| 1. |
- Fanourgakis, George S., et al.
(författare)
-
Evaluation of global simulations of aerosol particle and cloud condensation nuclei number, with implications for cloud droplet formation
- 2019
-
Ingår i: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 19:13, s. 8591-8617
-
Tidskriftsartikel (refereegranskat)abstract
- A total of 16 global chemistry transport models and general circulation models have participated in this study; 14 models have been evaluated with regard to their ability to reproduce the near-surface observed number concentration of aerosol particles and cloud condensation nuclei (CCN), as well as derived cloud droplet number concentration (CDNC). Model results for the period 2011-2015 are compared with aerosol measurements (aerosol particle number, CCN and aerosol particle composition in the submicron fraction) from nine surface stations located in Europe and Japan. The evaluation focuses on the ability of models to simulate the average across time state in diverse environments and on the seasonal and short-term variability in the aerosol properties. There is no single model that systematically performs best across all environments represented by the observations. Models tend to underestimate the observed aerosol particle and CCN number concentrations, with average normalized mean bias (NMB) of all models and for all stations, where data are available, of -24% and -35% for particles with dry diameters >50 and >120nm, as well as -36% and -34% for CCN at supersaturations of 0.2% and 1.0%, respectively. However, they seem to behave differently for particles activating at very low supersaturations (<0.1%) than at higher ones. A total of 15 models have been used to produce ensemble annual median distributions of relevant parameters. The model diversity (defined as the ratio of standard deviation to mean) is up to about 3 for simulated N3 (number concentration of particles with dry diameters larger than 3nm) and up to about 1 for simulated CCN in the extra-polar regions. A global mean reduction of a factor of about 2 is found in the model diversity for CCN at a supersaturation of 0.2% (CCN0.2) compared to that for N3, maximizing over regions where new particle formation is important. An additional model has been used to investigate potential causes of model diversity in CCN and bias compared to the observations by performing a perturbed parameter ensemble (PPE) accounting for uncertainties in 26 aerosol-related model input parameters. This PPE suggests that biogenic secondary organic aerosol formation and the hygroscopic properties of the organic material are likely to be the major sources of CCN uncertainty in summer, with dry deposition and cloud processing being dominant in winter. Models capture the relative amplitude of the seasonal variability of the aerosol particle number concentration for all studied particle sizes with available observations (dry diameters larger than 50, 80 and 120nm). The short-term persistence time (on the order of a few days) of CCN concentrations, which is a measure of aerosol dynamic behavior in the models, is underestimated on average by the models by 40% during winter and 20% in summer. In contrast to the large spread in simulated aerosol particle and CCN number concentrations, the CDNC derived from simulated CCN spectra is less diverse and in better agreement with CDNC estimates consistently derived from the observations (average NMB -13% and -22% for updraft velocities 0.3 and 0.6ms-1, respectively). In addition, simulated CDNC is in slightly better agreement with observationally derived values at lower than at higher updraft velocities (index of agreement 0.64 vs. 0.65). The reduced spread of CDNC compared to that of CCN is attributed to the sublinear response of CDNC to aerosol particle number variations and the negative correlation between the sensitivities of CDNC to aerosol particle number concentration (Nd=Na) and to updraft velocity (Nd=w). Overall, we find that while CCN is controlled by both aerosol particle number and composition, CDNC is sensitive to CCN at low and moderate CCN concentrations and to the updraft velocity when CCN levels are high. Discrepancies are found in sensitivities Nd=Na and Nd=w; models may be predisposed to be too "aerosol sensitive" or "aerosol insensitive" in aerosol-cloud-climate interaction studies, even if they may capture average droplet numbers well. This is a subtle but profound finding that only the sensitivities can clearly reveal and may explain intermodel biases on the aerosol indirect effect.
|
|
| 2. |
- Baker, Alex R., et al.
(författare)
-
Changing atmospheric acidity as a modulator of nutrient deposition and ocean biogeochemistry
- 2021
-
Ingår i: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 7:28
-
Tidskriftsartikel (refereegranskat)abstract
- Anthropogenic emissions to the atmosphere have increased the flux of nutrients, especially nitrogen, to the ocean, but they have also altered the acidity of aerosol, cloud water, and precipitation over much of the marine atmosphere. For nitrogen, acidity-driven changes in chemical speciation result in altered partitioning between the gas and particulate phases that subsequently affect long-range transport. Other important nutrients, notably iron and phosphorus, are affected, because their soluble fractions increase upon exposure to acidic environments during atmospheric transport. These changes affect the magnitude, distribution, and deposition mode of individual nutrients supplied to the ocean, the extent to which nutrient deposition interacts with the sea surface microlayer during its passage into bulk seawater, and the relative abundances of soluble nutrients in atmospheric deposition. Atmospheric acidity change therefore affects ecosystem composition, in addition to overall marine productivity, and these effects will continue to evolve with changing anthropogenic emissions in the future.
|
|
| 3. |
- Kalivitis, Nikos, et al.
(författare)
-
Formation and growth of atmospheric nanoparticles in the eastern Mediterranean : Results from long-term measurements and process simulations
- 2019
-
Ingår i: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 19:4, s. 2671-2686
-
Tidskriftsartikel (refereegranskat)abstract
- Atmospheric new particle formation (NPF) is a common phenomenon all over the world. In this study we present the longest time series of NPF records in the eastern Mediterranean region by analyzing 10 years of aerosol number size distribution data obtained with a mobility particle sizer. The measurements were performed at the Finokalia environmental research station on Crete, Greece, during the period June 2008-June 2018. We found that NPF took place on 27 % of the available days, undefined days were 23 % and non-event days 50 %. NPF is more frequent in April and May probably due to the terrestrial biogenic activity and is less frequent in August. Throughout the period under study, nucleation was observed also during the night. Nucleation mode particles had the highest concentration in winter and early spring, mainly because of the minimum sinks, and their average contribution to the total particle number concentration was 8 %. Nucleation mode particle concentrations were low outside periods of active NPF and growth, so there are hardly any other local sources of sub-25 nm particles. Additional atmospheric ion size distribution data simultaneously collected for more than 2 years were also analyzed. Classification of NPF events based on ion spectrometer measurements differed from the corresponding classification based on a mobility spectrometer, possibly indicating a different representation of local and regional NPF events between these two measurement data sets. We used the MALTE-Box model for simulating a case study of NPF in the eastern Mediterranean region. Monoterpenes contributing to NPF can explain a large fraction of the observed NPF events according to our model simulations. However the adjusted parameterization resulting from our sensitivity tests was significantly different from the initial one that had been determined for the boreal environment.
|
|
| 4. |
- Rastak, Narges, 1980-
(författare)
-
Aerosol-water interaction at sub and super-saturated regimes : From small scale molecular mechanisms to large scale atmospheric models
- 2018
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The term “atmospheric aerosol” refers to solid or liquid particles suspended in the atmosphere. Atmospheric aerosols influence the Earth’s energy budget directly by scattering and absorbing radiation (known as the direct aerosol effect) and indirectly by acting as cloud condensation nuclei (CCN) and ice nucleating particles and thereby modifying cloud properties (known as the indirect aerosol effect). The water-affinity of aerosols plays an important role on one hand in defining the aerosol water-content and optical properties, and on the other hand in determining the conditions at which the aerosols can act as CCN. Aerosol-water interactions thus affect both the direct as well as the indirect aerosol effects, leading to impacts on the Earth’s energy budget and ultimately climate. The role of aerosols and clouds in determining the radiative balance of the Earth is one of the largest sources of uncertainty in understanding climate change. Therefore, the main goal of this thesis was to improve the knowledge of aerosol-water interactions. In this thesis, we investigated the links between aerosol molecular composition, hygroscopic growth and CCN activation, with a focus on organic compounds. Specifically, we tested several commonly-used simplifying approaches for describing water uptake, CCN activation and their impact on aerosol radiative properties.The traditional Köhler theory that describes the equilibrium between droplet and vapor phase along with modifications of these theory were used to investigate the water affinity of aerosol particles. The modifications to this theory used in this study are as follows: complete dissolution, hygroscopicity parameter (κ), soluble fraction (ε), treatment of adsorption, counting for gas-particle partitioning of volatile organic compounds. Also a Solubility Basis Set (SBS) model was developed to investigate the CCN activation behavior of complex organic aerosols accounting for the distribution of solubilities present in these mixtures. Based on the theoretical approaches, a coupled hygroscopicity and radiative transfer model was developed to investigate the effect of hygroscopic growth and CCN activation of aerosol particles on radiative properties in Arctic and boreal forest environments. Finally on the global scale, we used two climate models (NorESM and ECHAM6-HAM2) to investigate the sensitivity of climate models to treatment of water uptake of organics.By using different thermodynamic modelling approaches it was found that an approach using assumptions of limited solubility of the SOA components and solubility distributions cannot alone explain the hygroscopic behavior of SOA at subsaturation, while they can explain the CCN activation behaviour of organic mixtures. Quantifying the hygroscopic behavior of SOA compounds below 90% Relative Humidity (RH) requires consideration of processes such as adsorptive water uptake, bulk to surface partitioning, gas-particle partitioning of the semivolatile vapors and non ideality of the liquid phases with decreasing relative humidity (RH). On the other hand, at supersaturation most SOA behave as nearly completely soluble in water. We found that the differences in water-affinity of SOA at sub- and supersaturated conditions can be explained by Liquid-Liquid Phase Separation (LLPS) effects. By using the coupled hygroscopicity and radiative transfer model, a great impact of water uptake of aerosol particles on direct radiative effect was found in Arctic and boreal forest environment. The climate impacts resulting from OA are currently estimated using model parameterizations of water uptake that drastically simplify this complexity of OA. We found that the single-parameter hygroscopicity framework commonly used in climate models, can introduce significant errors when quantifying the climate effects of OA. The results highlight the need for better constraints on the interactions between water vapor and OA and its molecular composition, as well as overall global OA mass loadings, including currently under-explored anthropogenic and marine OA sources.
|
|
| 5. |
- Schmale, Julia, et al.
(författare)
-
Collocated observations of cloud condensation nuclei, particle size distributions, and chemical composition
- 2017
-
Ingår i: Scientific Data. - : Springer Science and Business Media LLC. - 2052-4463. ; 4
-
Tidskriftsartikel (refereegranskat)abstract
- Cloud condensation nuclei (CCN) number concentrations alongside with submicrometer particle number size distributions and particle chemical composition have been measured at atmospheric observatories of the Aerosols, Clouds, and Trace gases Research InfraStructure (ACTRIS) as well as other international sites over multiple years. Here, harmonized data records from 11 observatories are summarized, spanning 98,677 instrument hours for CCN data, 157,880 for particle number size distributions, and 70,817 for chemical composition data. The observatories represent nine different environments, e.g., Arctic, Atlantic, Pacific and Mediterranean maritime, boreal forest, or high alpine atmospheric conditions. This is a unique collection of aerosol particle properties most relevant for studying aerosol-cloud interactions which constitute the largest uncertainty in anthropogenic radiative forcing of the climate. The dataset is appropriate for comprehensive aerosol characterization (e.g., closure studies of CCN), model-measurement intercomparison and satellite retrieval method evaluation, among others. Data have been acquired and processed following international recommendations for quality assurance and have undergone multiple stages of quality assessment.
|
|
