Mean Single Scattering Albedo Research Articles

Impact of long-range transport on black carbon source contribution and optical aerosol properties in two urban environments

Urban areas, as major sources of aerosol black carbon emissions, contribute to increased pollution levels in surrounding regions by air mass long-range transport, which should be taken into account in implementation of emission-reduction strategies. Properties of light-absorbing aerosol particles and a novel approach to assess the impact of long-range transport on black carbon (BC) pollution in two under-investigated urban environments: Warsaw (Poland, Central Europe) and Vilnius (Lithuania, North-Eastern Europe) are presented. During the warm season of May–August 2022, BC mass concentration and aerosol optical properties: the scattering Ångström exponent (SAE), absorption Ångström exponent (AAE), and single scattering albedo (SSA) were investigated. Generally, the mean BC mass concentration was higher at the more polluted site in Warsaw (1.07 μg/m3) than in Vilnius (0.77 μg/m3). The BC source apportionment to biomass burning (BCBB) and fossil fuel combustion (BCFF) showed similar contributions for both sites with BCBB (13–19%) being significantly lower than BCFF (81–87%). A uniform flow of air masses transporting aerosol particles over long distances to both sites was observed for 42% of the days. It affected BC mass concentration as follows: BC decrease was found similar at both sites (42% in Warsaw, 50% in Vilnius) but increase was twice higher in Vilnius (64%) than in Warsaw (30%). Despite variations in BC mass concentration, both sites exhibited a comparable abundance (90%) of submicron (SAE<1.3), BC-dominated (AAE<1.5) particles. The mean SSA was very low (0.69 ± 0.1 in Warsaw, 0.72 ± 0.1 in Vilnius), which indicates a very strong contribution of light-absorbing aerosol particles in both environments. The local episodes of biomass burning due to celebrations of May Days on 1st – 3rd May in Warsaw and Midsummer on 24th June in Vilnius showed similar aerosol properties in both cities (1.5<AAE<1.7, 1.7<SAE<2.2) but were highly different than any other during the entire campaign.

Combining POLDER-3 satellite observations and WRF-Chem numerical simulations to derive biomass burning aerosol properties over the southeast Atlantic region

Abstract. Aerosol absorption is a key property to assess the radiative impacts of aerosols on climate at both global and regional scales. The aerosol physico-chemical and optical properties remain not sufficiently constrained in climate models, with difficulties to properly represent both the aerosol load and their absorption properties in clear and cloudy scenes, especially for absorbing biomass burning aerosols (BBA). In this study we focus on biomass burning (BB) particle plumes transported above clouds over the southeast Atlantic (SEA) region off the southwest coast of Africa, in order to improve the representation of their physico-chemical and absorption properties. The methodology is based on aerosol regional numerical simulations from the WRF-Chem coupled meteorology–chemistry model combined with a detailed inventory of BB emissions and various sets of innovative aerosol remote sensing observations, both in clear and cloudy skies from the POLDER-3/PARASOL space sensor. Current literature indicates that some organic aerosol compounds (OC), called brown carbon (BrOC), primarily emitted by biomass combustion absorb the ultraviolet-blue radiation more efficiently than pure black carbon (BC). We exploit this specificity by comparing the spectral dependence of the aerosol single scattering albedo (SSA) derived from the POLDER-3 satellite observations in the 443–1020 nm wavelength range with the SSA simulated for different proportions of BC, OC and BrOC at the source level, considering the homogeneous internal mixing state of particles. These numerical simulation experiments are based on two main constraints: maintaining a realistic aerosol optical depth both in clear and above cloudy scenes and a realistic BC/OC mass ratio. Modelling experiments are presented and discussed to link the chemical composition with the absorption properties of BBA and to provide estimates of the relative proportions of black, organic and brown carbon in the African BBA plumes transported over the SEA region for July 2008. The absorbing fraction of organic aerosols in the BBA plumes, i.e. BrOC, is estimated at 2 % to 3 %. The simulated mean SSA are 0.81 (565 nm) and 0.84 (550 nm) in clear and above cloudy scenes respectively, in good agreement with those retrieved by POLDER-3 (0.85±0.05 at 565 nm in clear sky and at 550 nm above clouds) for the studied period.

Impact of Shutdown due to COVID-19 Pandemic on Aerosol Characteristics in Kanpur, India.

Background.Since March 2020, the number of confirmed COVID-19 positive cases have steadily risen in India. Various preventive measures have been taken to contain the spread of COVID-19. With restrictions on human activities, anthropogenic emissions driving air pollution levels have seen a reduction since March 23, 2020, when the government imposed the first nationwide shutdown. The landlocked Indo-Gangetic Plain (IGP) has many densely-populated cities, witnessing high levels of particulate matter due to both nature-driven and anthropogenic elements. Kanpur is an urban metropolis in the IGP with high aerosol loading, and this paper explores the impact of restricted anthropogenic activities on aerosol characteristics in Kanpur.Objectives.This study aims to investigate the change in aerosol optical depth level and its related parameters during the shutdown phases in Kanpur city compared to the same time periods in 2017–2019.Methods.Aerosol optical properties such as aerosol optical depth (AOD) at 500 nm, Angstrom exponent (AE), fine mode fraction (FMF) of AOD at 500 nm and single scattering albedo (SSA) at 440 nm were obtained from the Aerosol Robotic Network (AERONET) station operating in Kanpur from the 1st March to the 30th April for 2017–2020.Results.A significant decrease in aerosol loading was observed during the shutdown period compared to the pre-and partial shutdown periods in 2020 as well as during the same time periods of 2017–2019. Mean AOD, FMF and SSA were 0.37, 0.43 and 0.89, respectively, during the shutdown period in 2020. A 20–35% reduction in mean AOD levels was observed during the shutdown period in 2020 as compared to the same period in 2017–2019.Conclusions.The shutdown led to an improvement in air quality due to decreases in anthropogenic emissions. As fine particles, typically from urban and industrial emissions, dominate episodic air pollution events, this study can be further utilized by the scientific community and regulators to strengthen the emergency response action plan to check high pollution episodes in Kanpur city until cleaner technologies are in place.Competing Interests.The authors declare no completing financial interests.

Climatological aspects of size-resolved column aerosol optical properties over a rural site in the southern peninsular India

We present the long-term (2008–2018) climatological characteristics of column aerosol optical properties from the Sky Radiometer at Gadanki (13.48∘ N, 79.18∘ E), a typical rural site in southern peninsular India. A method to separate the fine and coarse mode spectral aerosol optical properties was presented. The climatological aspects related to the diurnal, monthly, and intra-annual variations of observed aerosol optical properties exhibited both seasonal and spectral dependence. While the size-resolved aerosol optical depth (AOD), absorption aerosol optical depth, and asymmetry parameter exhibited a decreasing trend, single scattering albedo (SSA) showed an increasing trend from shorter to longer wavelengths in all seasons with distinct magnitudes. Seasonal asymmetry in the diurnal variation of AOD and Angstrom exponent was observed. Intra-annual variability clearly indicated distinct fine and coarse mode dominance during winter and monsoon, respectively, while mixed contributions were found in other seasons. This is attributed to the influence of prevailing air masses on the column aerosol properties. Annual mean SSA values indicated the dominance of moderately absorbing (0.91) to weakly absorbing (0.95) aerosols. On evaluating the size-resolved AOD (at 500 nm) with those from extended Spectral Deconvolution Algorithm (SDA+), the observed differences are found to be remarkable for coarse mode AOD although the correlations for both fine and coarse AODs were observed to be 0.96. Lastly, the Dark Target and Deep Blue merged AOD (at 550 nm) from MODIS Terra/Aqua are evaluated to examine the consistency with ground measurements and both have shown a systematic underestimation with an increase in the magnitude of AOD strongly dependent on the seasons. These results form a basis for the comprehensive characterization of aerosol types and associated radiative impacts that are vital for understanding the regional climate change.

Mie Scattering Captures Observed Optical Properties of Ambient Biomass Burning Plumes Assuming Uniform Black, Brown, and Organic Carbon Mixtures

AbstractWe use a simple model of a spherical biomass burning aerosol particle containing an internal mixture of black carbon (BC) and organic carbon (OC) with an effective refractive index calculated as a volume fraction‐weighted mean of refractive indices. Brown carbon (BrC) is considered to be an OC with an imaginary part of refractive index at the blue end of the solar spectrum higher (in absolute value) than at the red end. Mie‐scattering formalism is employed to calculate absorption Ångström exponent (AAE) as a function of single‐scattering albedo (SSA) for a set of BC refractive indices, BC volume fraction, and a set of effective refractive indices of BC and BrC mixtures. Ambient plumes are characterized by their mean SSA at 405 nm wavelength and two‐wavelength (405 and 781 nm) AAE values. Comparing observed and model‐calculated AAE and SSA values identifies the BC refractive index and its volume fraction and an effective refractive index of the BC and BrC mixture. From these values, the imaginary part of BrC refractive index is calculated. For observed southwestern ambient fires, the imaginary part of BrC refractive index is ≤0.016. In contrast, fires in the Amazon and from Africa are dominated by BC. We also use the Mie scattering model to establish the upper limit on AAE of BC (AAE ≤ 1.4) and the upper limit of the BC and OC mixture (AAE ≤ 1.7). Any AAE(405/781) &gt; 1.7 requires the presence of BrC. We derive several relationships between AAE, SSA, BC fractions, and an imaginary part of BrC refractive index of ambient fires.

Inter-comparison of black carbon measurement methods for simulated open biomass burning emissions

Biomass burning (BB) is a major source of black carbon (BC), but comparing BC content of different smoke-impacted air masses may be uncertain if different measurement techniques are used to quantify the BC, or if non-BC fractions influence a given measurement. To investigate these potential issues, five instruments reporting BC were compared in well-mixed smoke during the FIREX laboratory campaign in 2016, including two filter-based absorption instruments; one in situ absorption instrument; a laser-induced incandescence instrument; and a thermal-optical instrument. BB aerosols were generated using fuels common to wildfires in the Western US in a relatively controlled environment, with BC concentrations ranging from roughly 10–100 μg m−3 (55 total fires). Applying the Bland-Altman graphical approach, systematic biases and proportional biases were identified between the selected reference instrument (in situ absorption) and the other four instruments. BC emission factors (EFBC) derived from the thermal-optical instrument, laser-induced incandescence instrument, and filter-based absorption instruments were, on average, 83%, 39% and 66%, greater than the in situ absorption instrument, respectively. To understand why these differences exist, principal component analysis combined with a K-means clustering algorithm was implemented to group different fires into three clusters based on several co-dependent fire-related parameters (modified combustion efficiency (MCE), single scattering albedo (SSA) at 870 nm, organic carbon/elemental carbon ratio (OC/EC ratio), and absorption Ångström exponents (AAE)); clusters are nominally referred to as “Black”, “Mixed”, and “Brown” based on the mean SSA and AAE values for each. The best agreement among all instruments was observed for the “Black” cluster (mean EFBC ratio = 1.89, for the fires with mean SSA = 0.31 and AAE = 1.44); this agreement worsened for the “Mixed” (mean EFBC ratio = 2.94, for the fires with mean SSA = 0.80 and AAE = 1.92) and “Brown” clusters (mean EFBC ratio = 3.12, for the fires with mean SSA = 0.96 and AAE = 2.50), likely due to the increased presence of externally (or internally) mixed aerosols that altered the chemical and optical properties of the aerosols. In general, the discrepancies observed among the BC instruments from this work agree with or slightly exceed the ones from previous ambient and laboratory studies. Care should be taken when interpreting different BC measurements in BB smoke because large artifacts can occur due to co-emitted materials.

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.

Measurement and analysis of surface aerosol optical properties over urban Nanjing in the Chinese Yangtze River Delta

Aerosol optical properties including aerosol scattering coefficient (σsp), absorption coefficient (σap), single scattering albedo (SSA), PM2.5 mass concentration and their relationship with meteorological factors were measured and analyzed from 1st March to 30th April 2011. The observations were conducted at an urban–industrial site in Nanjing located in the Yangtze River Delta (YRD) region, China. The averaged σsp at 550nm is found to be 329.3±321.6Mm−1 and occurred most frequently within the range of 100–200Mm−1. σap at 532nm is measured to be 28.1±17.6Mm−1, which is remarkably smaller than most of the measurements made over megacities in China and nearly equivalent to the observations made at rural sites in the YRD. The mean SSA at 532nm is 0.89±0.08 with the occurrence frequency of higher SSA (>0.9) accounting for about 60%, indicating that scattering-related aerosols are dominant over Nanjing during the study period. A bi-peak diurnal variation of σap and σsp is found with the maximum during late at night and early morning and the minimum in the afternoon, but the diurnal pattern of SSA is just the opposite to this. Significant correlation coefficients are noticed between different aerosol optical properties. σsp and σap showed a clear negative correlation with wind speed, temperature, and visibility. During the study period, the lower northwesterly and southeasterly winds are attributed to an increase in the aerosol optical properties. The σsp and σap increased significantly during severe haze and dust episodes caused by the accumulation of pollutants from both local and regional sources under favorable weather conditions.

Aerosol Optical Properties over Mount Song, a Rural Site in Central China

Seasonal variations of aerosol optical depth (AOD), Angstrom exponent (α), single scattering albedo (SSA), water vapor content (WVC), aerosol size distribution and refractive index at Mount Song, a rural site in Central China are analyzed using ground-based sunphotometer data for the period April 2012 to May 2014 for the first time. The results show that the area is highly polluted even the major anthropogenic emission sources are far away. Seasonal mean AOD is high (0.72 ± 0.52) during summer (June–August) season and low (0.51 ± 0.38) during autumn (September–November) season. The monthly mean α is very low (0.81 ± 0.30) in the month of April and very high (1.32 ± 0.23) in the month of September with annual average value 1.1. Analysis of the frequency distributions of AOD and α in each season indicates presence of fine-mode particles. Strong seasonal variations in SSA is likely due to local biomass burning and regional transport of anthropogenic aerosol particles, Seasonal mean SSA at 440 nm wavelength are 0.89 ± 0.04, 0.91 ± 0.06, 0.89 ± 0.07 and 0.92 ± 0.05, respectively during spring, summer, autumn and winter seasons. It is also shown that the scattering capacity of the fine-mode particles is relatively higher during summer compared to other seasons. The aerosol volume size distributions show pronounced seasonal variations in volume concentration, peak radius and the fine-mode particles are evidently dominant during summer season due to the hygroscopic growth. A distinct seasonal variations in aerosol parameters confirm the transport of polluted air mass at Mount Song.

Impacts of aerosol chemical compositions on optical properties in urban Beijing, China

The optical properties of aerosols and their chemical composition, including water-soluble ions, organic carbon (OC), and elemental carbon (EC) in PM2.5 and PM10, were measured from 26 May to 30 June of 2012 at an urban site in Beijing. The daily average concentrations of PM2.5 and PM10 were 103.2 and 159.6μg/m3, respectively. On average, the OC and EC contributed 20.1% and 4.3%, respectively, to PM2.5 and 16.3% and 3.9%, respectively, to PM10. Secondary ions (SO42−, NO3−, and NH4+) dominated the water-soluble ions and accounted for 57.9% and 62.6% of PM2.5 and PM10, respectively. The wind dependence of PM2.5, OC, SO42−, and NO3− implied that the pollution sources mainly came from south and southeast of Beijing during the summer. The monthly mean values of the scattering coefficient (σsc) and absorption coefficient (σab) at 525nm were 312.9 and 28.7Mm−1, respectively, and the mean single-scattering albedo (ω) was 0.85. The wind dependence of σsc revealed that this value was mainly influenced by regional transport during the summer, and the relationship between σab and wind indicated that a high σab resulted from the joint effects of local emissions and regional transport. The reconstructed σsc that was derived from the revised IMPROVE equation agreed well with the observations. The contribution of different chemical species to σsc was investigated under different pollution levels, and it was found that secondary inorganic aerosols accounted for a large part of σsc during pollution episodes (35.7%), while organic matter was the main contributor to σsc under clean conditions (33.6%).

Intercomparison of aerosol single-scattering albedo derived from AERONET surface radiometers and LARGE in situ aircraft profiles during the 2011 DRAGON-MD and DISCOVER-AQ experiments

Single-scattering albedo (SSA) retrievals obtained with CIMEL Sun-sky radiometers from the Aerosol Robotic Network (AERONET) aerosol monitoring network were used to make comparisons with simultaneous in situ sampling from aircraft profiles carried out by the NASA Langley Aerosol Group Experiment (LARGE) team in the summer of 2011 during the coincident DRAGON-MD (Distributed Regional Aerosol Gridded Observational Network-Maryland) and DISCOVER-AQ (Deriving Information on Surface conditions from Column and Vertically Resolved Observations Relevant to Air Quality) experiments. The single-scattering albedos (interpolated to 550 nm) derived from AERONET measurements for aerosol optical depth (AOD) at 440 nm ≥ 0.4 (mean SSA: 0.979) were on average 0.011 lower than the values derived from the LARGE profile measurements (mean SSA: 0.99). The maximum difference observed was 0.023 with all the observed differences within the combined uncertainty for the stated SSA accuracy (0.03 for AERONET; 0.02 for LARGE). Single-scattering albedo averages were also analyzed for lower aerosol loading conditions (AOD ≥ 0.2) and a dependence on aerosol optical depth was noted with significantly lower single-scattering albedos observed for lower AOD in both AERONET and LARGE data sets. Various explanations for the SSA trend were explored based on other retrieval products including volume median radius and imaginary refractive index as well as column water vapor measurements. Additionally, these SSA trends with AOD were evaluated for one of the DRAGON-MD study sites, Goddard Space Flight Center, and two other Mid-Atlantic AERONET sites over the long-term record dating to 1999.

Aerosol radiative forcing controls: Results from an Indian table-top mining region

Aerosol radiative forcing (ARF) over intense mining area in Indian monsoon trough region, computed based on the aerosol optical properties obtained through Prede (POM-1L) sky radiometer and radiative transfer model, are analysed for the year 2011 based on 21 clear sky days spread through seasons. Due to active mining and varied minerals ARF is expected to be significantly modulated by single scattering albedo (SSA). Our studies show that radiative forcing normalized by aerosol optical depth (AOD) is highly correlated with SSA (0.96) while ARF at the surface with AOD by 0.92. Our results indicate that for a given AOD, limits or range of ARF are determined by SSA, hence endorses the need to obtain SSA accurately, preferably derived through observations concurrent with AOD. Noticeably, ARF at the top-of the atmosphere is well connected to SSA (r = 0.77) than AOD (r = 0.6). Relation between observed black carbon and SSA are investigated. A possible over estimation of SSA by the inversion algorithm, SKYRAD.pack 4.2, used in the current study is also discussed. Choice of atmospheric profiles deviating from tropical to mid altitude summer or winter does not appear to be sensitive in ARF calculation by SBDART. Based on the 21 clear sky days, a multiple linear regression equation is obtained for ARFbot as a function of AOD and SSA with a bias of ±2.7 Wm−2. This equation is verified with an independent data set of seasonal mean AOD and SSA to calculate seasonal ARF that compares well with the modeled ARF within ±4 Wm−2.

Aerosol optical properties during firework, biomass burning and dust episodes in Beijing

In order to characterize the aerosol optical properties during different pollution episodes that occurred in Beijing, the aerosol loading, scattering, and size distributions are presented using solar and sky radiance measurements from 2001 to 2010 in this paper. A much higher aerosol loading than the background level was observed during the pollution episodes. The average aerosol optical depth (AOD) is largest during dust episodes coupled with the lowest Ångström exponent (α), while higher AOD and lower α were more correlated with firework and biomass burning days. The total mean AOD at 440, 675, 870 and 1020 nm were 0.24, 0.49, 0.64 and 1.38 in the clean, firework display, biomass burning and dust days, respectively. The mean α for dust days was 0.51 and exceeded 1.1 for the remaining episodes. The size distribution of the dusty periods was dominated by the coarse mode, but the coarse mode was similar magnitude to the fine mode during the firework and biomass burning days. The volume concentration of the coarse mode during the dust days increased by a magnitude of more than 2–8 times that derived in the other three aerosol conditions, suggesting that dust is the major contributor of coarse mode particles in Beijing. The single scattering albedo (SSA) values also increased during the pollution episodes. The overall mean SSA at the four wavelengths were 0.865, 0.911, 0.922 and 0.931 in clean, firework display, biomass burning, and dust days in Beijing, respectively. However, in the blue spectral range, the dust aerosols exhibited pronounced absorption.

South African EUCAARI measurements: seasonal variation of trace gases and aerosol optical properties

Abstract. In this paper we introduce new in situ observations of atmospheric aerosols, especially chemical composition, physical and optical properties, on the eastern brink of the heavily polluted Highveld area in South Africa. During the observation period between 11 February 2009 and 31 January 2011, the mean particle number concentration (size range 10–840 nm) was 6310 cm3 and the estimated volume of sub-10 μm particles 9.3 μm3 m−3. The aerosol absorption and scattering coefficients at 637 nm were 8.3 Mm−1 and 49.5 Mm−1, respectively. The mean single-scattering albedo at 637 nm was 0.84 and the Ångström exponent of scattering was 1.5 over the wavelength range 450–635 nm. The mean O3, SO2, NOx and H2S-concentrations were 37.1, 11.5, 15.1 and 3.2 ppb, respectively. The observed range of concentrations was large and attributed to the seasonal variation of sources and regional meteorological effects, especially the anticyclonic re-circulation and strong winter-time inversions. In a global context, the levels of gases and particulates were typical for continental sites with strong anthropogenic influence, but clearly lower than the most polluted areas of south-eastern Asia. Of all pollutants observed at the site, ozone is the most likely to have adverse environmental effects, as the concentrations were high also during the growing season. The measurements presented here will help to close existing gaps in the ground-based global atmosphere observation system, since very little long-term data of this nature is available for southern Africa.

Optical Characteristics of Southeast Asia's Regional Aerosols and Their Sources

ABSTRACT The dominant optical characteristics of Southeast Asia (SEA)'s regional aerosols were determined from the cluster analysis of the 26 AERONET aerosol inversion products, including aerosol light scattering/absorption indicators and aerosol size/shape parameters retrieved from 2003 to 2007. The data sets were acquired from four stations: Bac Giang in Vietnam and Mukdahan, Pimai, and Silpakorn University in Thailand. The cluster analysis showed agreement among the aerosol optical characteristics, land cover/uses, season as the surrogate of the prevailing winds, and observations from the literature. The results of this study showed that during the northeast prevailing winds from mid-September to December, the high aerosol exposure events were most frequently observed over the upwind station and less often over the downwind stations. This aerosol exhibited a single scattering albedo (SSA) of approximately 0.95 (440 nm), a relatively low refractive index, and a larger fine-mode size, suggesting it had the characteristics of urban/industrial aerosols reported in the literature. These aerosol sources were upwind from Bac Giang, probably in eastern China. From January to April, the aerosol exhibited a lower SSA of approximately 0.90, a higher refractive index, and a smaller fine-mode size, suggesting biomass burning smoke reported in the literature. The SEA urban aerosol exhibited a mean SSA of approximately 0.90 (440 nm) or lower, and the coarse-mode aerosol, possibly road dust or soil dust, played a role from October to January when seasonal winds are strongest. The results from a canonical discriminant function analysis suggest that the dominant SEA aerosol clusters tended to be separated by a canonical function positively correlated with SSA, the fine-mode asymmetry factor, and the overall fine-mode size and negatively correlated with the refractive index. IMPLICATIONS Previous SEA aerosol source contribution studies using traditional mass balance or positive matrix factorization analysis were focused on local urban sources, which may not represent the sources of the regional aerosol. Aerosol optical properties are a low-cost option for evaluating the large-scale atmospheric aerosols and their potential sources. Understanding optical characteristics of aerosol also helps assess its role in global climate, which is useful for air model developments and air pollution management.

The effect of the global UV irradiance measurement accuracy on the single scattering albedo retrieval

Abstract. The possibility of measuring aerosol optical absorption properties in the UV spectral range such as single scattering albedo (SSA), using remote sensing techniques, is currently an open scientific issue. We investigate the limitations on calculating column average SSA using a combination of global UV spectral measurements (that are comon in various UV monitoring stations worldwide) with radiative transfer modeling. To point out the difficulties in such a retrieval we have used the travelling reference spectroradiometer QASUME (Quality Assurance of Spectral Ultraviolet Measurements in Europe) results from 27 visits to UV monitoring stations around Europe. We have used the QASUME instrument as relative reference, analyzing absolute differences and also temporal and spectral deviations of UV irraidances, that are used as basic input for the SSA retrieval. The results comparing the mean SSA derived by all instruments, measuring synchronous UV spectra, showed that 5 were within ± 0.02 difference from the SSA calculated from the QASUME instrument, while 17 were within ± 0.04, for the Solar zenith angle of 60 degrees. As for the uncertainty that has been calculated using the 2σ standard deviation of the spectral measurements, a mean 0.072 and 0.10 (2σ) uncertainties have been calculated for 60° and 30°, respectively. Based on the fact that additional uncertainties would be introduced in the SSA retrieval from AOD model input accuracy, assymetry parameter assumptions, we show that only very few instrumnents could be able to detect long term SSA changes. However, such measurements/results ar useful in order to retrieve SSA at UV wavelengths, a product needed for various applications such as, inputs for modeling radiative forcing studies and satellite retrieval algorithms.

Dust aerosol optical properties retrieval and radiative forcing over northwestern China during the 2008 China‐U.S. joint field experiment

The Atmosphere Radiation Measurements Program's Ancillary Facility (AAF/SMART‐COMMIT) was deployed to Zhangye (39.082°N, 100.276°E), which is located in a semidesert area of northwest China, during the period of late April to mid June in 2008. We selected 11 cases to retrieve dust aerosol optical depth (AOD), Angstrom exponent, size distribution, single‐scattering albedo (SSA) and asymmetry parameter (ASY) from multifilter rotating shadowband radiometer (MFRSR) measurements. These cases are dominated by large particles with Angstrom exponent values ranging from 0.34 to 0.93. The values of AOD at 0.67 μm range from 0.07 to 0.25. The mean SSA value increases with wavelength from 0.76 ± 0.02 at 0.415 μm to 0.86 ± 0.01 at 0.870 μm, while the mean ASY value decreases from 0.74 ± 0.04 to 0.70 ± 0.02. Before estimating dust aerosol direct radiative forcing, a radiative closure experiment was performed to verify that the retrieved aerosol optical properties and other input parameters to the radiative transfer model appropriately represent atmospheric conditions. The daytime‐averaged differences between model simulations and ground observations are −8.5, −2.9, and −2.1 W m−2 for the total, diffuse, and direct normal fluxes, respectively. The mean difference in the instantaneous reflected solar fluxes at the top of atmosphere (TOA) between the model and CERES observations is 8.0 W m−2. The solar aerosol direct radiative forcing (ARF), averaged over a 24 h period, at the surface is −22.4 ± 8.9 W m−2, while the TOA ARF is small and has an average value of only 0.52 ± 1.69 W m−2. The daily averaged surface aerosol radiative forcing efficiency at 0.5 μm is −95.1 ± 10.3 W m−2τ−1. Our results illustrate that the primary role of dust aerosol is to alter the distribution of solar radiation within the climate system rather than to reflect solar energy to space. We assess the satellite aerosol optical depth products from Mutiangle Imaging Spectroradiometer (MISR) and Moderate Resolution Imaging Spectroradiometer (MODIS) observations by comparing them with our ground‐based retrievals. Reasonable agreements with the ground‐based observations are found for the MISR product and MODIS Deep Blue product.

Surface shortwave aerosol radiative forcing during the Atmospheric Radiation Measurement Mobile Facility deployment in Niamey, Niger

The Atmospheric Radiation Measurement (ARM) Program's Mobile Facility (AMF) was deployed to Niamey, Niger, during 2006. Niamey, which is located in sub‐Saharan Africa, is affected by both dust and biomass burning emissions. Column aerosol optical properties were derived from multifilter rotating shadowband radiometer, measurements and the vertical distribution of aerosol extinction was derived from a micropulse lidar during the two observed dry seasons (January–April and October–December). Mean aerosol optical depth (AOD) and single scattering albedo (SSA) at 500 nm during January–April were 0.53 ± 0.4 and 0.94 ± 0.05, while during October–December mean AOD and SSA were 0.33 ± 0.25 and 0.99 ± 0.01. Aerosol extinction profiles peaked near 500 m during the January–April period and near 100 m during the October–December period. Broadband shortwave surface fluxes and heating rate profiles were calculated using retrieved aerosol properties. Comparisons for noncloudy periods indicated that the remote sensing retrievals provided a reasonable estimation of the aerosol optical properties, with mean differences between calculated and observed fluxes of less than 5 W m−2 and RMS differences less than 25 W m−2. Sensitivity tests showed that the observed fluxes could be matched with variations of &lt;10% in the inputs to the radiative transfer model. The calculated 24‐h averaged SW instantaneous surface aerosol radiative forcing (ARF) was −21.1 ± 14.3 W m−2 and was estimated to account for 80% of the total radiative forcing at the surface. The ARF was larger during January–April (−28.5 ± 13.5 W m−2) than October–December (−11.9 ± 8.9 W m−2).

The measurement of aerosol optical properties at a rural site in Northern China

Abstract. Atmospheric aerosols constitute one of the largest sources of uncertainty in the estimation of radiative forcing for climate. From April 2003 to January 2005, in situ measurements of aerosol optical properties were conducted at a rural site in Northern China, Shangdianzi Global Atmosphere Watch (GAW) regional station (SDZ), about 150 km from Beijing. Mean values (standard deviation, S.D.) of scattering and absorption coefficients for the entire period are 174.6 Mm−1 (189.1 Mm-1) and 17.5 Mm−1 (13.4 Mm-1), respectively. These values are approximately one third of the reported values for scattering coefficients and one fifth of those for absorption coefficients obtained in the Beijing urban area. The mean single scattering albedo (SSA) for the entire period was estimated as 0.88 (0.05), which is about 0.07 higher than the values reported for the Beijing urban area, and also higher than the values (0.85) used in a reported climate simulation for China and India. Both the absorption and scattering coefficients showed a seasonal cycle with the lowest values in winter, while the highest values occurred in summer for absorption coefficients and in fall for scattering coefficients. The mean SSA values were lowest in spring and highest in winter. The daily variations of aerosol absorption and scattering coefficients were strongly influenced by synoptic changes throughout the observation period. A trajectory cluster analysis was applied to discern the source characteristics of aerosol optical properties for different air masses. The cluster-mean aerosol scattering coefficients, absorption coefficients and SSA were all high when the air masses moved from SW and SE-E directions to the site and aerosols were influenced with heavy pollution from the dense population centers and industrial areas. The cluster-mean SSA for air masses coming from the polluted areas was not only higher than those with trajectories from the "clean" directions, but also higher than the reported values for the regions with high pollution emissions (such as the Beijing urban area). This fact might reflect the substantial secondary aerosol production during transport. The characteristics of aerosol optical properties measured at this rural site suggest significant impacts of human activities on the regional aerosol.

Absorption properties of urban/suburban aerosols in China

The broadband diffuse radiation method is improved to retrieve the aerosol refractive index imaginary part (AIP) and broadband (400–1000 nm mean) single scattering albedo (SSA). In this method, four sets of SSA selection criteria are proposed for quality control. The method is used to retrieve AIP, SSA and absorptive optical thickness (AbOT) from routine bourly-exposed pyrheliometer and paranometer measurements over 11 sites (meteorological observatories) in China during 1998–2003. Apart from one suburban site (Ejin Qi), the other urban sites are all located around big or medium cities. As shown in the retrieval results, annual mean SSA during 1998–2003 changes from 0.941 (Wuhan) to 0.849 (Lanzhou), and AIP from 0.0054 to 0.0203. The 11-site average annual mean SSA and AIP are 0.898 and 0.0119, respectively. SSA during winter is smaller for most sites. There is an evidently positive correlation between SSA and aerosol optical thickness (AOT) for all sites. There is also a positive correlation between SSA and relative humidity for most sites, but a negative correlation for a few sites, such as Kashi and Urumqi in Northwest China.