Fine Mode Volume Fraction Research Articles

Variations in major aerosol components from long-term measurement of columnar aerosol optical properties at a SKYNET site downwind of Seoul, Korea

We investigated the variations in major aerosol components using optical properties from a SKYNET site downwind of metropolitan Seoul under prevailing westerlies. The study period was March 2007 through February 2017 during which numerous measures to reduce air pollutants were implemented in both Korea and China. Major aerosol components, including mineral dust (MD), secondary inorganic ions (SII), and carbonaceous materials were estimated from optical properties, such as fine mode volume fraction (FMVF), single scattering albedo (SSA), and the difference of absorption aerosol optical depth (dAAOD) between 400 and 870 nm. Monthly variations in aerosol characteristics estimated from the optical properties were mostly consistent with the previous studies based on in-situ measurement data. As many studies reported the reduction of particulate matter (PM) in Northeast Asia, AOD also decreased along with the reductions of MD and SII. Secondary formation during the long-range transport played an important role in elevating AOD. However, for the specific case of high FMVF, high SSA, or low dAAOD, potential source regions of aerosols tended to be distributed in the eastern Korean Peninsula and over the East Sea, as the effects of local emissions became larger than those of long-range transport. Although AOD variations successfully revealed the overall characteristics of PM on the whole, in some cases, the variations of optical properties were not clearly related to those of aerosol components.

Characteristics of columnar aerosol optical and microphysical properties retrieved from the sun photometer and its impact on radiative forcing over Skukuza (South Africa) during 1999-2010.

The detailed analysis of columnar optical and microphysical properties of aerosols obtained from the AErosol RObotic NETwork (AERONET) Cimel sun photometer operated at Skukuza (24.98° S, 31.60° E, 150m above sea level), South Africa was carried out using the level 2.0 direct sun and inversion products measured during 1999-2010. The observed aerosol optical depth (AOD) was generally low over the region, with high values noted in late winter (August) and mid-spring (September and October) seasons. The major aerosol types found during the study period were made of 3.74, 69.63, 9.34, 8.83, and 8.41% for polluted dust (PD), polluted continental (PC), non-absorbing (NA), slightly absorbing (SA), and moderately absorbing (MA) aerosols, respectively. Much attention was given to the aerosol fine- and coarse-modes deduced from the particle volume concentration, effective radius, and fine-mode volume fraction. The aerosol volume size distribution pattern was found to be bimodal with the fine-mode showing predominance relative to coarse-mode during the winter and spring seasons, owing to the onset of the biomass burning season. The mean values of total, fine-, and coarse-mode volume particle concentrations were 0.07±0.04, 0.03±0.03, and 0.04±0.02μm3μm-2, respectively, whereas the mean respective effective radii observed at Skukuza for the abovementioned modes were 0.35±0.17, 0.14±0.02, and 2.08±0.02μm. The averaged shortwave direct aerosol radiative forcing (ARF) observed within the atmosphere was found to be positive (absorption or heating effect), whereas the negative forcing in the surface and TOA depicted significant cooling effect due to more scattering type particles.

Aerosol properties of mineral dust and its mixtures in a regional background of north-central Iberian Peninsula

To broaden the knowledge about desert dust (DD) aerosols in western Mediterranean Basin, their fingerprints on optical and microphysical properties are analyzed during DD episodes in the north-central plateau of the Iberian Peninsula between 2003 and 2014. Aerosol columnar properties obtained from the AErosol RObotic NETwork (AERONET), such as aerosol optical depth (AOD), Ångström exponent (AE), volume particle size distribution, volume concentration (VC), sphericity, single scattering albedo, among others, are analyzed in order to provide a general characterization, being some of them compared to particle mass surface concentrations PM10, PM2.5, and their ratio, data obtained from EMEP network. The mean intensity of DD episodes exhibits: AOD440nm=0.27±0.12, PM10=24±18μg/m3, AE=0.94±0.40 and PM2.5/PM10=0.54±0.16. The AOD and PM10 annual cycles show maximum intensity in March and summer and minima in winter. A customized threshold of AE=1 distinguishes two types of dusty days, those with a prevailing desert character and those of mixed type, which is corroborated by sphericity values. Three well established intervals are obtained with the fine mode volume fraction (VCF/VCT). Coarse-mode-dominated cases (VCF/VCT≤0.2) present a mineral dust character: e.g., particle maximum concentration about 2μm, non-sphericity, stronger absorption power at shorter wavelengths, among others. The relevance of the fine mode is noticeable in mixtures with a predominance of particles about 0.2–0.3μm radii. Conditions characterized by 0.2<VCF/VCT<0.45 and VCF/VCT≥0.45 present a larger variability in all investigated aerosol properties. Relationships between AOD and columnar particle volume concentration give volume extinction efficiencies between 1.7 and 3.7μm2/μm3 depending on VCF/VCT. Aerosol scale height is obtained from relationships between surface and columnar concentrations displaying very large values up to 10km. The uncertainty associated with the transformation between AOD and PM10 can be partially reduced when the aerosol microphysical properties are known.

Identification of columnar aerosol types under high aerosol optical depth conditions for a single AERONET site in Korea

AbstractDominant aerosol types were classified using level 2 inversion products for the Anmyon Aerosol Robotic Network (AERONET) site in Korea for the period 1999–2007. The aerosol types were mineral dust (MD), MD mixed with carbon, and black carbon mixed coarse particles (BCCP) for coarse mode aerosols, black carbon (BC), organic carbon (OC), and secondary inorganic ions (SII) for fine mode aerosols, and mixed particles between. The classification was carried out using a clustering method based on parameters, including single scattering albedo (SSA), absorption Angstrom exponent (AAE), and fine mode volume fraction (FMVF). Among the seven aerosol types, MD was distinct, with the highest AAE and a very low FMVF and SII with the highest SSA and FMVF. BCCP was introduced to designate coarse particles mixed with BC, of which the AAE was lower than 1, despite a low FMVF. In addition to a large difference in AAE between BC and OC, the SSA of OC was larger than that of BC, indicating the effects of the white smoke produced from the smoldering phase of biomass burning. Monthly variations of the aerosol types were well interpreted by meteorology and emissions and coincided with those in the previous studies. Applying our results to well‐characterized global AERONET sites, we confirmed that the aerosol types at Anmyon were valid at other sites. However, the results also showed that the mean properties for aerosol types were influenced by the specific aerosols prevalent at the study sites.

Comparison of aerosol optical properties at the sub-arctic stations ALOMAR-Andenes, Abisko and Sodankylä in late spring and summer 2007

Aerosol concentration and aerosol type, retrieved from observations with CIMEL sun-photometers at three sub-arctic locations at the Scandinavian Peninsula are presented. The observations were made at ALOMAR-Andenes in Norway, Abisko in Sweden and Sodankylä in Finland. This field campaign took place in late spring and summer 2007 as part of the activities of the International Polar Year (IPY) within the POLARCAT project at ALOMAR and Abisko. Aerosol properties were characterized using the relationship between the aerosol optical depth and the Ångström Exponent. The characteristics of the predominant aerosol type and microphysics are largely determined by the location of the site (continental or coastal). During summer the fine mode particles dominate, as indicated by the fine mode volume fraction and the Ångström Exponent. The aerosol concentration was on average very low, except during an event in which long-range transported aerosols (dust and pollution) were detected.

Column-integrated aerosol microphysical properties from AERONET Sun photometer over southwestern Spain

Abstract. The aim of the present work is to carry out a detailed analysis of columnar microphysical properties obtained from Cimel sun-photometer measurements in the Southwest of Spain within the frame of the Aerosol Robotic Network (AERONET) – Iberian Network for aerosol measurements (RIMA). AERONET level 2 inversion products are analysed, in particular the particle size distribution together with their associated microphysical parameters for both fine and coarse modes: volume concentration, effective radius and the fine mode volume fraction. This work complements previous works based on aerosol optical depth (AOD) and the Ångström exponent (AE) for a global characterization of atmospheric aerosol in this area of southwestern Spain. The analysed dataset spans between February 2000 and October 2008. Time series and statistical analysis has been carried out for these parameters in order to assess their typical values and seasonality together with their relationships with the AOD and AE. Mean values of volume particle concentration are 0.06±0.07 μm3 μm−2 for total, 0.019±0.015 μm3 μm−2 for fine and 0.04±0.06 μm3 μm−2 for coarse mode; mean effective radii are 0.40±0.19 μm for total, 0.14±0.02 μm for fine and 1.96±0.41 μm for coarse mode. The most relevant features are the clear bimodality of the volume particle size distribution, with a slight dominance of the coarse mode in the overall climatology given the prevailing atmospheric conditions at the site (coastal marine). There is a clear prevalence of the coarse mode in summer months plus September and March, in coincidence with the occurrence of desert dust intrusions and highest AOD values. During desert dust outbreaks, the particle size distribution is practically monomodal with strong prevalence of the coarse mode which also shows a shift of the modal radius toward lower values. The size particle predominance defines the characteristic of the site and it has been analysed under two different parameters: the Ångström exponent and the fine mode volume fraction Vf/Vt. We investigated the relationship between them and also their relationship with the effective radius of the size distribution.

Aerosol characterization at the sub‐Arctic site Andenes (69°N, 16°E), by the analysis of columnar optical properties

AbstractSince 2002 sun photometer measurements were carried out at Andenes (Norway), in the ALOMAR observatory, to investigate the aerosol optical properties in the European sub‐Arctic region. From 2002 to 2005 measurements were performed during summer campaigns. Since 2006 a Cimel sun photometer has been permanently deployed at the site. The instrument is part of the Red Ibérica de medida de Aerosoles Atmosféricos aerosol robotic network (RIMA–AERONET). The aerosol optical depth (AOD) and the Ångström exponent (AE) were analysed to investigate the aerosol content, type and seasonality in this sub‐Arctic location. Back trajectories were used to provide information about the air‐mass origin, especially for cases of moderate turbidity produced by long‐range transported aerosols from mid‐latitudes. The AOD was in general very low, with mean AOD units of 0.10 ± 0.05 and characteristics of clean continental or marine type aerosols (AE = 1.2 ± 0.4). The lower mean monthly values were obtained in February (0.04) and November (0.06), and the maximum was found in May (0.12). Episodes of long‐range transported aerosol occurred at any time when observations were made, with the highest frequency in May, and originated in central and eastern Europe. The associated air masses transported anthropogenic pollution, biomass burning aerosols and in some cases also Saharan dust. A characterization of microphysical properties was performed, showing that the fine mode dominated the particle size distribution, with an average fine mode volume fraction of 0.69. In Arctic regions, enhanced levels of aerosol concentrations occur frequently in late winter and spring due Arctic haze. In our study such hazy periods were not observed at Andenes. Copyright © 2011 Royal Meteorological Society

Study of columnar aerosol size distribution in Hong Kong

Abstract. This paper presents studies on columnar aerosol optical properties in Hong Kong with focus on aerosol volume size distribution, which helps understand local aerosol properties, variation, hygroscopic growth and coagulation. Long-term ground measurements in the wet season in the years of 2002, 2003, 2004 and 2008 have been performed using a sun-sky radiometer. Data validation made using MODIS and local AERONET shows agreement. A bimodal size distribution is found with the fine mode centering at ~0.2 μm and coarse mode centering at ~3 μm respectively. The fine and coarse mode have close volume concentrations of nearly 50% fraction in composing local aerosols. Intercomparison of different years shows similar aerosol properties while a small increase of fine mode aerosol could be observed. A systematic shift of size distribution parameters is observed with different atmospheric conditions, where higher aerosol loadings and Angstrom exponent correspond to more fine mode aerosols. The fine mode is found to be more closely correlated with this shift than the coarse mode. A higher fine mode volume fraction and smaller median fine radius correspond to a larger Angstrom exponent. The fine mode aerosol hygroscopic growth is one of the main mechanisms for such systematic shifting. A third mode centering at ~1–2 μm could be discovered under high aerosol loading and high fine mode aerosol conditions. It becomes more pronounced with high aerosol optical depth and larger Angstrom exponent. Investigation of its variation with corresponding optical parameters and correlation with atmospheric conditions appears to support the hypothesis that it is mainly due to the fine mode aerosol hygroscopic growth and coagulation rather than the contribution from the coarse mode. While the very humid environment facilitates the aerosol hygroscopic growth, aerosol coagulation might further produce larger aerosols under high fine aerosol conditions. The continental outflow with transported aging aerosols and biomass burning might have also contributed to this additional mode.

Angstrom exponent and bimodal aerosol size distributions

Power laws have long been used to describe the spectral dependence of aerosol extinction, and the wavelength exponent of the aerosol extinction law is commonly referred to as the Angstrom exponent. The Angstrom exponent is often used as a qualitative indicator of aerosol particle size, with values greater than 2 indicating small particles associated with combustion byproducts, and values less than 1 indicating large particles like sea salt and dust. In this study, we investigate the relationship between the Angstrom exponent and the mode parameters of bimodal aerosol size distributions using Mie theory calculations and Aerosol Robotic Network (AERONET) retrievals. We find that Angstrom exponents based upon seven wavelengths (0.34, 0.38, 0.44, 0.5, 0.67, 0.87, and 1.02 μm) are sensitive to the volume fraction of aerosols with radii less then 0.6 μm but not to the fine mode effective radius. The Angstrom exponent is also known to vary with wavelength, which is commonly referred to as curvature; we show how the spectral curvature can provide additional information about aerosol size distributions for intermediate values of the Angstrom exponent. Curvature also has a significant effect on the conclusions that can be drawn about two‐wavelength Angstrom exponents; long wavelengths (0.67, 0.87 μm) are sensitive to fine mode volume fraction of aerosols but not fine mode effective radius, while short wavelengths (0.38, 0.44 μm) are sensitive to the fine mode effective radius but not the fine mode volume fraction.