Calculating the aerosol asymmetry factor based on measurements from the humidified nephelometer system

Abstract. The aerosol asymmetry factor (g) is one of the most important factors for assessing direct aerosol radiative forcing. However, little attention has been paid to the measurement and parameterization of g. In this study, the characteristics of g are studied based on field measurements over the North China Plain (NCP) using the Mie scattering theory. The results show that calculated g values for dry aerosol can vary over a wide range (between 0.54 and 0.67). Furthermore, when ambient relative humidity (RH) reaches 90 %, g is significantly enhanced by a factor of 1.2 due to aerosol hygroscopic growth. For the first time, a novel method of calculating g based on measurements from the humidified nephelometer system is proposed. This method can constrain the uncertainty of g to within 2.56 % for dry aerosol populations and 4.02 % for ambient aerosols, providing that aerosol hygroscopic growth is taken into account. Sensitivity studies show that aerosol hygroscopicity plays a vital role in the accuracy of predicting g.

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Method to calculate the aerosol asymmetry factor based on measurements from the humidified nephelometer system

10.5194/acp-2017-1148 ◽

2018 ◽

Author(s):

Gang Zhao ◽

Chunsheng Zhao ◽

Ye Kuang ◽

Yuxuan Bian ◽

Jiangchuan Tao ◽

...

Keyword(s):

Radiative Forcing ◽

Field Measurements ◽

Mie Scattering ◽

Asymmetry Factor ◽

Hygroscopic Growth ◽

Aerosol Radiative Forcing ◽

The North ◽

Ambient Relative Humidity ◽

Wide Range ◽

Aerosol Radiative

Abstract. The aerosol asymmetry factor (g) is one of the most important factors for assessing direct aerosol radiative forcing. So far, few studies have focused on the measurements and parameterization of g. The characteristics of g are studied based on field measurements over the North China Plain by using the Mie scattering theory. The results show that calculated g values can vary over a wide range (between 0.54 and 0.67). When ambient relative humidity (RH) reaches 90 %, g is significantly enhanced by a factor of 1.2 due to aerosol hygroscopic growth. Direct aerosol radiative forcing can be reduced by 40 % when g increases by 20 %. For the first time, a novel method to calculate g based on measurements from the humidified nephelometer system is proposed. This method can constrain the uncertainty of g within 2 % for dry aerosol populations and 4 % for ambient aerosols, taking into account aerosol hygroscopic growth. Sensitivity studies show that ambient RH and aerosol hygroscopicity are the most important factors that influence the accuracy of predicting g.

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A thermodynamic description for the hygroscopic growth of atmospheric aerosol particles

Atmospheric Chemistry and Physics ◽

10.5194/acp-18-14939-2018 ◽

2018 ◽

Vol 18 (20) ◽

pp. 14939-14948 ◽

Cited By ~ 7

Author(s):

Dimitri Castarède ◽

Erik S. Thomson

Keyword(s):

Radiative Forcing ◽

Thermodynamic Description ◽

Liquid Droplets ◽

Hygroscopic Growth ◽

Salt Particle ◽

Aerosol Radiative Forcing ◽

Condensation Nucleation ◽

Kohler Theory ◽

Kelvin Effect ◽

Brine Layer

Abstract. The phase state of atmospheric particulate is important to atmospheric processes, and aerosol radiative forcing remains a large uncertainty in climate predictions. That said, precise atmospheric phase behavior is difficult to quantify and observations have shown that “precondensation” of water below predicted saturation values can occur. We propose a revised approach to understanding the transition from solid soluble particles to liquid droplets, typically described as cloud condensation nucleation – a process that is traditionally captured by Köhler theory, which describes a modified equilibrium saturation vapor pressure due to (i) mixing entropy (Raoult's law) and (ii) droplet geometry (Kelvin effect). Given that observations of precondensation are not predicted by Köhler theory, we devise a more complete model that includes interfacial forces giving rise to predeliquescence, i.e., the formation of a brine layer wetting a salt particle at relative humidities well below the deliquescence point.

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Method to measure the size-resolved real part of aerosol refractive index using differential mobility analyzer in tandem with single-particle soot photometer

Atmospheric Measurement Techniques ◽

10.5194/amt-12-3541-2019 ◽

2019 ◽

Vol 12 (7) ◽

pp. 3541-3550 ◽

Cited By ~ 3

Author(s):

Gang Zhao ◽

Weilun Zhao ◽

Chunsheng Zhao

Keyword(s):

Refractive Index ◽

Single Particle ◽

Radiative Forcing ◽

Differential Mobility ◽

Ambient Aerosols ◽

Aerosol Radiative Forcing ◽

Abstract Knowledge ◽

Wide Range ◽

Aerosol Radiative Effects ◽

Aerosol Radiative

Abstract. Knowledge on the refractive index of ambient aerosols can help reduce the uncertainties in estimating aerosol radiative forcing. A new method is proposed to retrieve the size-resolved real part of the refractive index (RRI). The main principle of deriving the RRI is measuring the scattering intensity by a single-particle soot photometer (SP2) of size-selected aerosols. This method is validated by a series of calibration experiments using the components of the known RRI. The retrieved size-resolved RRI covers a wide range, from 200 to 450 nm, with uncertainty of less than 0.02. Measurements of the size-resolved RRI can improve the understanding of the aerosol radiative effects.

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Reply to “Comment on ‘Rethinking the Lower Bound on Aerosol Radiative Forcing’”

Journal of Climate ◽

10.1175/jcli-d-17-0034.1 ◽

2017 ◽

Vol 30 (16) ◽

pp. 6585-6589 ◽

Cited By ~ 4

Author(s):

Bjorn Stevens ◽

Stephanie Fiedler

Keyword(s):

Twentieth Century ◽

Lower Bound ◽

Radiative Forcing ◽

Climate Models ◽

Anthropogenic Aerosols ◽

Anthropogenic Aerosol ◽

Aerosol Radiative Forcing ◽

Clear Sky ◽

The North ◽

Aerosol Forcing

Kretzschmar et al., in a comment in 2017, use the spread in the output of aerosol–climate models to argue that the models refute the hypothesis (presented in a paper by Stevens in 2015) that for the mid-twentieth-century warming to be consistent with observations, then the present-day aerosol forcing, [Formula: see text] must be less negative than −1 W m−2. The main point of contention is the nature of the relationship between global SO2 emissions and [Formula: see text] In contrast to the concave (log-linear) relationship used by Stevens and in earlier studies, whereby [Formula: see text] becomes progressively less sensitive to SO2 emissions, some models suggest a convex relationship, which would imply a less negative lower bound. The model that best exemplifies this difference, and that is most clearly in conflict with the hypothesis of Stevens, does so because of an implausible aerosol response to the initial rise in anthropogenic aerosol precursor emissions in East and South Asia—already in 1975 this model’s clear-sky reflectance from anthropogenic aerosol over the North Pacific exceeds present-day estimates of the clear-sky reflectance by the total aerosol. The authors perform experiments using a new (observationally constrained) climatology of anthropogenic aerosols to further show that the effects of changing patterns of aerosol and aerosol precursor emissions during the late twentieth century have, for the same global emissions, relatively little effect on [Formula: see text] These findings suggest that the behavior Kretzschmar et al. identify as being in conflict with the lower bound in Stevens arises from an implausible relationship between SO2 emissions and [Formula: see text] and thus provides little basis for revising this lower bound.

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Impact of aerosol hygroscopic growth on retrieving aerosol extinction coefficient profiles from elastic-backscatter lidar signals

10.5194/acp-2017-240 ◽

2017 ◽

Author(s):

Gang Zhao ◽

Chunsheng Zhao ◽

Ye Kuang ◽

Jiangchuan Tao ◽

Wangshu Tan ◽

...

Keyword(s):

Extinction Coefficient ◽

Enhancement Factor ◽

Field Measurements ◽

Relative Bias ◽

Aerosol Extinction ◽

Hygroscopic Growth ◽

The North ◽

True Value ◽

Lidar Measurements ◽

Aerosol Extinction Coefficient

Abstract. Light detection and ranging (lidar) measurements have been widely used to profile ambient aerosol extinction coefficient (σext). Particle extinction-to-backscatter ratio (lidar ratio, LR), which highly depends on aerosol dry particle number size distribution (PNSD) and aerosol hygroscopicity, is introduced to retrieve the σext profile from elastic-backscatter lidar signals. Conventionally, a constant column integrated LR that is estimated from aerosol optical depth is used by the retrieving algorithms. In this paper, the influences of aerosol PNSD, aerosol hygroscopic growth and relative humidity (RH) profiles on the variation of LR are investigated based on the datasets from field measurements in the North China Plain (NCP). Results show that LR has an enhancement factor of 2.2 when RH reaches 92 %. Simulation results indicate that both the magnitude and vertical structures of the σext profiles by using column-related LR method are significantly biased from the original σext profile. The relative bias, which is mainly influenced by RH and PNSD, can reach up to 40 % when RH at the top of the mixed layer is above 90 %. A new algorithm for retrieving σext profiles and a new scheme of LR enhancement factor by RH in the NCP are proposed in this study. The relative bias between the σext profile retrieved with this new algorithm and the ideal true value is reduced to below 13 %.

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Impact of aerosol hygroscopic growth on retrieving aerosol extinction coefficient profiles from elastic-backscatter lidar signals

Atmospheric Chemistry and Physics ◽

10.5194/acp-17-12133-2017 ◽

2017 ◽

Vol 17 (19) ◽

pp. 12133-12143 ◽

Cited By ~ 13

Author(s):

Gang Zhao ◽

Chunsheng Zhao ◽

Ye Kuang ◽

Jiangchuan Tao ◽

Wangshu Tan ◽

...

Keyword(s):

Extinction Coefficient ◽

Enhancement Factor ◽

Field Measurements ◽

Relative Bias ◽

Aerosol Extinction ◽

Hygroscopic Growth ◽

The North ◽

True Value ◽

Lidar Measurements ◽

Aerosol Extinction Coefficient

Abstract. Light detection and ranging (lidar) measurements have been widely used to profile the ambient aerosol extinction coefficient (σext). The particle extinction-to-backscatter ratio (lidar ratio, LR), which strongly depends on the aerosol dry particle number size distribution (PNSD) and aerosol hygroscopicity, is introduced to retrieve the σext profile from elastic-backscatter lidar signals. Conventionally, a constant column-integrated LR that is estimated from aerosol optical depth is used by the retrieving algorithms. In this paper, the influences of aerosol PNSD, aerosol hygroscopic growth and relative humidity (RH) profiles on the variation in LR are investigated based on the datasets from field measurements in the North China Plain (NCP). Results show that LR has an enhancement factor of 2.2 when RH reaches 92 %. Simulation results indicate that both the magnitude and vertical structures of the σext profiles by using the column-related LR method are significantly biased from the original σext profile. The relative bias, which is mainly influenced by RH and PNSD, can reach up to 40 % when RH at the top of the mixed layer is above 90 %. A new algorithm for retrieving σext profiles and a new scheme of LR enhancement factor by RH in the NCP are proposed in this study. The relative bias between the σext profile retrieved with this new algorithm and the ideal true value is reduced to below 13 %.

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Optical particle counter measurement of marine aerosol hygroscopic growth

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-7-12381-2007 ◽

2007 ◽

Vol 7 (4) ◽

pp. 12381-12415 ◽

Cited By ~ 2

Author(s):

J. R. Snider ◽

M. D. Petters

Keyword(s):

Growth Factors ◽

Relative Humidity ◽

Scattered Light ◽

Particle Diameter ◽

Mie Scattering ◽

Size Spectrum ◽

Hygroscopic Growth ◽

Marine Stratocumulus ◽

Ambient Relative Humidity ◽

Light Scattering Detection

Abstract. A technique is developed for the determination of the hygroscopic growth factor of dry particles with diameter between 0.3 and 0.6 µm and is applied to measurements made during the second Dynamics and Chemistry of Marine Stratocumulus experiment. Two optical particle counters are utilized, one measures the aerosol size spectrum at ambient relative humidity and the other simultaneously dries the aerosol prior to light scattering detection. Growth factors are based on measurements made in the region of the Mie scattering curve where scattered light intensity increases monotonically with dry and wet particle diameter, i.e. D<0.9 µm. Factors influencing the accuracy of the measurement are evaluated, including particle drying, refractive index and shape. Growth factors at 90±3% ambient relative humidity in marine airmasses 400 km west of San Diego, California range between 1.5 and 1.8. This suggests that a significant fraction of the particle mass, between 40 and 70%, is either non-hygroscopic or weakly hygroscopic.

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Opportunistic Experiments to Constrain Aerosol Effective Radiative Forcing

10.5194/acp-2021-559 ◽

2021 ◽

Author(s):

Matthew Christensen ◽

Andrew Gettelman ◽

Jan Cermak ◽

Guy Dagan ◽

Michael Diamond ◽

...

Keyword(s):

Forest Fires ◽

Radiative Forcing ◽

Volcanic Eruptions ◽

Point Sources ◽

Natural Experiments ◽

Experimental Conditions ◽

Anthropogenic Aerosol ◽

Aerosol Radiative Forcing ◽

Wide Range ◽

Aerosol Radiative

Abstract. Aerosol-cloud interactions (ACI) are considered to be the most uncertain driver of present-day radiative forcing due to human activities. The non-linearity of cloud-state changes to aerosol perturbations make it challenging to attribute causality in observed relationships of aerosol radiative forcing. Using correlations to infer causality can also be challenging when meteorological variability also drives both aerosol and cloud changes independently. Natural and anthropogenic aerosol perturbations from well defined sources provide “opportunistic experiments” (also known as natural experiments) to investigate ACI in cases where causality may be more confidently inferred. These perturbations cover a wide range of locations and spatio-temporal scales, including point sources such as volcanic eruptions or industrial sources, plumes from biomass burning or forest fires, and tracks from individual ships or shipping corridors. We review the different experimental conditions and conduct a synthesis of the available satellite data sets and field campaigns to place these opportunistic experiments on a common footing, facilitating new insights and a clearer understanding of key uncertainties in aerosol radiative forcing. Strong liquid water path increases due to aerosol perturbations are largely ruled out by averaging across experiments. Cloud albedo perturbations are strongly sensitive to background meteorological conditions. Opportunistic experiments have significantly improved process level understanding of ACI, but it remains unclear how reliably the relationships found can be scaled to the global level, thus, demonstrating a need for deeper investigation in order to improve assessments of aerosol radiative forcing and climate change.

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Role of black carbons mass size distribution in the direct aerosol radiative forcing

10.5194/acp-2019-385 ◽

2019 ◽

Author(s):

Gang Zhao ◽

Jiangchuan Tao ◽

Ye Kuang ◽

Chuanyang Shen ◽

Yingli Yu ◽

...

Keyword(s):

Size Distribution ◽

Radiative Forcing ◽

Climate Models ◽

Field Measurements ◽

Radiative Effects ◽

Aerosol Radiative Forcing ◽

Mass Size Distribution ◽

Mass Size ◽

Mixing States ◽

Aerosol Radiative

Abstract. Large uncertainties exist when estimating radiative effects of ambient black carbon (BC) aerosol. Previous studies about the BC aerosol radiative forcing mainly focus on the BC aerosols’ mass concentrations and mixing states, while the effects of BC mass size distribution (BCMSD) were not well considered. In this paper, we developed a method by measuring the BCMSD by using a differential mobility analyzer in tandem with an aethalometer. A comprehensive method of multiple charging corrections is proposed and implemented in measuring the BCMSD. Good agreement is obtained between the BC mass concentration integrated from this system and that measured in bulk phase, demonstrating the reliability of our proposed method. Characteristics of the BCMSD and corresponding radiative effects are studied based on field measurements conducted in the North China Plain by using our own designed measurement system. Results show that the BCMSD have two modes and the mean peak diameters of the two modes are 150 nm and 503 nm respectively. The BCMSD of coarser mode varies significantly under different pollution conditions with peak diameter varying between 430 nm and 580 nm, which gives rise to significant variation in aerosol buck optical properties. The aerosol direct aerosol radiative forcing is estimated to vary by 22.5 % for different measured BCMSDs, which shares the same magnitude to the variation associated with assuming different aerosol mixing states (21.5 %). Our study reveals that the BCMSD matters as well as their mixing state in estimating the direct aerosol radiative forcing. Knowledge of the BCMSD should be fully considered in climate models.

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The Climatic Importance of Uncertainties in Regional Aerosol–Cloud Radiative Forcings over Recent Decades

Journal of Climate ◽

10.1175/jcli-d-15-0127.1 ◽

2015 ◽

Vol 28 (17) ◽

pp. 6589-6607 ◽

Cited By ~ 11

Author(s):

Leighton A. Regayre ◽

Kirsty J. Pringle ◽

Lindsay A. Lee ◽

Alexandru Rap ◽

Jo Browse ◽

...

Keyword(s):

Time Scales ◽

Radiative Forcing ◽

Large Scale ◽

Regional Patterns ◽

Aerosol Radiative Forcing ◽

Aerosol Cloud ◽

The North ◽

Aerosol Forcing ◽

Aerosol Emission ◽

Surface Temperatures

Abstract Regional patterns of aerosol radiative forcing are important for understanding climate change on decadal time scales. Uncertainty in aerosol forcing is likely to vary regionally and seasonally because of the short aerosol lifetime and heterogeneous emissions. Here the sensitivity of regional aerosol cloud albedo effect (CAE) forcing to 31 aerosol process parameters and emission fluxes is quantified between 1978 and 2008. The effects of parametric uncertainties on calculations of the balance of incoming and outgoing radiation are found to be spatially and temporally dependent. Regional uncertainty contributions of opposite sign cancel in global-mean forcing calculations, masking the regional importance of some parameters. Parameters that contribute little to uncertainty in Earth’s global energy balance during recent decades make significant contributions to regional forcing variance. Aerosol forcing sensitivities are quantified within 11 climatically important regions, where surface temperatures are thought to influence large-scale climate effects. Substantial simulated uncertainty in CAE forcing in the eastern Pacific leaves open the possibility that apparent shifts in the mean ENSO state may result from a forced aerosol signal on multidecadal time scales. A likely negative aerosol CAE forcing in the tropical North Atlantic calls into question the relationship between Northern Hemisphere aerosol emission reductions and CAE forcing of sea surface temperatures in the main Atlantic hurricane development region on decadal time scales. Simulated CAE forcing uncertainty is large in the North Pacific, suggesting that the role of the CAE in altering Pacific tropical storm frequency and intensity is also highly uncertain.

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