scholarly journals The radiative impact of out-of-cloud aerosol hygroscopic growth during the summer monsoon in southern West Africa

2019 ◽  
Vol 19 (3) ◽  
pp. 1505-1520 ◽  
Author(s):  
Sophie L. Haslett ◽  
Jonathan W. Taylor ◽  
Konrad Deetz ◽  
Bernhard Vogel ◽  
Karmen Babić ◽  
...  
Keyword(s):  
Optical Properties ◽  
Chemical Composition ◽  
Relative Humidity ◽  
West Africa ◽  
Monsoon Season ◽  
Hygroscopic Growth ◽  
Aerosol Direct Effect ◽  
Humidity Measurements ◽  
Original Size ◽  
Solid Liquid

Abstract. Water in the atmosphere can exist in the solid, liquid or gas phase. At high humidities, if the aerosol population remains constant, more water vapour will condense onto the particles and cause them to swell, sometimes up to several times their original size. This significant change in size and chemical composition is termed hygroscopic growth and alters a particle's optical properties. Even in unsaturated conditions, this can change the aerosol direct effect, for example by increasing the extinction of incoming sunlight. This can have an impact on a region's energy balance and affect visibility. Here, aerosol and relative humidity measurements collected from aircraft and radiosondes during the Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa (DACCIWA) campaign were used to estimate the effect of highly humid layers of air on aerosol optical properties during the monsoon season in southern West Africa. The effects of hygroscopic growth in this region are of particular interest due to the regular occurrence of high humidity and the high levels of pollution in the region. The Zdanovskii, Stokes and Robinson (ZSR) mixing rule is used to estimate the hygroscopic growth of particles under different conditions based on chemical composition. These results are used to estimate the aerosol optical depth (AOD) at λ=525 nm for 63 relative humidity profiles. The median AOD in the region from these calculations was 0.36, the same as that measured by sun photometers at the ground site. The spread in the calculated AODs was less than the spread from the sun photometer measurements. In both cases, values above 0.5 were seen predominantly in the mornings and corresponded with high humidities. Observations of modest variations in aerosol load and composition are unable to explain the high and variable AODs observed using sun photometers, which can only be recreated by accounting for the very elevated and variable relative humidities (RHs) in the boundary layer. Most importantly, the highest AODs present in the mornings are not possible without the presence of high RH in excess of 95 %. Humid layers are found to have the most significant impact on AOD when they reach RH greater than 98 %, which can result in a wet AOD more than 1.8 times the dry AOD. Unsaturated humid layers were found to reach these high levels of RH in 37 % of observed cases. It can therefore be concluded that the high AODs present across the region are driven by the high humidities and are then moderated by changes in aerosol abundance. Aerosol concentrations in southern West Africa are projected to increase substantially in the coming years; results presented here show that the presence of highly humid layers in the region is likely to enhance the consequent effect on AOD significantly.

scholarly journals The radiative impact of out-of-cloud aerosol hygroscopic growth during the summer monsoon in southernWest Africa

2018 ◽  
Author(s):  
Sophie L. Haslett ◽  
Jonathan W. Taylor ◽  
Konrad Deetz ◽  
Bernhard Vogel ◽  
Karmen Babić ◽  
...  
Keyword(s):  
Optical Properties ◽  
Chemical Composition ◽  
Relative Humidity ◽  
West Africa ◽  
Frequency Distribution ◽  
Monsoon Season ◽  
Hygroscopic Growth ◽  
Aerosol Direct Effect ◽  
Humidity Measurements ◽  
Original Size

Abstract. Water in the atmosphere exists as both vapour and liquid water contained in particles. At high humidities, more water vapour condenses onto particles and causes them to swell, sometimes up to several times their original size. This significant change in size and chemical composition is termed hygroscopic growth and alters a particle's optical properties. Even in unsaturated conditions, this can change the aerosol direct effect, for example by increasing the extinction of incoming sunlight. This can have an impact on a region's energy balance and affect visibility. Here, aerosol and relative humidity measurements collected from aircraft and radiosondes during the Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) campaign were used to estimate the effect of highly humid layers of air on aerosol optical properties during the monsoon season in southern West Africa. The effects of hygroscopic growth in this region are of particular interest due to the regular occurrence of high humidity and the high levels of pollution in the region. The Zdanovskii, Stokes and Robinson (ZSR) mixing rule is used to estimate the hygroscopic growth of particles under different conditions based on chemical composition. These results are used to estimate the aerosol optical depth (AOD) for 63 relative humidity profiles. A static aerosol profile was assumed. Therefore, these results show the extent of the AOD frequency distribution that can be explained by humidity alone, rather than predicting actual AOD values. The median AOD in the region from these calculations was 0.46, which compares to a median of 0.36 measured by sun photometers. The shape of the AOD frequency distribution was largely comparable to that of the sun photometer measurements, demonstrating that relative humidity is able to account for a large part of the region's AOD variability. Humid layers are found to have the most significant impact on AOD when they reach relative humidities greater than 98 %, which can result in a wet AOD up to seven times larger than the dry AOD. Unsaturated humid layers were found to reach these high levels of relative humidity in 37 % of observed cases. Aerosol concentrations in southern West Africa are projected to increase substantially in the coming years; results presented here show that the presence of highly humid layers in the region is likely to enhance the consequent effect on AOD significantly.

scholarly journals Impact of relative humidity and particles number size distribution on aerosol light extinction in the urban area of Guangzhou

2013 ◽  
Vol 13 (3) ◽  
pp. 1115-1128 ◽  
Author(s):  
Z. J. Lin ◽  
J. Tao ◽  
F. H. Chai ◽  
S. J. Fan ◽  
J. H. Yue ◽  
...  
Keyword(s):  
Optical Properties ◽  
Chemical Composition ◽  
Relative Humidity ◽  
Temporal Variation ◽  
Size Distribution ◽  
Urban Area ◽  
Water Soluble ◽  
Light Extinction ◽  
Hygroscopic Growth ◽  
Number Size Distribution

Abstract. In the urban area of Guangzhou, observations on aerosol light extinction effect were conducted at a monitoring site of the South China Institute of Environmental Sciences (SCIES) during April 2009, July 2009, October 2009 and January 2010. The main goal of these observations is to recognise the impact of relative humidity (RH) and particles number distribution on aerosol light extinction. PM2.5 was sampled by Model PQ200 air sampler; ions and OC/EC in PM2.5 were identified by the Dionex ion chromatography and the DRI model 2001 carbon analyser, respectively; particles number size distribution was measured by TSI 3321 APS, while total light scattering coefficient was measured by TSI 3563 Nephelometer. Chemical composition of PM2.5 was reconstructed by the model ISORROPIA II. As a result, possible major components in PM2.5 were (NH4)2SO4, Na2SO4, K2SO4, NH4NO3, HNO3, water, POM and EC. Regarding ambient RH, mass concentration of PM2.5 ranged from 26.1 to 279.1 μg m−3 and had an average of 94.8, 44.6, 95.4 and 130.8 μg m−3 in April, July, October and January, respectively. With regard to the total mass of PM2.5, inorganic species, water, POM, EC and the Residual accounted for 34–47%, 19–31%, 14–20%, 6–8% and 8–17%, respectively. Under the assumption of "internal mixture", optical properties of PM0.5–20 were estimated following the Mie Model. Optical refractive index, hygroscopic growth factor and the dry aerosol density required by the Mie Model were determined with an understanding of chemical composition of PM2.5. With these three parameters and the validated particles number size distribution of PM0.5–20, the temporal variation trend of optical property of PM0.5–20 was estimated with good accuracy. The highest average of bep,pm0.5–20 was 300 Mm−1 in April while the lowest one was 78.6 Mm−1 in July. Regarding size distribution of bep,pm0.5–20, peak value was almost located in the diameter range between 0.5 and 1.0 μm. Furthermore, hygroscopic growth of optical properties of PM0.5–20 largely depended on RH. As RH increased, bep,pm0.5–20 grew and favoured a more rapid growth when aerosol had a high content of inorganic water-soluble salts. Averagely, fbep,pm0.5–20 enlarged 1.76 times when RH increased from 20% to 90%. With regard to the temporal variation of ambient RH, fbep,pm0.5–20 was 1.29, 1.23, 1.14 and 1.26 on average in April, July, October and January, respectively.

scholarly journals Radiation fog formation alerts using attenuated backscatter power from automatic Lidars and ceilometers

2016 ◽  
Author(s):  
Martial Haeffelin ◽  
Quentin Laffineur ◽  
Juan-Antonio Bravo-Aranda ◽  
Marc-Antoine Drouin ◽  
Juan-Andrés Casquero-Vera ◽  
...  
Keyword(s):  
Relative Humidity ◽  
Temporal Evolution ◽  
Weather Prediction ◽  
Atmospheric Conditions ◽  
Hygroscopic Growth ◽  
Radiation Fog ◽  
Low Visibility ◽  
Humidity Measurements ◽  
Ground Current ◽  
Growth Laws

Abstract. Radiation fog occurs over many locations around the world in stable atmospheric conditions. Air traffic at busy airports can be significantly disrupted because low visibility at the ground makes it unsafe to take off, land and taxi on the ground. Current numerical weather prediction forecasts are able to predict general conditions favorable for fog formation, but not the exact time or location of fog occurrence. A selected set of observations available in near realtime at strategic locations could also be useful to track the evolution of key processes and key parameters that drive fog formation. Such observations could complement the information predicted by NWP models that is made available to airport forecasters in support of their fog forecast. This paper presents an experimental setup based on collocated automatic lidar and ceilometer measurements, relative humidity measurements and horizontal visibility measurements to study hygroscopic growth of fog condensation nuclei. This process can take several minutes to hours and can be tracked using lidar or ceilometer attenuated backscatter profiles. Based on hygroscopic growth laws we derive a set of parameters that can be used to provide alerts minutes to hours prior to formation of radiation fog. We present an algorithm that uses the temporal evolution of attenuated backscatter measurements to derive pre-fog formation alerts. The performance of the algorithm is tested on 45 independent pre-fog situations at two locations (near Paris, France and Brussels, Belgium). We find that pre-fog alerts occur predominantly 10–50 min prior to fog formation at an altitude ranging 0 to 100 m above ground. In a few cases, alerts can occur up to 100 min prior to fog formation. Alert durations are found to be sensitive to relative humidity conditions found a few hours prior to the fog.

scholarly journals Absorbing aerosols at high relative humidity: linking hygroscopic growth to optical properties

2012 ◽  
Vol 12 (12) ◽  
pp. 5511-5521 ◽  
Author(s):  
J. Michel Flores ◽  
R. Z. Bar-Or ◽  
N. Bluvshtein ◽  
A. Abo-Riziq ◽  
A. Kostinski ◽  
...  
Keyword(s):  
Optical Properties ◽  
Growth Factors ◽  
Relative Humidity ◽  
Atmospheric Chemistry ◽  
Theoretical Calculations ◽  
Complex Refractive Index ◽  
High Relative Humidity ◽  
Mixing Rule ◽  
Hygroscopic Growth ◽  
Absorbing Aerosols

Abstract. One of the major uncertainties in the understanding of Earth's climate system is the interaction between solar radiation and aerosols in the atmosphere. Aerosols exposed to high humidity will change their chemical, physical, and optical properties due to their increased water content. To model hydrated aerosols, atmospheric chemistry and climate models often use the volume weighted mixing rule to predict the complex refractive index (RI) of aerosols when they interact with high relative humidity, and, in general, assume homogeneous mixing. This study explores the validity of these assumptions. A humidified cavity ring down aerosol spectrometer (CRD-AS) and a tandem hygroscopic DMA (differential mobility analyzer) are used to measure the extinction coefficient and hygroscopic growth factors of humidified aerosols, respectively. The measurements are performed at 80% and 90%RH at wavelengths of 532 nm and 355 nm using size-selected aerosols with different degrees of absorption; from purely scattering to highly absorbing particles. The ratio of the humidified to the dry extinction coefficients (fRHext(%RH, Dry)) is measured and compared to theoretical calculations based on Mie theory. Using the measured hygroscopic growth factors and assuming homogeneous mixing, the expected RIs using the volume weighted mixing rule are compared to the RIs derived from the extinction measurements. We found a weak linear dependence or no dependence of fRH(%RH, Dry) with size for hydrated absorbing aerosols in contrast to the non-monotonically decreasing behavior with size for purely scattering aerosols. No discernible difference could be made between the two wavelengths used. Less than 7% differences were found between the real parts of the complex refractive indices derived and those calculated using the volume weighted mixing rule, and the imaginary parts had up to a 20% difference. However, for substances with growth factor less than 1.15 the volume weighted mixing rule assumption needs to be taken with caution as the imaginary part of the complex RI can be underestimated.

scholarly journals Radiation fog formation alerts using attenuated backscatter power from automatic lidars and ceilometers

2016 ◽  
Vol 9 (11) ◽  
pp. 5347-5365 ◽  
Author(s):  
Martial Haeffelin ◽  
Quentin Laffineur ◽  
Juan-Antonio Bravo-Aranda ◽  
Marc-Antoine Drouin ◽  
Juan-Andrés Casquero-Vera ◽  
...  
Keyword(s):  
Relative Humidity ◽  
Numerical Weather Prediction ◽  
Weather Prediction ◽  
Atmospheric Conditions ◽  
Hygroscopic Growth ◽  
Radiation Fog ◽  
Numerical Weather ◽  
Humidity Measurements ◽  
Ground Current ◽  
Growth Laws

Abstract. Radiation fog occurs over many locations around the world in stable atmospheric conditions. Air traffic at busy airports can be significantly disrupted because low visibility at the ground makes it unsafe to take off, land and taxi on the ground. Current numerical weather prediction forecasts are able to predict general conditions favorable for fog formation, but not the exact time or location of fog occurrence. A selected set of observations available in near-real time at strategic locations could also be useful to track the evolution of key processes and key parameters that drive fog formation. Such observations could complement the information predicted by numerical weather prediction (NWP) models that is made available to airport forecasters in support of their fog forecast. This paper presents an experimental setup based on collocated automatic lidar and ceilometer measurements, relative humidity measurements and horizontal visibility measurements to study hygroscopic growth of fog condensation nuclei. This process can take several minutes to hours, and can be tracked using lidar- or ceilometer-attenuated backscatter profiles. Based on hygroscopic growth laws we derive a set of parameters that can be used to provide alerts minutes to hours prior to formation of radiation fog. We present an algorithm that uses the temporal evolution of attenuated backscatter measurements to derive pre-fog formation alerts. The performance of the algorithm is tested on 45 independent pre-fog situations at two locations (near Paris, France, and Brussels, Belgium). We find that an alert for pre-fog conditions predominantly occurs 10–50 min prior to fog formation at an altitude ranging 0 to 100 m above ground. In a few cases, alerts can occur up to 100 min prior to fog formation. Alert durations are found to be sensitive to the relative humidity conditions found a few hours prior to the fog.

Nitrate-driven high PM2.5 episodes in Seoul during pre-monsoon season

2020 ◽  
Author(s):  
Joonhyoung Park ◽  
Saehee Lim ◽  
Meehye Lee ◽  
Taehyung Lee ◽  
Moon-soo Park ◽  
...  
Keyword(s):  
Chemical Composition ◽  
Liquid Water ◽  
Nitrate Concentration ◽  
Monsoon Season ◽  
Liquid Water Content ◽  
The Other ◽  
Formation Processes ◽  
University Campus ◽  
Hygroscopic Growth ◽  
Main Driver

<p>Ammonium nitrate (NH<sub>4</sub>NO<sub>3</sub>) is the main driver of high PM<sub>2.5</sub> episodes in Seoul, but its formation processes are not fully understood yet. Intensive experiments were conducted at the Korea University campus in Seoul during June ~ August 2018 and April ~ June 2019, when the chemical composition of PM<sub>2.5</sub> including Na<sup>+</sup>, SO<sub>4</sub><sup>2-</sup>, NH<sub>3</sub>, NO<sub>3</sub><sup>-</sup>, Cl<sup>-</sup>, Ca<sup>2+</sup>, K<sup>+</sup>, Mg<sup>2+</sup>, OC and EC, and its gaseous precursors including NO<sub>X</sub>, HNO<sub>3</sub> and SO<sub>2</sub> were continuously measured. The concentrations of PM<sub>2.5</sub> and its major constituents were noticeably higher in pre-monsoon (June) than summer monsoon (July~August) period. In particular, nitrate concentration was much higher (6.9 μg/m<sup>3</sup>) during the high PM<sub>2.5</sub> episode (24-hr average PM<sub>2.5</sub> > 35 μg/m<sup>3</sup>) in June compared to those of non-episode (3.1 μg/m<sup>3</sup>) and the other two months (0.74 μg/m<sup>3</sup>). Aerosol liquid water content (ALWC) was calculated using ISORROPIA II model, ALWC was higher during the episode than non-episode and the highest ALWC was found concurrently with the highest NO<sub>3</sub><sup>-</sup> concentration (18.2 μg/m<sup>3</sup>) at night. Concurrent increases of nitrate and ALWC cause aqueous-phase formation and hygroscopic growth of aerosol, which lead to high PM<sub>2.5</sub> concentration. In addition, ALWC was more rapidly increased with the number of accumulation mode particles larger than 100 nm in diameter at higher RH and nitrate concentration. In this study, PM<sub>2.5</sub> mass and nitrate were elevated after the NO<sub>X</sub> peak in the morning as well as at dawn. The surface of pre-existing particles was found to be prerequisite for nitrate driven PM<sub>2.5</sub> episode.</p>

scholarly journals COMPARISON BETWEEN KELVIN RADII AND BULK HYGROSCOPICITY OF MASS AND VOLUME BASED HYGROSCOPICITY PARAMETER FOR ATMOSPHERIC AEROSOLS

2020 ◽  
Vol 4 (2) ◽  
pp. 337-349
Author(s):  
Bello Saadu ◽  
B. I. Tijjani ◽  
S. Bello
Keyword(s):  
Refractive Index ◽  
Optical Properties ◽  
Regression Analysis ◽  
Growth Factors ◽  
Relative Humidity ◽  
Multiple Regression Analysis ◽  
Atmospheric Aerosols ◽  
Atmospheric Modeling ◽  
Hygroscopic Growth ◽  
Microphysical Properties

A mass based and volume based hygroscopicities models were applied to the data extracted from Optical Properties of Aerosols and Clouds (OPAC).The microphysical properties obtained were radii, density, refractive index, mass and volume of the atmospheric aerosols of continental average, continental clean, continental polluted, maritime tropical, maritime polluted, maritime clean, at eight different relative humidity of 0%, 50%, 70%, 80%, 90%, 95%, 98% and 99%.Using the microphysical properties, hygroscopic growth factors and effective radii of the mixtures were determined while the parameters Av and Bv for volume based and Am and Bm for mass based of the aerosols were determined using multiple regression analysis with SPSS 16.0 at each relative humidity. Although it was discovered that Bv is more dominant than Av, the R2 for all the models are greater than 90%. The significances are less than 0.05, Am>Av and Bm<Bv, therefore the two models are good for atmospheric modeling.

Structural/Property Relationships for Optical Materials

1993 ◽  
Vol 329 ◽  
Author(s):  
Vivien D.
Keyword(s):  
Crystal Structure ◽  
Optical Properties ◽  
Electronic Structure ◽  
Chemical Composition ◽  
Field Strength ◽  
Optical Materials ◽  
Site Occupancy ◽  
Laser Materials ◽  
Crystal Field Strength ◽  
The Matrix

AbstractIn this paper the relationships between the crystal structure, chemical composition and electronic structure of laser materials, and their optical properties are discussed. A brief description is given of the different laser activators and of the influence of the matrix on laser characteristics in terms of crystal field strength, symmetry, covalency and phonon frequencies. The last part of the paper lays emphasis on the means to optimize the matrix-activator properties such as control of the oxidation state and site occupancy of the activator and influence of its concentration.

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