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  • 1.
    Freud, Eyal
    et al.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM). The Hebrew University of Jerusalem, Israel.
    Koussevitzky, Hagai
    Goren, Tom
    Rosenfeld, Daniel
    Cloud microphysical background for the Israel-4 cloud seeding experiment2015In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 158-159, p. 122-138Article in journal (Refereed)
    Abstract [en]

    The modest amount of rainfall in Israel occurs in winter storms that bring convective clouds from the Mediterranean Sea when the cold post frontal air interacts with its relatively warm surface. These clouds were seeded in the Israel-1 and Israel-2 cloud glaciogenic seeding experiments, which have shown statistically significant positive effect of added rainfall of at least 13% in northern Israel, whereas the Israel-3 experiment showed no added rainfall in the south. This was followed by operational seeding in the north since 1975. The lack of physical evidence for the causes of the positive effects in the north caused a lack of confidence in the statistical results and led to the Israel-4 randomized seeding experiment in northern Israel. This experiment started in the winter of 2013/14. The main difference from the previous experiments is the focus on the orographic clouds in the catchment of the Sea of Galilee. The decision to commence the experiment was partially based on evidence supporting the existence of seeding potential, which is reported here. Aircraft and satellite microphysical and dynamic measurements of the clouds document the critical roles of aerosols, especially sea spray, on cloud microstructure and precipitation forming processes. It was found that the convective clouds over sea and coastal areas are naturally seeded hygroscopically by sea spray and develop precipitation efficiently. The diminution of the large sea spray aerosols farther inland along with the increase in aerosol concentrations causes the clouds to develop precipitation more slowly. The short time available for the precipitation forming processes in super-cooled orographic clouds over the Golan Heights farthest inland represents the best glaciogenic seeding potential.

  • 2.
    Glantz, Paul
    et al.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Nilsson, E. D.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    von Hoyningen-Huene, W.
    Estimating a relationship between aerosol optical thickness and surface wind speed over the ocean (vol 92, pg 58, 2009)2011In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 101, no 4, p. 956-956Article in journal (Refereed)
  • 3.
    Glantz, Paul
    et al.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Nilsson, E.D.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    von Hoyningen-Huene, W.
    Estimating a relationship between aerosol optical thickness and surface wind speed over the ocean2009In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 92, no 1, p. 58-68Article in journal (Refereed)
    Abstract [en]

    Retrieved aerosol optical thickness (ACT) based on data obtained by the Sea-viewing Wide Field Sensor (SeaWiFS) is combined with surface wind speed, obtained at the European Centre for Medium-Range Weather Forecasts (ECMWF), over the North Pacific for September 2001. In this study a cloud-screening approach is introduced in an attempt to exclude pixels partly or fully covered by clouds. The relatively broad swath width through which the nadir-viewing SeaWiFS scanned over the North Pacific means that the ACT can be estimated according to a relatively large range of wind speeds for each of the scenes analyzed. The sensitivity in AOT due to sea salt and hygroscopic growth of the marine aerosols was also investigated. The validation approach is based on previous parameterization in combination with the environmental quantities wind speed, RH and boundary layer height (BLH), estimated at the ECMWF. In this study a factor of 2 higher AOT is obtained between the highest wind speed (12 m s(-1)) and the lowest wind speed range (0-4 m s(-1)) for September 2001 over remote ocean areas. This is supported by the validation of the results. The enhancement in AOT is explained by a combination of hygroscopic growth of the marine aerosols (similar to 40%) and an increase in the sea salt particle mass concentrations (similar to 60%), caused by a wind-driven water vapor and sea salt flux respectively. Reasonable agreement (within 1 to 52%) occurs also between satellite-retrieved aerosol optical thickness and AOT observed at two AERONET (AErosol RObotic NETwork) ground-based remote sensing stations. The overall variability is also observed by this comparison. Finally, possible reasons why relatively large standard deviations occur around the mean values of AOT, when all data is taken into consideration in the analyses for September 2001, are discussed.

  • 4.
    Hultin, Kim
    et al.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Krejci, Radovan
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Pinhassi, Jarone
    Gomez-Consarnau, Laura
    Mårtensson, Monica
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Hagström, Åke
    Nilsson, Douglas
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Aerosol and bacterial emissions from Baltic Seawater2011In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 99, no 1, p. 1-14Article in journal (Refereed)
    Abstract [en]

    Factors influencing the production of primary marine aerosol are of great importance to better understand the marine aerosols' impact on our climate. Bubble-bursting from whitecaps is considered the most effective mechanism for sea spray production, and a way of sea–air transfer for some bacterial species.

    Two coastal sites in the Baltic Sea were used to investigate aerosol and bacterial emissions from the bubble-bursting process by letting a jet of water hit a water surface within an experimental tank, mimicking the actions of breaking waves.

    The aerosol size distribution spectra from the two sites were similar and conservative in shape where the modes were centered at about 200 nm dry diameter. We found a distinct decrease in bubbled aerosol production with increasing water temperature. A clear diurnal cycle in bubbled aerosol production was observed, anticorrelated with both water temperature and dissolved oxygen, which to our knowledge has never been shown before. A link between decreasing aerosol production in daytime and phytoplankton activity is likely to be an important factor. Colony-forming bacteria were transferred to the atmosphere via the bubble-bursting process, with a linear relationship to their seawater concentration.

  • 5.
    Salih, Abubakr A. M.
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Elagib, Nadir Ahmed
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Zhang, Qiong
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Characterization of the Sahelian-Sudan rainfall based on observations and regional climate models2018In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 202, p. 205-218Article in journal (Refereed)
    Abstract [en]

    The African Sahel region is known to be highly vulnerable to climate variability and change. We analyze rainfall in the Sahelian Sudan in terms of distribution of rain-days and amounts, and examine whether regional climate models can capture these rainfall features. Three regional models namely, Regional Model (REMO), Rossby Center Atmospheric Model (RCA) and Regional Climate Model (RegCM4), are evaluated against gridded observations (Climate Research Unit, Tropical Rainfall Measuring Mission, and ERA-interim reanalysis) and rain gauge data from six arid and semi-arid weather stations across Sahelian Sudan over the period 1989 to 2008. Most of the observed rain-days are characterized by weak (0.1-1.0 mm/day) to moderate ( > 1.0-10.0 mm/day) rainfall, with average frequencies of 18.5% and 48.0% of the total annual rain-days, respectively. Although very strong rainfall events ( > 30.0 mm/day) occur rarely, they account for a large fraction of the total annual rainfall (28-42% across the stations). The performance of the models varies both spatially and temporally. RegCM4 most closely reproduces the observed annual rainfall cycle, especially for the more arid locations, but all of the three models fail to capture the strong rainfall events and hence underestimate its contribution to the total annual number of rain days and rainfall amount. However, excessive moderate rainfall compensates this underestimation in the models in an annual average sense. The present study uncovers some of the models' limitations in skillfully reproducing the observed climate over dry regions, will aid model users in recognizing the uncertainties in the model output and will help climate and hydrological modeling communities in improving models.

  • 6. Sporre, Moa K.
    et al.
    Glantz, Paul
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Tunved, Peter
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Swietlicki, Erik
    Kulmala, Markku
    Lihavainen, Heikki
    A study of the indirect aerosol effect on subarctic marine liquid low-level clouds using MODIS cloud data and ground-based aerosol measurements2012In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 116, p. 56-66Article in journal (Refereed)
    Abstract [en]

    Cloud microphysics is substantially affected by aerosol loading and the resulting changes in the reflective properties of the clouds can significantly affect the global radiation budget. A study of how marine low-level clouds over Barents Sea and the northern parts of the Norwegian Sea are affected by air mass origin has been performed by combining ground-based aerosol measurements with satellite cloud retrievals. Aerosol number size distributions have been obtained from measurement stations in northern Finland, and a trajectory model has been used to estimate the movement of the air masses. To identify anthropogenic influences on the clouds, the dataset has been divided according to aerosol loading. The clean air masses arrived to the investigation area from the north and the polluted air masses arrived from the south. Satellite derived microphysical and optical cloud parameters from the Moderate Resolution Imaging Spectrometer (MODIS) have then been analyzed for days when the trajectories coincided with marine low-level clouds over the investigated area. The cloud optical thickness (tau), cloud depth (H) and droplet number concentration (N-d) were significantly higher for the polluted days compared to the clean conditions, while the opposite was found for the cloud droplet effective radius (r(e)). The H and N-d were derived from the satellite retrievals of tau and r(e). Furthermore, calculations of the aerosol cloud interaction relationship (ACI), relating N-d to boundary layer aerosol concentrations, resulted in a value of 0.17, which is in line with previous remote sensing studies. The results demonstrate that ground-based aerosol measurements can be combined with satellite cloud observations to study the indirect aerosol effect, and that the microphysics of marine sub-polar clouds can be considerably affected by continental aerosols.

  • 7.
    Syed, Faisal Saeed
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Yoo, J. H.
    Körnich, Heiner
    Stockholm University, Faculty of Science, Department of Meteorology .
    Kucharski, F.
    Are intraseasonal summer rainfall events micro monsoon onsets over the western edge of the South-Asian monsoon?2010In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 98, no 2-4, p. 341-346Article in journal (Refereed)
    Abstract [en]

    A thermodynamic structure leading the active phase (AP) of the Western Edge of the South-Asian Monsoon (WESAM) is investigated The APs seems to have significant contribution in the mean seasonal rainfall in the region A few days before APs the upper level warm anomaly appears over the north Hindu Kush-Himalaya (HKH) region and It is reinforced by surface heating yielding the column average warming The height anomalies are baroclinic with the low-level anticyclone being located at the east of warming The low-level anticyclone causes the moisture convergence at the core WESAM region As the region keeps warming, the height anomalies and associated low-level convergence become stronger The AP starts when the low-level moisture convergence is strong enough to overcome the preexisting stable atmospheric condition due to the upper level warming The proposed mechanism of APs has some resemblance with large scale south Asian monsoon onset, whereas conventional south Asian monsoon intraseasonal oscillations (ISOs) do not show clear relationship with APs of WESAM

  • 8.
    Targino, Admir Créso
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology .
    Noone, Kevin J.
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Airborne in situ characterization of dry urban aerosol optical properties around complex topography2006In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 79, no 2, p. 148-174Article in journal (Refereed)
    Abstract [en]

    In situ data from the 1997 Southern California Ozone Study—NARSTO2 were used to describe the aerosol optical properties in an urban area whose aerosol distribution is modified as the aerosols are advected over the surrounding topography. The data consist of measurements made with a nephelometer and absorption photometer onboard the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Pelican aircraft. The cases investigated in this study include vertical profiles flown over coastal sites as well as sites located along some important mountain ranges in southern California.

    The vertical distribution of the aerosol in the Los Angeles Basin showed a complex configuration, directly related with the local meteorological circulations and the surrounding topography. High spatial and temporal variability in air pollutant concentrations within a relatively small area was found, as indicated by the aerosol scattering and absorption coefficient data. The results suggest that in areas with such complex terrain, a high spatial resolution is required in order to adequately describe the aerosol optical quantities.

    Principal components analysis (PCA) has been applied to aerosol chemical samples in order to identify the major aerosol types in the Los Angeles Basin. The technique yielded four components that accounted for 78% of the variance in the data set. These were indicative of marine aerosols, urban aerosols, trace elements and secondary aerosol components of traffic emissions and agricultural activities.

    A Monte Carlo radiation transfer model has been employed to simulate the effects that different aerosol vertical profiles have on the attenuation of solar energy. The cases examined were selected using the results of the PCA and in situ data were used to describe the atmospheric optical properties in the model. These investigations comprise a number of sensitivity tests to evaluate the effects on the results of the location of the aerosol layers as well as the vertical resolution of the Monte Carlo model.

  • 9.
    Tiwari, Suresh
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. Indian Institute of Tropical Meteorology, India.
    Dumka, U. C.
    Hopke, P. K.
    Tunved, Peter
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Srivastava, A. K.
    Bisht, D. S.
    Chakrabarty, R. K.
    Atmospheric heating due to black carbon aerosol during the summer monsoon period over Ballia: A rural environment over Indo-Gangetic Plain2016In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 178, p. 393-400Article in journal (Refereed)
    Abstract [en]

    Black carbon (BC) aerosols are one of the most uncertain drivers of global climate change. The prevailing view is that BC mass concentrations are low in rural areas where industrialization and vehicular emissions are at a minimum. As part of a national research program called the Ganga Basin Ground Based Experiment-2014 under the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) Phase-III of Ministry of Earth Sciences, Government of India, the continuous measurements of BC and particulate matter (PM) mass concentrations, were conducted in a rural environment in the highly-polluted Indo-Gangetic Plain region during 16th June to 15th August (monsoon period), 2014. The mean mass concentration of BC was 4.03 (+/- 0.85) mu g m(-3) with a daily variability between 2.4 and 5.64 mu g m(-3), however, the mean mass PM concentrations [near ultrafine (PM1.0), fine (PM2.5) and inhalable (PM1.0)] were 29.1(+/- 16.2), 34.7 (+/- 19.9) and 43.7 (+/- 283) mu g m(-3), respectively. The contribution of BC in PM1.0 was approximately 13%, which is one of the highest being recorded. Diurnally, the BC mass concentrations were highest (mean: 5.89 mu g m(-3)) between 20:00 to 22:00 local time (LT) due to the burning of biofuels/biomass such as wood, dung, straw and crop residue mixed with dung by the local residents for cooking purposes. The atmospheric direct radiative forcing values due to the composite and BC aerosols were determined to be +78.3, +44.9, and +45.0 W m(-2) and +42.2, +35.4 and +34.3 W m(-2) during the months ofJune, July and August, respectively. The corresponding atmospheric heating rates (AHR) for composite and BC aerosols were 2.21,1.26 and 1.26; and 1.19, 0.99 and 0.96 K day(-1) for the month ofJune, July and August, respectively, with a mean of 1.57 and 1.05 K day(-1) which was 33% lower AHR (BC) than for the composite particles during the study period. This high AHR underscores the importance of absorbing aerosols such as BC contributed by residential cooking using biofuels in India. Our study demonstrates the need for immediate, effective regulations and policies that mitigate the emission of BC particles from domestic cooking in rural areas of India.

  • 10.
    Tiwari, Suresh
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. Indian Institute of Tropical Meteorology, India.
    Tunved, Peter
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Hopke, Philip K.
    Srivastava, A. K.
    Bisht, D. S.
    Pandey, A. K.
    Observations of ambient trace gas and PM10 concentrations at Patna, Central Ganga Basin during 2013-2014: The influence of meteorological variables on atmospheric pollutants2016In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 180, p. 138-149Article in journal (Refereed)
    Abstract [en]

    Atmospheric pollutants including ozone (O-3), sulfur dioxide (SO2), oxides of nitrogen (NOx), carbon monoxide (CO), and inhalable particulate matter (PM10) were measured in the central Indo-Gangetic Basin (IGB) at Patna, India, from 1st March 2013 to 31st December 2014, and significant variability was observed in the temporal patterns of these pollutant concentrations. The mean O-3, SO2, NO, NO2, CO (trace gases: TG), and PM10 (PM) concentrations were 14.5 +/- 4.8, 5.9 +/- 4.8, 23.1 +/- 22, 20.6 +/- 14.6 ppb, 1.5 +/- 0.7 ppm, and 192.0 +/- 132.8 ng/m(3), respectively, over the study period. The highest concentrations of these species were during the post-monsoon and winter seasons except O-3 and SO2 that showed the highest concentrations during the pre-monsoon. The lowest concentrations of TG and PM were observed during the monsoon season as a result of scavenging by rain. NO and NO2 along with PM concentrations decreased by similar to 76,19, and 63% when the wind speed (WS) was >0.5 m/s. However, for O-3, an opposite trend was observed with similar to 14% higher concentrations. The WS was negatively correlated with PM during the winter (-0.48) and post-monsoon (-0.32) seasons. In order to investigate the source region of TG and PM, 5-day air mass back trajectories were computed. The dominance of the air masses (92, 53, and 49%) were from the IGB is highly polluted during the winter, pre-monsoon, and post-monsoon, respectively. The TG and PM were observed much higher during these periods. During the biomass burning period (post-monsoon), the trajectory analysis showed that the TG and PM concentrations were around three-fold higher (flow from the IGB) than the other seasons. To improve air quality over IGB, the mitigation measures should be designed to reduce emissions from both local and regional sources.

  • 11. Vuollekoski, Henri
    et al.
    Nieminen, Tuomo
    Paasonen, Pauli
    Sihto, Sanna-Liisa
    Boy, Michael
    Manninen, Hanna
    Lehtinen, Kari
    Kerminen, Veli-Matti
    Kulmala, Markku
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Atmospheric nucleation and initial steps of particle growth: Numerical comparison of different theories and hypotheses2010In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 98, no 2-4, p. 229-236Article in journal (Refereed)
    Abstract [en]

    New particle formation in boreal forest environment in Hyytiala Finland was studied in an aerosol dynamical model Basing on the concepts of activation or kinetic collision of two preexisting clusters at 1 5 nm several semi-empirical nucleation rate formulae were parameterized The mechanisms had linear or squared dependence on the concentration of sulphuric acid or a low volatile organic vapor or both and they all showed good agreement with field measurements A new method for examining the power dependence of apparent formation rate (at 2 nm or 3 nm) on sulphuric acid concentration was developed The new method produced exponents 1 6-2 1 for cluster activation and exponents 2 4-3 1 for kinetic collision which suggests that the activation scenarios are in better agreement with experimental observations that imply exponents around 1-2 However, It was found that if low volatile organic vapors with concentrations exceeding that of sulphuric acid are present, they have a major role in shaping the temporal behavior of the apparent formation rates causing error in the exponent analyses Finally, a sensitivity study showed that the analyzed exponents grew even further if the size of the critical cluster was assumed smaller that 1 5 nm

  • 12. Zdun, Agnieszka
    et al.
    Rozwadowska, Anna
    Kratzer, Susanne
    Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.
    The impact of air mass advection on aerosol optical properties over Gotland (Baltic Sea)2016In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 182, p. 142-155Article in journal (Refereed)
    Abstract [en]

    In the present paper, measurements of aerosol optical properties from the Gotland station of the AERONET network, combined with a two-stage cluster analysis of back trajectories of air masses moving over Gotland, were used to identify the main paths of air mass advection to the Baltic Sea and to relate them to aerosol optical properties, i.e. the aerosol optical thickness at the wavelength lambda = 500 nm, AOT (500) and the Angstrom exponent for the spectral range from 440 to 870 nm, alpha(440,870). One-to six-day long back trajectories ending at 300, 500 and 3000 m above the station were computed using the HYSPLIT model. The study shows that in the Gotland region, variability in aerosol optical thickness AOT(500) is more strongly related to advections in the boundary layer than to those in the free troposphere. The observed variability in AOT(500) was best explained by the advection speeds and directions given by clustering of 4-day backward trajectories of air arriving in the boundary layer at 500 m above the station. 17 clusters of 4-day trajectories arriving at altitude 500 m above the Gotland station (sea level) derived using two-stage cluster analysis differ from each other with respect to trajectory length, the speed of air mass movement and the direction of advection. They also show different cluster means of AOT(500) and alpha(440,870). The cluster mean AOT(500) ranges from 0342 +/- 0.012 for the continental clusters M2 (east-southeast advection with moderate speed) and 0294 +/- 0.025 for S5 (slow south-southeast advection) to 0.064 +/- 0.002 and 0.069 +/- 0.002 for the respective marine clusters L3 (fast west-northwest advection) and M3 (north-northwest advection with moderate speed). The cluster mean a(440,870) varies from 1.65-1.70 for the short-trajectory clusters to 0.98 +/- 0.03 and 1.06 +/- 0.03 for the Arctic marine cluster L4 (fast inflow from the north) and marine cluster L5 (fast inflow from the west) respectively.

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