Light Scattering Enhancement Factor Research Articles

The Impacts of Dust Storms With Different Transport Pathways on Aerosol Chemical Compositions and Optical Hygroscopicity of Fine Particles in the Yangtze River Delta

AbstractAerosol physicochemical properties during two dust storms (DS1: March 30–31, and DS2: May 7–8) are measured in 2021 in Nanjing, aiming to investigate the impacts of dust storms on aerosol chemical compositions and optical hygroscopicity in the Yangtze River Delta (YRD). During DS1, the dust air masses are transported in the lower atmosphere and pass through the inlands and sea areas before reaching the YRD region. During DS2, the dust air masses are transported in the upper atmosphere and pass through the inlands only. Both of them are accompanied by an increase in black carbon (BC) mass fraction and a decrease in nitrate mass fraction in fine particles (PM2.5, particles with diameters less than 2.5 μm) near the surface. However, the impacts on the mass fractions of organics or sulfate are adverse between DS1 and DS2. During dust‐influence periods the enlarged mass ratios of sulfate to nitrate (greater than 4) promote the occurrence of deliquescent behavior of ambient aerosols although dust aerosols can significantly suppress aerosol optical hygroscopicity. To improve model simulations of aerosol optical hygroscopicity, it is necessary to use a segment parameterization to describe aerosol light scattering enhancement factor during dust‐influence periods. The closure study of optical hygroscopicity parameters with two different methods reveals the impact of aerosol scattering Ångström exponent on the estimation of optical hygroscopicity parameter. The results highlight that the impact of dust storms on aerosol chemical compositions and optical hygroscopicity are different for DS events with different transport pathways.

Small angle approximation for the lidar return from clouds and fogs

The paper is aimed at the inter-comparison of the small angle approximation (SAA) for the lidar return with Monte Carlo calculations at small receiver field of views, where the SAA can be used for the evaluation of lidar signals from clouds and fogs. The diffraction component of light scattering by a local volume of a cloudy medium is fully taken into account and the geometrical part of light scattering field has been neglected. It has been found that the derived approximation provides useful approximation for the simulation of lidar signals from a cloudy medium close to its boundary, where the contribution of small-angle light scattering and propagation on the path from the cloud boundary to the observation point and back is significant. It has been found that the multiple light scattering enhancement factor A in the framework of the SSA for clouds with the same half - width parameter μ of the droplet size distribution depends just on two main parameters: optical depth τand the scale parameter s, which is equal to the ratio of the effective scattering angle to the receiver field of view.

Relative Humidity Dependence of Growth Factor and Real Refractive Index for Sea Salt/Malonic Acid Internally Mixed Aerosols

AbstractSea salt aerosols (SSAs) often mix with organic matter via sea‐to‐air emission, photochemical oxidation, and atmospheric transport, which change particle hygroscopicity and real refractive index (RRI), thereby the radiative effects. However, the influence of water uptake and organic matters on radiative effects is not well understood. In this study, relative humidity (RH)‐dependent growth factor (GF), light scattering enhancement factor, RRI (nmea), and radiative forcing efficiency (RFE) in the visible spectrum of SSAs/malonic acid (MA) internally mixed aerosols were investigated. Our results showed that MA‐mediated SSAs readily absorbed water even at low RH but suppressed hygroscopic growth at high RH. The discrepancy between Zdanovskii‐Stokes‐Robinson predictions and measured GF indicated that the suppressing effect on size caused by produced malonate salts in chloride depletion became more evident as MA content increased. RRI in the visible spectrum decreased as MA got enriched and approached toward water's RRI at high RH, resulting in a less cooling effect. The increasing RH reduced RFE both by decreasing RRI and increasing particle size substantially. Ignoring the effect of MA on particle size and RRI under varied RH, errors in RFE at 525 nm up to 25% could be induced. Volume mixing rule performs well in predicting the RRI of mixed aerosols in dry conditions, while the retrieved difference of 0.061 between volume mixing RRI nvol and nmea would make the error in RFE up to 1.15 W·m−2·Aerosol optical depth−1 at 87% RH. Our study helps understand the impact of water content and dicarboxylic acids on the direct radiative effects of SSAs.

A global model–measurement evaluation of particle light scattering coefficients at elevated relative humidity

Abstract. The uptake of water by atmospheric aerosols has a pronounced effect on particle light scattering properties, which in turn are strongly dependent on the ambient relative humidity (RH). Earth system models need to account for the aerosol water uptake and its influence on light scattering in order to properly capture the overall radiative effects of aerosols. Here we present a comprehensive model–measurement evaluation of the particle light scattering enhancement factor f(RH), defined as the particle light scattering coefficient at elevated RH (here set to 85 %) divided by its dry value. The comparison uses simulations from 10 Earth system models and a global dataset of surface-based in situ measurements. In general, we find a large diversity in the magnitude of predicted f(RH) amongst the different models, which can not be explained by the site types. Based on our evaluation of sea salt scattering enhancement and simulated organic mass fraction, there is a strong indication that differences in the model parameterizations of hygroscopicity and model chemistry are driving at least some of the observed diversity in simulated f(RH). Additionally, a key point is that defining dry conditions is difficult from an observational point of view and, depending on the aerosol, may influence the measured f(RH). The definition of dry also impacts our model evaluation, because several models exhibit significant water uptake between RH = 0 % and 40 %. The multisite average ratio between model outputs and measurements is 1.64 when RH = 0 % is assumed as the model dry RH and 1.16 when RH = 40 % is the model dry RH value. The overestimation by the models is believed to originate from the hygroscopicity parameterizations at the lower RH range which may not implement all phenomena taking place (i.e., not fully dried particles and hysteresis effects). This will be particularly relevant when a location is dominated by a deliquescent aerosol such as sea salt. Our results emphasize the need to consider the measurement conditions in such comparisons and recognize that measurements referred to as dry may not be dry in model terms. Recommendations for future model–measurement evaluation and model improvements are provided.

Understanding and improving model representation of aerosol optical properties for a Chinese haze event measured during KORUS-AQ

Abstract. KORUS-AQ was an international cooperative air quality field study in South Korea that measured local and remote sources of air pollution affecting the Korean Peninsula during May–June 2016. Some of the largest aerosol mass concentrations were measured during a Chinese haze transport event (24 May). Air quality forecasts using the WRF-Chem model with aerosol optical depth (AOD) data assimilation captured AOD during this pollution episode but overpredicted surface particulate matter concentrations in South Korea, especially PM2.5, often by a factor of 2 or larger. Analysis revealed multiple sources of model deficiency related to the calculation of optical properties from aerosol mass that explain these discrepancies. Using in situ observations of aerosol size and composition as inputs to the optical properties calculations showed that using a low-resolution size bin representation (four bins) underestimates the efficiency with which aerosols scatter and absorb light (mass extinction efficiency). Besides using finer-resolution size bins (8–16 bins), it was also necessary to increase the refractive indices and hygroscopicity of select aerosol species within the range of values reported in the literature to achieve better consistency with measured values of the mass extinction efficiency (6.7 m2 g−1 observed average) and light-scattering enhancement factor (f(RH)) due to aerosol hygroscopic growth (2.2 observed average). Furthermore, an evaluation of the optical properties obtained using modeled aerosol properties revealed the inability of sectional and modal aerosol representations in WRF-Chem to properly reproduce the observed size distribution, with the models displaying a much wider accumulation mode. Other model deficiencies included an underestimate of organic aerosol density (1.0 g cm−3 in the model vs. observed average of 1.5 g cm−3) and an overprediction of the fractional contribution of submicron inorganic aerosols other than sulfate, ammonium, nitrate, chloride, and sodium corresponding to mostly dust (17 %–28 % modeled vs. 12 % estimated from observations). These results illustrate the complexity of achieving an accurate model representation of optical properties and provide potential solutions that are relevant to multiple disciplines and applications such as air quality forecasts, health impact assessments, climate projections, solar power forecasts, and aerosol data assimilation.

Calculating ambient aerosol surface area concentrations using aerosol light scattering enhancement measurements

Aerosol surface area concentration (SA) is crucial for studying atmospheric chemical reactions happened in aerosol water or on aerosol surface. However, there is no commercial instrument that can provide direct measurements of ambient SA. In this paper, we propose a method to calculate ambient SA based only on measurements of a three-wavelength humidified nephelometer system, which measures aerosol optical properties at three wavelengths under dry state and different relative humidity (RH) conditions. Two critical steps are required in this method: (1) Calculating surface area concentration of ambient aerosols in dry state, SA(dry), using a trained random forest machine learning model based only on optical properties measured by the “dry” nephelometer. The proposed machine learning method is evaluated with particle number size distributions (PNSD) datasets from eight field campaigns conducted on the North China Plain during different seasons. The square of correlation coefficients between predicted and calculated SA(dry) for PNSD is about 0.99, the average ratio between predicted and calculated SA(dry) is 1.01 and 70% of data points has a relative difference less than 10%. (2) Calculating the surface area growth factor fS(RH) of ambient aerosol particles due to water uptake using the proposed fS(RH) parameterization scheme fS(RH)=(1+κSRH100−RH)23, and the hygroscopicity parameter κS can be calculated using measured aerosol light scattering enhancement factor and Ångstro¨m exponent. The ambient SA values during two field campaigns which are conducted on the North China Plain (Wangdu campaign and Gucheng campaign, in summer and winter respectively) are calculated using the proposed method. The ambient SA ranges from 42 to 1871 μm2/cm3 with an average of 319 μm2/cm3 during Wangdu campaign, and ranges from 19 to 4156 μm2/cm3 with an average of 788 μm2/cm3 during Gucheng campaign. Drastic daily variations of ambient SA are observed during these two campaigns. The results demonstrate that aerosol hygroscopic growth impacts significantly on variations in ambient SA especially under high RH conditions. The fS(RH) ranges from near 1 to 4 with an average of 1.4 during Wangdu campaign, and ranges from near 1 to 2.7 with an average of 1.3 during Gucheng campaign. The results indicate that the larger RH, the more sensitive fS(RH) becomes to variations in κS which highlights that real-time measurements of aerosol hygroscopicity are required for accurate calculations of ambient SA. The advantage of the proposed method is that the ambient SA can be obtained solely based on measurements of a three-wavelength humidified nephelometer system, facilitating real-time measurements of ambient SA and promoting studies in aerosol heterogeneous reactions.

Recent Progress of Aerosol Light-scattering Enhancement Factor Studies in China

Assessment of the radiative forcing of aerosols in models still lacks sufficient input data for aerosol hygroscopicity. The light scattering enhancement factor [f(RH, λ)] is a crucial parameter for describing aerosol hygroscopic growth properties. In this paper, we provide a survey of f(RH, λ) studies in China for the past seven years, including instrument developments of humidified nephelometers, ambient f(RH, λ) measurements in China, f(RH, λ) parameterization schemes, and f(RH, λ) applications in aerosol measurements. Comparisons of different f(RH, λ) parameterizations are carried out to check their performance in China using field measurement datasets. We also summary the parameterization schemes for predicting f(RH, λ) with aerosol chemical compositions. The recently developed methods to observe other aerosol properties using f(RH, λ) measurements, such as calculating the aerosol hygroscopicity parameter, cloud condensation nuclei number concentration, aerosol liquid water content, and aerosol asymmetry factor, are introduced. Suggestions for further research on f(RH, λ)in China are given.

A parameterization for the light scattering enhancement factor with aerosol chemical compositions

The light scattering enhancement factor f (RH), defined as the ratio of the light scattering coefficient (σSP) observed under an elevated relative humidity (RH) conditions to that under dry conditions, is a crucial parameter for estimating aerosol direct radiative effects and atmospheric visibility. In this paper, a new f (RH) parameterization scheme considering both the influence of aerosol chemical compositions and that of particle number size distribution (PNSD) is proposed based on in situ measurements in the North China Plain. The development of this parameterization scheme includes three steps. First, aerosol hygroscopicity parameter κ is parameterized with aerosol chemical compositions. Then, the ratio between f (RH) fitted parameter κsca (f(RH)=1+κscaRH100−RH) and κ is introduced to correlate f (RH) with κ. The ratio, influenced mostly by PNSD, is described as a function of scattering Ångström exponent (SAE) because the SAE can represent the predominant size of aerosol particles to some extent. Finally, f (RH) cycle is parameterized with κsca. Validation results show that a good consistency between the parameterized and measured f (RH = 80%) is achieved and the correlation coefficient is 0.80. This parameterization scheme between aerosol chemistry and f (RH) can be used in chemical transport models for reducing uncertainties in estimations of direct aerosol radiative forcing and air quality assessments.

A novel method for calculating ambient aerosol liquid water content based on measurements of a humidified nephelometer system

Abstract. Water condensed on ambient aerosol particles plays significant roles in atmospheric environment, atmospheric chemistry and climate. Before now, no instruments were available for real-time monitoring of ambient aerosol liquid water contents (ALWCs). In this paper, a novel method is proposed to calculate ambient ALWC based on measurements of a three-wavelength humidified nephelometer system, which measures aerosol light scattering coefficients and backscattering coefficients at three wavelengths under dry state and different relative humidity (RH) conditions, providing measurements of light scattering enhancement factor f(RH). The proposed ALWC calculation method includes two steps: the first step is the estimation of the dry state total volume concentration of ambient aerosol particles, Va(dry), with a machine learning method called random forest model based on measurements of the “dry” nephelometer. The estimated Va(dry) agrees well with the measured one. The second step is the estimation of the volume growth factor Vg(RH) of ambient aerosol particles due to water uptake, using f(RH) and the Ångström exponent. The ALWC is calculated from the estimated Va(dry) and Vg(RH). To validate the new method, the ambient ALWC calculated from measurements of the humidified nephelometer system during the Gucheng campaign was compared with ambient ALWC calculated from ISORROPIA thermodynamic model using aerosol chemistry data. A good agreement was achieved, with a slope and intercept of 1.14 and −8.6 µm3 cm−3 (r2 = 0.92), respectively. The advantage of this new method is that the ambient ALWC can be obtained solely based on measurements of a three-wavelength humidified nephelometer system, facilitating the real-time monitoring of the ambient ALWC and promoting the study of aerosol liquid water and its role in atmospheric chemistry, secondary aerosol formation and climate change.

A new method for calculating number concentrations of cloud condensation nuclei based on measurements of a three-wavelength humidified nephelometer system

Abstract. The number concentration of cloud condensation nuclei (CCN) plays a fundamental role in cloud physics. Instrumentations of direct measurements of CCN number concentration (NCCN) based on chamber technology are complex and costly; thus a simple way for measuring NCCN is needed. In this study, a new method for NCCN calculation based on measurements of a three-wavelength humidified nephelometer system is proposed. A three-wavelength humidified nephelometer system can measure the aerosol light-scattering coefficient (σsp) at three wavelengths and the light-scattering enhancement factor (fRH). The Ångström exponent (Å) inferred from σsp at three wavelengths provides information on mean predominate aerosol size, and hygroscopicity parameter (κ) can be calculated from the combination of fRH and Å. Given this, a lookup table that includes σsp, κ and Å is established to predict NCCN. Due to the precondition for the application, this new method is not suitable for externally mixed particles, large particles (e.g., dust and sea salt) or fresh aerosol particles. This method is validated with direct measurements of NCCN using a CCN counter on the North China Plain. Results show that relative deviations between calculated NCCN and measured NCCN are within 30 % and confirm the robustness of this method. This method enables simplerNCCN measurements because the humidified nephelometer system is easily operated and stable. Compared with the method using a CCN counter, another advantage of this newly proposed method is that it can obtain NCCN at lower supersaturations in the ambient atmosphere.

First results from light scattering enhancement factor over central Indian Himalayas during GVAX campaign

The present work examines the influence of relative humidity (RH), physical and optical aerosol properties on the light-scattering enhancement factor [f(RH=85%)] over central Indian Himalayas during the Ganges Valley Aerosol Experiment (GVAX). The aerosol hygroscopic properties were measured by means of DoE/ARM (US Department of Energy, Atmospheric Radiation Measurement) mobile facility focusing on periods with the regular instrumental operation (November–December 2011). The measured optical properties include aerosol light-scattering (σsp) and absorption (σap) coefficients and the intensive parameters i.e., single scattering albedo (SSA), scattering Ångström exponent (SAE), absorption Ångström exponent (AAE) and light scattering enhancement factor (f(RH)=σsp(RH, λ)/σsp(RHdry, λ)). The measurements were separated for sub-micron (<1μm, D1μm) and particles with diameter<10μm (D10μm) in order to examine the influence of particle size on f(RH) and enhancement rate (γ). The particle size affects the aerosol hygroscopicity since mean f(RH=85%) of 1.27±0.12 and 1.32±0.14 are found for D10μm and D1μm, respectively. These f(RH) values are relatively low suggesting the enhanced presence of soot and carbonaceous particles from biomass burning activities, which is verified via backward air-mass trajectories. Similarly, the light-scattering enhancement rates are ~0.20 and 0.17 for the D1μm and D10μm particles, respectively. However, a general tendency for increasing f(RH) and γ is shown for higher σsp and σap values indicating the presence of rather aged smoke plumes, coated with industrial aerosols over northern India, with mean SSA, SAE and AAE values of 0.92, 1.00 and 1.15 respectively. On the other hand, a moderate-to-small dependence of f(RH) and γ on SAE, AAE, and SSA was observed for both particle sizes. Furthermore, f(RH) exhibits an increasing tendency with the number of cloud condensation nuclei (NCCN) indicating larger particle hygroscopicity but without significant dependence on the activation ratio.

A novel method for deriving the aerosol hygroscopicity parameter based only on measurements from a humidified nephelometer system

Abstract. Aerosol hygroscopicity is crucial for understanding roles of aerosol particles in atmospheric chemistry and aerosol climate effects. Light-scattering enhancement factor f(RH, λ) is one of the parameters describing aerosol hygroscopicity, which is defined as f(RH, λ) = σsp(RH, λ)∕σsp(dry, λ), where σsp(RH, λ) or σsp(dry, λ) represents σsp at wavelength λ under certain relative humidity (RH) or dry conditions. Traditionally, an overall hygroscopicity parameter κ can be retrieved from measured f(RH, λ), hereinafter referred to as κf(RH), by combining concurrently measured particle number size distribution (PNSD) and mass concentration of black carbon. In this paper, a new method is proposed to directly derive κf(RH) based only on measurements from a three-wavelength humidified nephelometer system. The advantage of this newly proposed approach is that κf(RH) can be estimated without any additional information about PNSD and black carbon. This method is verified with measurements from two different field campaigns. Values of κf(RH) estimated from this new method agree very well with those retrieved by using the traditional method: all points lie near the 1 : 1 line and the square of correlation coefficient between them is 0.99. The verification results demonstrate that this newly proposed method of deriving κf(RH) is applicable at different sites and in seasons of the North China Plain and might also be applicable in other regions around the world.

Impact of relative humidity on visibility degradation during a haze event: A case study

Light scattering of aerosols depends on ambient relative humidity (RH) since hygroscopic particles absorb significant water at high RH, and this results in low visibility. This paper used custom-made parallel nephelometers (PNEPs) to measure aerosol light scattering enhancement factor ƒ(RH), and utilized data including visibility, PM2.5, black carbon, water-soluble ions mass concentrations and surface meteorological parameters, in conjunction with background weather conditions, to analyze a haze event in Guangzhou during 8th–15th Dec. 2013. Unfavorable weather conditions, such as high RH and low wind speed, were observed during the haze event. The hourly average mass concentration of PM2.5 was 127μg/m3, with concentration of 192.4μg/m3 on 9th and 196μg/m3 on 13th. The ƒ(RH) did not exhibit significant changes during this haze process, with value of ƒ(80%)=1.58±0.07. Although the mass fraction of water-soluble ions to PM2.5 decreased after 12th Dec., the aerosol hygroscopicity might not have changed significantly since the mass fraction of nitrate became more dominant, which has stronger ability to take up water. The best-fitted parameterized function for ƒ(RH) is ƒ(RH)=0.731+0.1375∗(1–RH/100)−1+0.00719∗(1–RH/100)−2. Combining the fixed parameterization of ƒ(RH) above, the visibility was calculated with the measured light scattering and absorption coefficient of particles and gas under dry condition, as well as ambient RH. The predicted visibility range agrees well with the measurements without precipitation. Using ISORROPIA II model, the calculated aerosol liquid water content (ALWC) at ambient RH varied consistently with the PM2.5 under lower RH, while it was more influenced by high RH. This work also show that high RH accompanied with precipitation will enhance aerosol hygroscopic growth effect, leading to further visibility degradation, even if PM2.5 mass decreased due to precipitation.

Optical and hygroscopic properties of long‐range transported haze plumes observed at Deokjeok Island off the west coast of the Korean Peninsula under the Asian continental outflows

AbstractAn intensive field campaign was conducted on Deokjeok Island off the west coast of the Korean Peninsula during spring 2009 to characterize the optical and hygroscopic properties of Asian continental outflows. A slightly high wavelength dependence of light absorption coefficient, α of 1.6 ± 0.05 (average ± 1· standard deviation), and a low humidity‐dependent light scattering enhancement factor at 80% relative humidity, f(80%) (2.0 ± 0.2), were obtained when air masses originated from the northern part of China (N China), compared to those obtained when air masses originated from the eastern part of China (E China) (α = 1.4 ± 0.1; f(80%) = 2.4 ± 0.2). The relatively high α and low f(80%) during the N China compared to those during the E China were consistent with a relatively high mass ratio of organic aerosol to sum of SO42−, NO3−, and NH4+ during the N China (1.01 ± 0.17) compared to the E China episode (0.25 ± 0.13). This result indicates the importance of organic aerosol on aerosol optical and hygroscopic properties of haze plumes. Single scattering albedo (SSA) of dry particulate matter with a diameter less than or equal to 2.5 µm (PM2.5) (0.92 ± 0.01) and mass scattering efficiency (MSE) of dry PM2.5 at 550 nm wavelength during the E China episode (3.6 ± 0.3 m2 g−1) were higher than those previously obtained at the air mass source regions in China (SSA = ~0.8; MSE = ~3.0 m2 g−1), implying that optical properties of PM2.5 were significantly altered during long‐range atmospheric transport.

Aerosol hygroscopicity parameter derived from the light scattering enhancement factor measurements in the North China Plain

Abstract. The relative humidity (RH) dependence of aerosol light scattering is an essential parameter for accurate estimation of the direct radiative forcing induced by aerosol particles. Because of insufficient information on aerosol hygroscopicity in climate models, a more detailed parameterization of hygroscopic growth factors and resulting optical properties with respect to location, time, sources, aerosol chemistry and meteorology are urgently required. In this paper, a retrieval method to calculate the aerosol hygroscopicity parameter, κ, is proposed based on the in situ measured aerosol light scattering enhancement factor, namely f(RH), and particle number size distribution (PNSD) obtained from the HaChi (Haze in China) campaign. Measurements show that f(RH) increases sharply with increasing RH, and that the time variance of f(RH) is much greater at higher RH. A sensitivity analysis reveals that the f(RH) is more sensitive to the aerosol hygroscopicity than PNSD. f(RH) for polluted cases is distinctly higher than that for clean periods at a specific RH. The derived equivalent κ, combined with the PNSD measurements, is applied in the prediction of the cloud condensation nuclei (CCN) number concentration. The predicted CCN number concentration with the derived equivalent κ agrees well with the measured ones, especially at high supersaturations. The proposed calculation algorithm of κ with the f(RH) measurements is demonstrated to be reasonable and can be widely applied.

Tracking sources of severe haze episodes and their physicochemical and hygroscopic properties under Asian continental outflow: Long-range transport pollution, postharvest biomass burning, and Asian dust

[1] Aerosol physicochemical and hygroscopic properties were measured from 12 October to 21 November 2005 at a downwind area of the Asian continental outflow (Gwangju, Korea) to characterize severe haze episodes. Using optically measured elemental carbon (EC) at 660 nm (Opt.EC) and 880 nm (BC) wavelengths and Mie theory, it was estimated that the higher BC/Opt.EC ratio during the cloudy day of the long-range transport (LTP) period was mainly due to EC particle growth from in-cloud processing with secondary aerosols such as sulfate and organic aerosols. Single scattering albedo (SSA) of biomass burning (BB) aerosol increased sharply from 0.89 to 0.94 under a relative humidity >70%, suggesting that organic aerosols emitted from rice straw burning contained high amounts of hydrophilic compounds. The contribution of aerosol water content to the total light extinction coefficient (bext) was determined as 51.4% and 68.4% during the BB and BB + LTP periods, respectively, indicating that the haze episodes were highly enhanced by an increase in aerosol water content. The Asian dust event was characterized by the highest SSA (0.92 ± 0.02), the lowest mass scattering efficiency of fine particles (2.5 ± 1.0 m2 g−1), and the lowest hygroscopic nature (humidity-dependent light scattering enhancement factor, f(80%), which is defined by the ratio of light scattering coefficient at 80% relative humidity to that at dry condition, = ∼1.37). Based on the Angstrom exponent (α) values observed at the source region of the Asian continent and the downwind area of South Korea during the BB + LTP period, it was found that the α value of urban aerosols decreased ∼11% for 1–2 days of the transport, probably due to the increase in particle size through water uptake. Increasing rates of surface PM10 mass concentrations at western coastal areas of the South Korean peninsula were in the range 2.4–14.4 μgm−3 h−1 at the beginning of the BB + LTP period (24 October 2005, 0700–2300 LT). Based on in situ and remote measurement techniques, Asian aerosol outflow over the South Korean peninsula is of the order of 388–3789 tons h−1 at the beginning of the LTP event.