has now been reported for the free troposphere. Given the potentially significant effect on anthropogenic forcing, effort should be made to better understand such naturally driven aerosol processes

Molecular understanding of sulphuric acid-amine particle nucleation in the atmosphere

4 pages 359-363 in the print version, additional 7 pages online.Peer reviewe

The effect of acid-base clustering and ions on the growth of atmospheric nano-particles

The growth of freshly formed aerosol particles can be the bottleneck in their survival to cloud condensation nuclei. It is therefore crucial to understand how particles grow in the atmosphere. Insufficient experimental data has impeded a profound understanding of nano-particle growth under atmospheric conditions. Here we study nano-particle growth in the CLOUD (Cosmics Leaving OUtdoors Droplets) chamber, starting from the formation of molecular clusters. We present measured growth rates at sub-3 nm sizes with different atmospherically relevant concentrations of sulphuric acid, water, ammonia and dimethylamine. We find that atmospheric ions and small acid-base clusters, which are not generally accounted for in the measurement of sulphuric acid vapour, can participate in the growth process, leading to enhanced growth rates. The availability of compounds capable of stabilizing sulphuric acid clusters governs the magnitude of these effects and thus the exact growth mechanism. We bring these observations into a coherent framework and discuss their significance in the atmosphere.Peer reviewe