scholarly journals The impact of aerosol hygroscopic growth on the single-scattering albedo and its application on the NO<sub>2</sub> photolysis rate coefficient

2014 ◽  
Vol 14 (11) ◽  
pp. 16351-16386
Author(s):  
J. C. Tao ◽  
C. S. Zhao ◽  
N. Ma ◽  
P. F. Liu
Keyword(s):  
Boundary Layer ◽  
Aerosol Particle ◽  
Rate Coefficient ◽  
Aerosol Particles ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Hygroscopic Growth ◽  
Particle Population ◽  
The Impact ◽  
Upper Boundary

Abstract. Hygroscopic growth of aerosol particles can significantly affect their single-scattering albedo (ω), and consequently alters the aerosol effect on tropospheric photochemistry. In this study, the impact of aerosol hygroscopic growth on the ω and its application on NO2 photolysis rate coefficient (JNO2) are investigated for a typical aerosol particle population in the North China Plain (NCP). The variations of aerosol optical properties with relative humidity (RH) are calculated using a Mie-theory aerosol optical model, on the basis of field measurements of number size distribution and hygroscopic growth factor from 2009 HaChi (Haze in China) project. Results demonstrate that ambient ω has pronounced diurnal patterns and is highly sensitive to the ambient RHs. Ambient ω in the NCP can be described by a dry state ω value of 0.863, increasing with the RH following a characteristic RH dependence curve. The Monte Carlo simulation shows that the uncertainty of ω from the propagation of uncertainties in the input parameters decreases from 0.03 (at dry state) to 0.01 (RHs > 90%). The impact of hygroscopic growth on ω is further applied in the calculation of the radiative transfer process. Hygroscopic growth of the studied aerosol particle population generally inhibits the photolysis of NO2 at the ground level, whereas accelerates it above the upper boundary layer. Compared with dry state, the calculated JNO2 at RH of 98% at the height of 1 km increases by 30.4%, because of the enhancement of ultraviolet radiation by the humidified scattering-dominant aerosol particles. The increase of JNO2 due to the aerosol hygroscopic growth above the upper boundary layer may affect the tropospheric photochemical processes and this needs to be taken into account in the atmospheric chemical models.

scholarly journals The impact of aerosol hygroscopic growth on the single-scattering albedo and its application on the NO<sub>2</sub> photolysis rate coefficient

2014 ◽  
Vol 14 (22) ◽  
pp. 12055-12067 ◽  
Author(s):  
J. C. Tao ◽  
C. S. Zhao ◽  
N. Ma ◽  
P. F. Liu
Keyword(s):  
Boundary Layer ◽  
Aerosol Particle ◽  
Rate Coefficient ◽  
Aerosol Particles ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Hygroscopic Growth ◽  
Particle Population ◽  
Photolysis Rate ◽  
The Impact

Abstract. Hygroscopic growth of aerosol particles can significantly affect their single-scattering albedo (ω), and consequently alters the aerosol effect on tropospheric photochemistry. In this study, the impact of aerosol hygroscopic growth on ω and its application to the NO2 photolysis rate coefficient (JNO2) are investigated for a typical aerosol particle population in the North China Plain (NCP). The variations of aerosol optical properties with relative humidity (RH) are calculated using a Mie theory aerosol optical model, on the basis of field measurements of number–size distribution and hygroscopic growth factor (at RH values above 90%) from the 2009 HaChi (Haze in China) project. Results demonstrate that ambient ω has pronouncedly different diurnal patterns from ω measured at dry state, and is highly sensitive to the ambient RHs. Ambient ω in the NCP can be described by a dry state ω value of 0.863, increasing with the RH following a characteristic RH dependence curve. A Monte Carlo simulation shows that the uncertainty of ω from the propagation of uncertainties in the input parameters decreases from 0.03 (at dry state) to 0.015 (RHs > 90%). The impact of hygroscopic growth on ω is further applied in the calculation of the radiative transfer process. Hygroscopic growth of the studied aerosol particle population generally inhibits the photolysis of NO2 at the ground level, whereas accelerates it above the moist planetary boundary layer. Compared with dry state, the calculated JNO2 at RH of 98% at the height of 1 km increases by 30.4%, because of the enhancement of ultraviolet radiation by the humidified scattering-dominant aerosol particles. The increase of JNO2 due to the aerosol hygroscopic growth above the upper boundary layer may affect the tropospheric photochemical processes and this needs to be taken into account in the atmospheric chemical models.

scholarly journals Aerosol particle characteristics measured in the United Arab Emirates and their response to mixing in the boundary layer

2021 ◽  
Author(s):  
Jutta Kesti ◽  
John Backman ◽  
Ewan James O'Connor ◽  
Anne Hirsikko ◽  
Eija Asmi ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Aerosol Particle ◽  
United Arab Emirates ◽  
Wind Direction ◽  
Aerosol Particles ◽  
Surface Wind ◽  
Number Concentration ◽  
Particle Properties ◽  
Boundary Layer Height

Abstract. Aerosol particles play an important in role in the microphysics of clouds and hence on their likelihood to precipitate. In the changing climate already dry areas such as the United Arab Emirates (UAE) are predicted to become even drier. Comprehensive observations of the daily and seasonal variation in aerosol particle properties in such locations are required reducing the uncertainty in such predictions. We analyse observations from a one-year measurement campaign at a background location in the United Arab Emirates to investigate the properties of aerosol particles in this region, study the impact of boundary layer mixing on background aerosol particle properties measured at the surface and study the temporal evolution of the aerosol particle cloud formation potential in the region. We used in-situ aerosol particle measurements to characterise the aerosol particle composition, size, number and cloud condensation nuclei (CCN) properties, in-situ SO2 measurements as an anthropogenic signature and a long-range scanning Doppler lidar to provide vertical profiles of the horizontal wind and turbulent properties to monitor the evolution of the boundary layer. Anthropogenic sulphate dominated the aerosol particle mass composition in this location. There was a clear diurnal cycle in the surface wind direction, which had a strong impact on aerosol particle total number concentration, SO2 concentration and black carbon mass concentration. Local sources were the predominant source of black carbon, as concentrations clearly depended on the presence of turbulent mixing, with much higher values during calm nights. The measured concentrations of SO2, instead, were highly dependent on the surface wind direction as well as on the depth of the boundary layer when entrainment from the advected elevated layers occurred. The wind direction at the surface or of the elevated layer suggests that the cities of Dubai, Abu Dhabi and other coastal conurbations were the remote sources of SO2. We observed new aerosol particle formation events almost every day (on four days out of five on average). Calm nights had the highest CCN number concentrations and lowest κ values and activation fractions. We did not observe any clear dependence of CCN number concentration and κ parameter on the height of the daytime boundary layer, whereas the activation fraction did show a slight increase with increasing boundary layer height, due to the change in the shape of the aerosol particle size distribution where the relative portion of larger aerosol particles increased with increasing boundary layer height. We believe that this indicates that size is more important than chemistry for aerosol particle CCN activation at this site. The combination of instrumentation used in this campaign enabled us to identify periods when anthropogenic pollution from remote sources that had been transported in elevated layers was present, and had been mixed down to the surface in the growing boundary layer.

Global OMI Aerosol Single Scattering Albedo evaluation using ground-based AERONET

2020 ◽  
Author(s):  
Periklis Drakousis ◽  
Marios-Bruno Korras-Carraca ◽  
Hiren Jethva ◽  
Omar Torres ◽  
Nikos Hatzianastassiou
Keyword(s):  
Optical Properties ◽  
Climate Models ◽  
Spatial Scales ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Absolute Difference ◽  
Ozone Monitoring Instrument ◽  
Seasonal Basis ◽  
Radiance Data ◽  
The Impact

&lt;p&gt;Aerosol measurements are carried out worldwide in order to reduce the uncertainties about the impact of aerosols on climate. Over the past two decades, different methods (ground- or satellite-based) for measuring aerosol properties have been developed, covering a variety of approaches with different temporal and spatial scales, which can be considered complementary. Aerosol optical properties are essential for assessing the effects of aerosols on radiation and climate. Aerosol single scattering albedo (SSA), along with optical depth and asymmetry parameter, is one of the three key optical properties that are necessary for radiation transfer and climate models. At the same time, SSA strongly depends on different aerosol types, thus enabling the identification of these different aerosol particles. However, despite the strong need for aerosol SSA products with global and climatological coverage, and the significant progress in retrieving SSA from satellite measurements, the satellite SSA retrievals are still subjected to uncertainties.&lt;/p&gt;&lt;p&gt;In this study, we perform an evaluation of the OMAERUVd (PGE Version V1.8.9.1) daily L3 (1&amp;#176; x 1&amp;#176; latitude-longitude) aerosol SSA data, which are based on the enhanced two-channel OMAERUV algorithm that essentially uses the ultraviolet radiance data from Aura/Ozone Monitoring Instrument (OMI), through comparisons against daily SSA products from 541 globally distributed Aerosol Robotic Network (AERONET) stations for a 15-year period (2005-2019). The comparison is performed between the available OMAERUVd SSA data at 354 nm, 388 nm, and 500 nm, and the AERONET SSA data at 440 nm (or 443 nm). The comparison is made on an annual and seasonal basis in order to reveal possible seasonally dependent patterns, as well as on a climatological and a year-to-year basis. The statistical metrics, such as Coefficient of Correlation (R) and Bias, are computed for individual AERONET stations as well as for all stations. The effect of availability of common OMI and AERONET data pairs on the comparison is assessed by making comparisons when at least 10, 50 and 100 common pairs are available.&lt;/p&gt;&lt;p&gt;The results show that about 50% (75%) of OMI-AERONET matchups agree within the absolute difference of &amp;#177;0.03 (&amp;#177;0.05) for the 500 nm OMI SSA and the 440 nm (or 443 nm) AERONET SSA. The corresponding percentage for the 388 nm OMI SSA and the 440 nm (or 443 nm) AERONET SSA increases to 58% (81%), while the corresponding numbers for the 354 nm SSA OMI and the 440 nm (or 443 nm) AERONET are 43% (67%). It is found that in overall, OMI tends mainly to overestimate (underestimate) SSA for the 500 nm (354 nm) products in comparison to AERONET 440 nm (or 443 nm) with a total bias of 0.025 (-0.024), or 2.7% (2.6%) in relative percentage terms with respect to AERONET (mean AERONET value equal to 0.908), and an overall R value of 0.399 (0.386). At 388 nm, OMI tends to retrieve higher SSA over regions where biomass burning occurs, against lower SSA values elsewhere, with overall bias and R values equal to -0.002 (0.22%) and 0.395, respectively.&lt;/p&gt;

scholarly journals Aerosol particle characteristics measured in the United Arab Emirates and their response to mixing in the boundary layer

2022 ◽  
Vol 22 (1) ◽  
pp. 481-503
Author(s):  
Jutta Kesti ◽  
John Backman ◽  
Ewan J. O'Connor ◽  
Anne Hirsikko ◽  
Eija Asmi ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Aerosol Particle ◽  
United Arab Emirates ◽  
Wind Direction ◽  
Aerosol Particles ◽  
Surface Wind ◽  
Number Concentration ◽  
Particle Properties ◽  
Boundary Layer Height

Abstract. Aerosol particles play an important role in the microphysics of clouds and hence in their likelihood to precipitate. In the changing climate already-dry areas such as the United Arab Emirates (UAE) are predicted to become even drier. Comprehensive observations of the daily and seasonal variation in aerosol particle properties in such locations are required, reducing the uncertainty in such predictions. We analyse observations from a 1-year measurement campaign at a background location in the United Arab Emirates to investigate the properties of aerosol particles in this region, study the impact of boundary layer mixing on background aerosol particle properties measured at the surface, and study the temporal evolution of the aerosol particle cloud formation potential in the region. We used in situ aerosol particle measurements to characterise the aerosol particle composition, size, number, and cloud condensation nuclei (CCN) properties; in situ SO2 measurements as an anthropogenic signature; and a long-range scanning Doppler lidar to provide vertical profiles of the horizontal wind and turbulent properties to monitor the evolution of the boundary layer. Anthropogenic sulfate dominated the aerosol particle mass composition in this location. There was a clear diurnal cycle in the surface wind direction, which had a strong impact on aerosol particle total number concentration, SO2 concentration, and black carbon mass concentration. Local sources were the predominant source of black carbon as concentrations clearly depended on the presence of turbulent mixing, with much higher values during calm nights. The measured concentrations of SO2, instead, were highly dependent on the surface wind direction as well as on the depth of the boundary layer when entrainment from the advected elevated layers occurred. The wind direction at the surface or of the elevated layer suggests that the oil refineries and the cities of Dubai and Abu Dhabi and other coastal conurbations were the remote sources of SO2. We observed new-aerosol-particle formation events almost every day (on 4 d out of 5 on average). Calm nights had the highest CCN number concentrations and lowest κ values and activation fractions. We did not observe any clear dependence of CCN number concentration and κ parameter on the height of the daytime boundary layer, whereas the activation fraction did show a slight increase with increasing boundary layer height due to the change in the shape of the aerosol particle size distribution where the relative portion of larger aerosol particles increased with increasing boundary layer height. We believe that this indicates that size is more important than chemistry for aerosol particle CCN activation at this site. The combination of instrumentation used in this campaign enabled us to identify periods when anthropogenic pollution from remote sources that had been transported in elevated layers was present and had been mixed down to the surface in the growing boundary layer.

scholarly journals Ice nucleation by viruses and their potential for cloud glaciation

2021 ◽  
Vol 18 (14) ◽  
pp. 4431-4444
Author(s):  
Michael P. Adams ◽  
Nina S. Atanasova ◽  
Svetlana Sofieva ◽  
Janne Ravantti ◽  
Aino Heikkinen ◽  
...  
Keyword(s):  
Aerosol Particles ◽  
Ice Nucleation ◽  
Marine Atmosphere ◽  
Biological Origin ◽  
Virus Particles ◽  
Particle Population ◽  
Minor Contribution ◽  
Different Types ◽  
A Minor ◽  
The Impact

Abstract. In order to effectively predict the formation of ice in clouds we need to know which subsets of aerosol particles are effective at nucleating ice, how they are distributed and where they are from. A large proportion of ice-nucleating particles (INPs) in many locations are likely of biological origin, and some INPs are extremely small, being just tens of nanometres in size. The identity and sources of such INPs are not well characterized. Here, we show that several different types of virus particles can nucleate ice, with up to about 1 in 20 million virus particles able to nucleate ice at −20 ∘C. In terms of the impact on cloud glaciation, the ice-nucleating ability (the fraction which are ice nucleation active as a function of temperature) taken together with typical virus particle concentrations in the atmosphere leads to the conclusion that virus particles make a minor contribution to the atmospheric ice-nucleating particle population in the terrestrial-influenced atmosphere. However, they cannot be ruled out as being important in the remote marine atmosphere. It is striking that virus particles have an ice-nucleating activity, and further work should be done to explore other types of viruses for both their ice-nucleating potential and to understand the mechanism by which viruses nucleate ice.

A comprehensive database of the optical properties of irregular aerosol particles for radiative transfer simulations

2021 ◽  
Author(s):  
Masanori Saito ◽  
Ping Yang ◽  
Jiachen Ding ◽  
Xu Liu
Keyword(s):  
Optical Properties ◽  
Radiative Transfer ◽  
Aerosol Particle ◽  
Volcanic Ash ◽  
Aerosol Particles ◽  
Single Scattering ◽  
Dust Aerosol ◽  
Index Of Refraction ◽  
Particle Sizes

AbstractA database (TAMUdust2020) of the optical properties of irregular aerosol particles is developed for applications to radiative transfer simulations involving aerosols, particularly dust and volcanic ash particles. The particle shape model assumes an ensemble of irregular hexahedral geometries to mimic complex aerosol particle shapes in nature. State-of-the-art light scattering computational capabilities are employed to compute the single-scattering properties of these particles for wide ranges of values of the size parameter, the index of refraction, and the degree of sphericity. The database therefore is useful for various radiative transfer applications over a broad spectral region from ultraviolet to infrared. Overall, agreement between simulations and laboratory/in-situ measurements is achieved for the scattering phase matrix and backscattering of various dust aerosol and volcanic ash particles. Radiative transfer simulations of active and passive spaceborne sensor signals for dust plumes with various aerosol optical depths and the effective particle sizes clearly demonstrate the applicability of the database for aerosol studies. In particular, the present database includes, for the first time, robust backscattering of nonspherical particles spanning the entire range of aerosol particle sizes, which shall be useful to appropriately interpret lidar signals related to the physical properties of aerosol plumes. Furthermore, thermal infrared simulations based on in-situ measured refractive indices of dust aerosol particles manifest the effects of the regional variations of aerosol optical properties. This database includes a user-friendly interface to obtain user-customized aerosol single-scattering properties with respect to spectrally dependent complex refractive index, size, and the degree of sphericity.

Global variability of aerosol optical properties retrieved from the network of GAW near-surface observatories

2020 ◽  
Author(s):  
Alessandro Bigi ◽  
Martine Collaud Coen ◽  
Elisabeth J. Andrews ◽  
Clémence Rose ◽  
Cathrine Lund Myhre ◽  
...  
Keyword(s):  
Optical Properties ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Scattering Coefficient ◽  
Near Surface ◽  
World Data ◽  
Data Centre ◽  
The Impact ◽  
Urban Sites ◽  
Aerosol Properties

&lt;p&gt;Atmospheric aerosols are known to play a key role in Earth&amp;#8217;s radiative budget, although the quantification of their climate forcing is still highly uncertain. In order to improve the scientific understanding of their climatic effect, in-situ ground-based aerosol properties observations are needed by the research community. Such data would also allow the global assessment of the effect of environmental policies over both the short and the long term.&lt;/p&gt;&lt;p&gt;To develop a robust and consistent view over time of the worldwide variability of aerosol properties, data resulting from a fully-characterized value chain, including uncertainty estimation, is needed.&lt;/p&gt;&lt;p&gt;The present work is part of a wider project, having among its goals the investigation of the variability of climate-relevant aerosol properties observed at all sites connected to the Global Atmospheric Watch network, whose data are publicly available from the World Data Centre for Aerosols and follow the aforementioned specifications.&lt;/p&gt;&lt;p&gt;This work focuses on aerosol optical proprieties, i.e. the aerosol light scattering coefficient (&amp;#963;&lt;sub&gt;sp&lt;/sub&gt;), the aerosol light absorption coefficient (&amp;#963;&lt;sub&gt;ap&lt;/sub&gt;), single scattering albedo (&amp;#969;&lt;sub&gt;o&lt;/sub&gt;) and both scattering and absorption &amp;#197;ngstr&amp;#246;m exponents (&amp;#229;&lt;sub&gt;sp&lt;/sub&gt; and &amp;#229;&lt;sub&gt;ap&lt;/sub&gt;).&lt;/p&gt;&lt;p&gt;The analysis includes 108 yearly datasets collected either during 2016 or 2017 at different sites: 53 for absorption and 55 for scattering coefficient datasets, respectively. For 29 of these sites it was also possible to compute single scattering albedo.&lt;/p&gt;&lt;p&gt;The spatial variability in extensive and intensive optical properties was analysed in terms of each site&amp;#8217;s geographical location (either polar, continental, coastal or mountain) and its footprint (from pristine to urban, representing increasing levels of anthropogenic influence).&lt;/p&gt;&lt;p&gt;The results highlight the impact of anthropogenic emissions and biomass burning on absolute levels and annual variability. The effect of sea spray or long range transport of dust is also evident for several sites, along with the influence of regional emissions. The largest seasonality in aerosol loading was observed at mountain sites under mixed footprint conditions, while the lowest seasonality occurred at urban sites. Urban sites also exhibited the highest &amp;#963;sp and &amp;#963;ap values. The lowest levels in &amp;#963;&lt;sub&gt;sp&lt;/sub&gt; and &amp;#963;&lt;sub&gt;ap&lt;/sub&gt; were observed at some polar sites, along with few coastal and mountain sites, despite their typically mixed footprint.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Acknowledgements&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;The authors acknowledge WMO-GAW World Data Centre on Aerosol for providing data available at http://ebas.nilu.no&lt;/p&gt;

scholarly journals Ice nucleation by viruses and their potential for cloud glaciation

2021 ◽  
Author(s):  
Michael P. Adams ◽  
Nina S. Atanasova ◽  
Svetlana Sofieva ◽  
Janne Ravantti ◽  
Aino Heikkinen ◽  
...  
Keyword(s):  
Aerosol Particles ◽  
Ice Nucleation ◽  
Marine Atmosphere ◽  
Biological Origin ◽  
Virus Particles ◽  
Particle Population ◽  
Minor Contribution ◽  
Different Types ◽  
A Minor ◽  
The Impact

Abstract. In order to effectively predict the formation of ice in clouds we need to know which subsets of aerosol particles are effective at nucleating ice, how they are distributed and where they are from. A large proportion of ice-nucleating particles (INPs) in many locations are likely of biological origin, and some INPs are extremely small being just tens of nanometers in size. The identity and sources of such INPs are not well characterized. Here, we show that several different types of virus particles can nucleate ice, with up to about one in twenty million virus particles able to nucleate ice at −20 °C. In terms of the impact on cloud glaciation, the ice-nucleating ability (the fraction which are ice nucleation active as a function of temperature) taken together with typical virus particle concentrations in the atmosphere lead to the conclusion that virus particles make a minor contribution to the atmospheric ice-nucleating particle population in the terrestrial influenced atmosphere. However, they cannot be ruled out as being important in the remote marine atmosphere. It is striking that virus particles have an ice-nucleating activity and further work should be done to explore other types of viruses for both their ice-nucleating potential and to understand the mechanism by which viruses nucleate ice.

scholarly journals Roll vortices induce new particle formation bursts in the planetary boundary layer

2020 ◽  
Author(s):  
Janne Lampilahti ◽  
Hanna Elina Manninen ◽  
Katri Leino ◽  
Riikka Väänänen ◽  
Antti Manninen ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Planetary Boundary Layer ◽  
Particle Production ◽  
Aerosol Particles ◽  
Regional Scale ◽  
Particle Formation ◽  
Forest Site ◽  
New Particle Formation ◽  
Roll Vortices ◽  
The Impact

Abstract. Recent studies have shown the importance of new particle formation (NPF) to global cloud concensation nuclei (CCN) production, as well as to air pollution in megacities. In addition to the necessary presence of low-volatility vapors that can form the new aerosol particles, both numerical and observational studies have shown that the dynamics of the planetary boundary layer (BL) plays an important role in NPF. Evidence from field observations suggests that roll vortices might be favorable for inducing NPF in a convective BL. However, direct observations and estimates on the potential importance of this phenomenon to the production of new aerosol particles are lacking. Here we show that rolls frequently induce NPF bursts along the horizontal circulations, and that the small clusters and particles originating from these bursts grow in size similar to particles typically ascribed to regional-scale atmospheric NPF. We outline a method to identify roll-induced NPF from measurements and, based on the collected data, estimate the impact of roll vortices on the overall aerosol particle production due to NPF at a boreal forest site (83 ± 34 % and 26 ± 8 % overall enhancement in particle formation for 3-nm and 10-nm particles respectively). We conclude that the formation of roll vortices should be taken into account when estimating particle number budgets in the atmospheric BL.

scholarly journals A global study of hygroscopicity-driven light-scattering enhancement in the context of other in situ aerosol optical properties

2021 ◽  
Vol 21 (17) ◽  
pp. 13031-13050
Author(s):  
Gloria Titos ◽  
María A. Burgos ◽  
Paul Zieger ◽  
Lucas Alados-Arboledas ◽  
Urs Baltensperger ◽  
...  
Keyword(s):  
Light Scattering ◽  
Radiative Forcing ◽  
Incident Light ◽  
Single Scattering ◽  
Single Scattering Albedo ◽  
Clear Pattern ◽  
Opposite Pattern ◽  
Aerosol Radiative Forcing ◽  
Dry Conditions ◽  
The Impact

Abstract. The scattering and backscattering enhancement factors (f(RH) and fb(RH)) describe how aerosol particle light scattering and backscattering, respectively, change with relative humidity (RH). They are important parameters in estimating direct aerosol radiative forcing (DARF). In this study we use the dataset presented in Burgos et al. (2019) that compiles f(RH) and fb(RH) measurements at three wavelengths (i.e., 450, 550 and 700 nm) performed with tandem nephelometer systems at multiple sites around the world. We present an overview of f(RH) and fb(RH) based on both long-term and campaign observations from 23 sites representing a range of aerosol types. The scattering enhancement shows a strong variability from site to site, with no clear pattern with respect to the total scattering coefficient. In general, higher f(RH) is observed at Arctic and marine sites, while lower values are found at urban and desert sites, although a consistent pattern as a function of site type is not observed. The backscattering enhancement fb(RH) is consistently lower than f(RH) at all sites, with the difference between f(RH) and fb(RH) increasing for aerosol with higher f(RH). This is consistent with Mie theory, which predicts higher enhancement of the light scattering in the forward than in the backward direction as the particle takes up water. Our results show that the scattering enhancement is higher for PM1 than PM10 at most sites, which is also supported by theory due to the change in scattering efficiency with the size parameter that relates particle size and the wavelength of incident light. At marine-influenced sites this difference is enhanced when coarse particles (likely sea salt) predominate. For most sites, f(RH) is observed to increase with increasing wavelength, except at sites with a known dust influence where the spectral dependence of f(RH) is found to be low or even exhibit the opposite pattern. The impact of RH on aerosol properties used to calculate radiative forcing (e.g., single-scattering albedo, ω0, and backscattered fraction, b) is evaluated. The single-scattering albedo generally increases with RH, while b decreases. The net effect of aerosol hygroscopicity on radiative forcing efficiency (RFE) is an increase in the absolute forcing effect (negative sign) by a factor of up to 4 at RH = 90 % compared to dry conditions (RH < 40 %). Because of the scarcity of scattering enhancement measurements, an attempt was made to use other more commonly available aerosol parameters (i.e., ω0 and scattering Ångström exponent, αsp) to parameterize f(RH). The majority of sites (75 %) showed a consistent trend with ω0 (higher f(RH = 85 %) for higher ω0), while no clear pattern was observed between f(RH = 85 %) and αsp. This suggests that aerosol ω0 is more promising than αsp as a surrogate for the scattering enhancement factor, although neither parameter is ideal. Nonetheless, the qualitative relationship observed between ω0 and f(RH) could serve as a constraint on global model simulations.

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