| 1. |
- Pajunoja, Aki, et al.
(författare)
-
Adsorptive uptake of water by semisolid secondary organic aerosols
- 2015
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 42:8, s. 3063-3068
-
Tidskriftsartikel (refereegranskat)abstract
- Aerosol climate effects are intimately tied to interactions with water. Here we combine hygroscopicity measurements with direct observations about the phase of secondary organic aerosol (SOA) particles to show that water uptake by slightly oxygenated SOA is an adsorption-dominated process under subsaturated conditions, where low solubility inhibits water uptake until the humidity is high enough for dissolution to occur. This reconciles reported discrepancies in previous hygroscopicity closure studies. We demonstrate that the difference in SOA hygroscopic behavior in subsaturated and supersaturated conditions can lead to an effect up to about 30% in the direct aerosol forcinghighlighting the need to implement correct descriptions of these processes in atmospheric models. Obtaining closure across the water saturation point is therefore a critical issue for accurate climate modeling.
|
|
| 2. |
- 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.
|
|
| 3. |
- Rastak, Narges, et al.
(författare)
-
Microphysical explanation of the RH-dependent water affinity of biogenic organic aerosol and its importance for climate
- 2017
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 44:10, s. 5167-5177
-
Tidskriftsartikel (refereegranskat)abstract
- A large fraction of atmospheric organic aerosol (OA) originates from natural emissions that are oxidized in the atmosphere to form secondary organic aerosol (SOA). Isoprene (IP) and monoterpenes (MT) are the most important precursors of SOA originating from forests. The climate impacts from OA are currently estimated through parameterizations of water uptake that drastically simplify the complexity of OA. We combine laboratory experiments, thermodynamic modeling, field observations, and climate modeling to (1) explain the molecular mechanisms behind RH-dependent SOA water-uptake with solubility and phase separation; (2) show that laboratory data on IP- and MT-SOA hygroscopicity are representative of ambient data with corresponding OA source profiles; and (3) demonstrate the sensitivity of the modeled aerosol climate effect to assumed OA water affinity. We conclude that the commonly used single-parameter hygroscopicity framework can introduce significant error when quantifying the climate effects of organic aerosol. The results highlight the need for better constraints on the overall global OA mass loadings and its molecular composition, including currently underexplored anthropogenic and marine OA sources. Plain Language Summary The interaction of airborne particulate matter (aerosols) with water is of critical importance for processes governing climate, precipitation, and public health. It also modulates the delivery and bioavailability of nutrients to terrestrial and oceanic ecosystems. We present a microphysical explanation to the humidity-dependent water uptake behavior of organic aerosol, which challenges the highly simplified theoretical descriptions used in, e.g., present climate models. With the comprehensive analysis of laboratory data using molecular models, we explain the microphysical behavior of the aerosol over the range of humidity observed in the atmosphere, in a way that has never been done before. We also demonstrate the presence of these phenomena in the ambient atmosphere from data collected in the field. We further show, using two state-of-the-art climate models, that misrepresenting the water affinity of atmospheric organic aerosol can lead to significant biases in the estimates of the anthropogenic influence on climate.
|
|
| 4. |
- Rastak, Narges, et al.
(författare)
-
Modeling Aerosol Water Uptake in The Arctic Based on The kappa-Kohler Theory
- 2013
-
Ingår i: Nucleation and Atmospheric Aerosols. - : American Institute of Physics (AIP). - 9780735411524 ; , s. 702-705
-
Konferensbidrag (refereegranskat)abstract
- Water uptake or hygroscopicity is one of the most fundamental properties of atmospheric aerosols. Aerosol particles containing soluble materials can grow in size by absorbing water in ambient atmosphere. This property is measured by a parameter known as growth factor (GF), which is defined as the ratio of the wet diameter to the dry diameter. Hygroscopicity controls the size of an aerosol particle and therefore its optical properties in the atmosphere. Hygroscopic growth depends on the dry size of the particle, its chemical composition and the relative humidity in the ambient air (Fitzgerald, 1975; Pilinis et al., 1995). One of the typical problems in aerosol studies is the lack of measurements of aerosol size distributions and optical properties in ambient conditions. The gap between dry measurements and the real humid atmosphere is filled in this study by utilizing a hygroscopic model which calculates the hygroscopic growth of aerosol particles at Mt Zeppelin station, Ny Alesund, Svalbard during 2008.
|
|
| 5. |
- Rastak, Narges, et al.
(författare)
-
Seasonal variation of aerosol water uptake and its impact on the direct radiative effect at Ny-Alesund, Svalbard
- 2014
-
Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 14:14, s. 7445-7460
-
Tidskriftsartikel (refereegranskat)abstract
- In this study we investigated the impact of water uptake by aerosol particles in ambient atmosphere on their optical properties and their direct radiative effect (ADRE, W m(-2)) in the Arctic at Ny-Alesund, Svalbard, during 2008. To achieve this, we combined three models, a hygroscopic growth model, a Mie model and a radiative transfer model, with an extensive set of observational data. We found that the seasonal variation of dry aerosol scattering coefficients showed minimum values during the summer season and the beginning of fall (July-August-September), when small particles (< 100 nm in diameter) dominate the aerosol number size distribution. The maximum scattering by dry particles was observed during the Arctic haze period (March-April-May) when the average size of the particles was larger. Considering the hygroscopic growth of aerosol particles in the ambient atmosphere had a significant impact on the aerosol scattering coefficients: the aerosol scattering coefficients were enhanced by on average a factor of 4.30 +/- 2.26 (mean +/- standard deviation), with lower values during the haze period (March-April-May) as compared to summer and fall. Hygroscopic growth of aerosol particles was found to cause 1.6 to 3.7 times more negative ADRE at the surface, with the smallest effect during the haze period (March-April-May) and the highest during late summer and beginning of fall (July-August-September).
|
|
| 6. |
- Riipinen, Ilona, et al.
(författare)
-
Connecting the solubility and CCN activation of complex organic aerosols : a theoretical study using solubility distributions
- 2015
-
Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 15:11, s. 6305-6322
-
Tidskriftsartikel (refereegranskat)abstract
- We present a theoretical study investigating the cloud activation of multicomponent organic particles. We modeled these complex mixtures using solubility distributions (analogous to volatility distributions in the VBS, i.e., volatility basis set, approach), describing the mixture as a set of surrogate compounds with varying water solubilities in a given range. We conducted Khler theory calculations for 144 different mixtures with varying solubility range, number of components, assumption about the organic mixture thermodynamics and the shape of the solubility distribution, yielding approximately 6000 unique cloud condensation nucleus (CCN)-activation points. The results from these comprehensive calculations were compared to three simplifying assumptions about organic aerosol solubility: (1) complete dissolution at the point of activation; (2) combining the aerosol solubility with the molar mass and density into a single effective hygroscopicity parameter kappa; and (3) assuming a fixed water-soluble fraction eff. The complete dissolution was able to reproduce the activation points with a reasonable accuracy only when the majority (70-80 %) of the material was dissolved at the point of activation. The single-parameter representations of complex mixture solubility were confirmed to be powerful semi-empirical tools for representing the CCN activation of organic aerosol, predicting the activation diameter within 10% in most of the studied supersaturations. Depending mostly on the condensedphase interactions between the organic molecules, material with solubilities larger than about 0.1-100 g L-1 could be treated as soluble in the CCN activation process over atmospherically relevant particle dry diameters and supersaturations. Our results indicate that understanding the details of the solubility distribution in the range of 0.1-100 g L-1 is thus critical for capturing the CCN activation, while resolution outside this solubility range will probably not add much information except in some special cases. The connections of these results to the previous observations of the CCN activation and the molecular properties of complex organic mixture aerosols are discussed. The presented results help unravel the mechanistic reasons behind observations of hygroscopic growth and CCN activation of atmospheric secondary organic aerosol (SOA) particles. The proposed solubility distribution framework is a promising tool for modeling the interlinkages between atmospheric aging, volatility and water uptake of atmospheric organic aerosol.
|
|
| 7. |
- Tesche, Matthias, et al.
(författare)
-
Reconciling aerosol light extinction measurements from spaceborne lidar observations and in situ measurements in the Arctic
- 2014
-
Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 14:15, s. 7869-7882
-
Tidskriftsartikel (refereegranskat)abstract
- In this study we investigate to what degree it is possible to reconcile continuously recorded particle light extinction coefficients derived from dry in situ measurements at Zeppelin station (78.92 degrees N, 11.85 degrees E; 475 m above sea level), Ny-lesund, Svalbard, that are recalculated to ambient relative humidity, as well as simultaneous ambient observations with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite. To our knowledge, this represents the first study that compares spaceborne lidar measurements to optical aerosol properties from short-term in situ observations (averaged over 5 h) on a case-by-case basis. Finding suitable comparison cases requires an elaborate screening and matching of the CALIOP data with respect to the location of Zeppelin station as well as the selection of temporal and spatial averaging intervals for both the ground-based and spaceborne observations. Reliable reconciliation of these data cannot be achieved with the closest-approach method, which is often used in matching CALIOP observations to those taken at ground sites. This is due to the transport pathways of the air parcels that were sampled. The use of trajectories allowed us to establish a connection between spaceborne and ground-based observations for 57 individual overpasses out of a total of 2018 that occurred in our region of interest around Svalbard (0 to 25 degrees E, 75 to 82 degrees N) in the considered year of 2008. Matches could only be established during winter and spring, since the low aerosol load during summer in connection with the strong solar background and the high occurrence rate of clouds strongly influences the performance and reliability of CALIOP observations. Extinction coefficients in the range of 2 to 130 Mm(-1) at 532 nm were found for successful matches with a difference of a factor of 1.47 (median value for a range from 0.26 to 11.2) between the findings of in situ and spaceborne observations (the latter being generally larger than the former). The remaining difference is likely to be due to the natural variability in aerosol concentration and ambient relative humidity, an insufficient representation of aerosol particle growth, or a misclassification of aerosol type (i.e., choice of lidar ratio) in the CALIPSO retrieval.
|
|
| 8. |
- Zabori, Julia, et al.
(författare)
-
Size-resolved cloud condensation nuclei concentration measurements in the Arctic : two case studies from the summer of 2008
- 2015
-
Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 15:23, s. 13803-13817
-
Tidskriftsartikel (refereegranskat)abstract
- The Arctic is one of the most vulnerable regions affected by climate change. Extensive measurement data are needed to understand the atmospheric processes governing this vulnerability. Among these, data describing cloud formation potential are of particular interest, since the indirect effect of aerosols on the climate system is still poorly understood. In this paper we present, for the first time, size-resolved cloud condensation nuclei (CCN) data obtained in the Arctic. The measurements were conducted during two periods in the summer of 2008: one in June and one in August, at the Zeppelin research station (78 degrees 54'N, 11 degrees 53'E) in Svalbard. Trajectory analysis indicates that during the measurement period in June 2008, air masses predominantly originated from the Arctic, whereas the measurements from August 2008 were influenced by mid-latitude air masses. CCN supersaturation (SS) spectra obtained on the 27 June, before size-resolved measurements were begun, and spectra from the 21 and 24 August, conducted before and after the measurement period, revealed similarities between the 2 months. From the ratio between CCN concentration and the total particle number concentration (CN) as a function of dry particle diameter (D-p) at a SS of 0.4 %, the activation diameter (D-50), corresponding to CCN / CN = 0.50, was estimated. D-50 was found to be 60 and 67 nm for the examined periods in June and August 2008, respectively. Corresponding D-50 hygroscopicity parameter (kappa) values were estimated to be 0.4 and 0.3 for June and August 2008, respectively. These values can be compared to hygroscopicity values estimated from bulk chemical composition, where kappa was calculated to be 0.5 for both June and August 2008. While the agreement between the 2 months is reasonable, the difference in kappa between the different methods indicates a size dependence in the particle composition, which is likely explained by a higher fraction of inorganics in the bulk aerosol samples.
|
|
