Aerosol Index Research Articles

Altitude profiles of cloud condensation nuclei characteristics across the Indo-Gangetic Plain prior to the onset of the Indian summer monsoon

Abstract. Concurrent measurements of the altitude profiles of the concentration of cloud condensation nuclei (CCN), as a function of supersaturation (ranging from 0.2 % to 1.0 %), and aerosol optical properties (scattering and absorption coefficients) were carried out aboard an instrumented aircraft across the Indo-Gangetic Plain (IGP) just prior to the onset of the Indian summer monsoon (ISM) of 2016. The experiment was conducted under the aegis of the combined South-West Asian Aerosol–Monsoon Interactions and Regional Aerosol Warming Experiment (SWAAMI–RAWEX) campaign. The measurements covered coastal, urban and arid environments. In general, the CCN concentration was highest in the central IGP, decreasing spatially from east to west above the planetary boundary layer (PBL), which is ∼1.5 km for the IGP during pre-monsoon period. Despite this, the CCN activation efficiency at 0.4 % supersaturation was, interestingly, the highest over the eastern IGP (∼72 %), followed by that in the west (∼61 %), and it was the least over the central IGP (∼24 %) within the PBL. In general, higher activation efficiency is noticed above the PBL than below it. The central IGP showed remarkably low CCN activation efficiency at all altitudes, which appears to be associated with high black carbon (BC) mass concentration there, indicating the role of anthropogenic sources in suppressing the CCN efficiency. These first-ever CCN measurements over the western IGP, encompassing “the Great Indian Desert” also known as “the Thar Desert”, showed high CCN efficiency, ∼61 % at 0.4 % supersaturation, indicating the hygroscopic nature of the dust. The vertical structure of CCN properties is found to be air mass dependent, with higher activation efficiency even over the central IGP during the prevalence of marine air mass. Wet scavenging associated with precipitation episodes seems to have reduced the CCN activation efficiency below cloud level. An empirical relation has emerged between the CCN concentration and the scattering aerosol index (AI), which would facilitate the prediction of CCN from aerosol optical properties.

Identification of Black Dragon Forest Fire in Amur River Basin Using Satellite Borne NDVI Data and Its Impact on Long Range Transport of Pollutants: A Case Study

The Greater Hinggan Forest was the world’s largest stand of evergreens, along the Black Dragon River (also known as Amur), which forms the border between Chinese Manchuria and Soviet Siberia. Black Dragon fire ranks as one of the worst environmental disasters of the 20th century and it burned about 18 million acres of conifer forest. In the 2nd week of May, 1987, we observe more than 10K rise in brightness temperature over a wide region in the China-Russia border. The weekly mean NDVI data shows the changes in greenness after the forest fire broke out. The NDVI value is positive with persistent greenness and vegetation in the Amur River valley, but from the 2nd week of May onwards the reddish patch appears to spread over the entire region, indicates the burned areas. In addition, we observe the impact of Black Dragon forest fire on tropospheric ozone concentration, aerosol index away from the location over North Pacific Ocean. A clear increase in atmospheric pollutants can be noticed after the forest fire event and the long range transports are confirmed with 72 hours NOAA HYSPLIT forward trajectory analysis.

Pollution Trends in China from 2000 to 2017: A Multi-Sensor View from Space

Satellite sensors can provide unique views of global pollution information from space. In particular, a series of aerosol and trace gas monitoring instruments have been operating for more than a decade, providing the opportunity to analyze temporal trends of major pollutants on a large scale. In this study, we integrate aerosol products from MODIS (MODIS Resolution Imaging Spectroradiometer, all abbreviations and their definitions are listed alphabetically in Abbreviations) and MISR (Multi-angle Imaging Spectroradiometer), the AAI (Absorbing Aerosol Index) product from OMI (Ozone Monitoring Instrument), column SO2 and NO2 concentrations from OMI, and tropospheric column ozone concentration from OMI/MLS (Microwave Limb Sounder) to study temporal changes in major pollutants over China. MODIS and MISR consistently revealed that column AOD (Aerosol Optical Depth) increased from 2000, peaked around 2007, and started to decline afterward, except for northwest and northeast China, where a continuous upward trend was found. Extensive negative trends in both SO2 and NO2 have also been found over major pollution source regions since ~2005. On the other hand, the OMI AAI exhibited significant increases over north China, especially the northeast and northwest regions. These places also have a decreased Angstrom exponent as revealed by MISR, indicating an increased fraction of large particles. In general, summer had the largest AOD, SO2, and NO2 trends, whereas AAI trends were strongest for autumn and winter. A multi-regression analysis showed that much of the AOD variance over major pollution source regions could be explained by SO2, NO2, and AAI combined, and that the SO2 and NO2 reduction was likely to be responsible for the negative AOD trends, while the AOD increase over NE and NW China may be associated with an increase of coarse particles revealed by increased AAI and decreased AE. In contrast to aerosols, tropospheric ozone exhibited a steady increase from 2005 throughout China. This indicates that although the recent emission control effectively reduced aerosol pollutants, ozone remains a challenging issue and may dominate future air pollution.

Air quality changes in Ukraine during the April 2020 wildfire event

The paper analyzes air quality changes in Ukraine during a wildfire event in April 2020 and a dust storm episode during the 16th of April 2020. The wildfire event contained two episodes of active fires and huge pollutants' emission: 4-14 April and 16-21 April, respectively. Using the Sentinel-5P data of CO and NO 2 column number density and ground-based measurements, there was estimated air quality deterioration. Advection of polluted air masses and analysis of affected territories were made in combination with a Web-based HYSPLIT model. Satellite data described air quality changes better than in-situ measurements. Data intercomparison showed better coincidence in regions that were not affected by wildfire emissions. The paper described the dust storm event based on absorbing aerosol index (AAI) data that occurred between two wildfire episodes.

Fires in ecosystems and influence on the atmosphere

Introduction. Fires in ecosystems, mostly after open burning, affect Ukrainian territory each year causing flora and fauna damage, soil degradation, pollutants emission, which impact air quality and human health. Fires influence the atmosphere by adding burned products and its further direct and indirect effects. Despite majority of fires are open burning, research of forest fire emissions prevail among Ukrainian scientists. Therefore, the study aimed to analyze the influence of all-type fires in Ukrainian ecosystems on substances fluxes to the atmosphere and possible changes of meteorological processes. Data and methodology. The study uses GFED4 data and inventories for analyses of forest and agricultural burned fraction, carbon and dry matter emissions for the period of 1997–2016. Additional data includes absorbed aerosol index derived from OMI (Aura) instrument and ground-based meteorological measurements. Results. Burning fraction indicates the 10 to 30% of area influencing in case of active fires. More than 90% of fires in Ukrainian ecosystems happened on the agricultural lands. The highest trends of active fires appear on the western and northern part of Ukraine, whereas burned fraction on the central territories reached up to 60% decreasing per decade. Most fires happened during two periods: March – April and July – September. The most severe fires occurred in 1999, 2001, 2005, 2007, 2008 and 2012. Average emissions in Ukraine vary from 0.2 to 1.0 g·m2·month-1 for carbon and from 0.001 to 0.003 kg·m2·month-1 for dry matter. There are three localizations of huge burning products emissions, where maximal average values reach 1.8 g·m2·month-1 for carbon and 0.005 kg·m2·month-1 for dry matter. The biggest one occurred in the Polissia forest region. Despite the maximal emission from forest fires, open burning results the biggest coverage and air quality deteriorating. Absorbing aerosol index (AAI) could be good indicator of fires in Ukrainian ecosystems and burning products emissions. Overall, AAI with values more than 0.2 correspond to dry matter emissions of 0.005–0.01 kg·m2·month-1. If AAI exceed 0.4 usual dry matter emissions exceed 0.02 kg·m2·month-1. The study finds local scale changes of air temperature and days with precipitation due to huge burning products emissions. In case of monthly average AAI exceed 1.2 during fires events, positive air temperature anomaly at the ground decrease from 0.7 to 0.1°C. The main reason is absorption of solar radiation in the atmosphere. During the next month after intensive fires in ecosystems, days with precipitation have twofold decrease: from 13-14 to 7 days with precipitation more than 0 mm, and from 2-3 to 1 day with precipitation more than 5 mm. The reason might be changes of cloudiness formation due to elevated concentrations of carbonaceous aerosols. The results obtained for atmospheric changes is planned to be verified and compared using online integrated atmospheric modelling.

Absorbing aerosol influence on temperature maxima: An observation based study over India

Increasing trends in summer-time temperature maxima (Tmax) over India, show consequent increases in the intensity and frequency of heatwave events in recent years. Heat waves have been largely attributed to large-scale meteorological blocking, characterized by subsidence, clear skies and low soil moisture, in observational studies, or greenhouse gas enhancements in model studies. While radiative effects of absorbing aerosols are acknowledged, the association of absorbing aerosols with temperature maxima has not been investigated comprehensively. In the current study, statistical tools (such as correlation and Granger causality) were applied to long term (1979–2013) satellite and ground based observations to evaluate influence of absorbing aerosols on Tmax in north-west India (Tmax-NW). Regional absorbing aerosol index (AAI) in the north-west (AAI-NW) and central-India (AAI-CI) showed co-variability with Tmax-NW, implying connections to both local and non-local absorbing aerosols. The effects persisted on seasonal and heatwave event scales, becoming stronger on heatwave days with presence of enhanced AAI loadings. Causal effects of AAI-NW and AAI-CI were identified on Tmax-NW with a lag of 1–11 days, across multiple years, thereby establishing the influence of absorbing aerosols on heatwave events. The absence of confounding effects of surface pressure on these links suggests that, even during heat wave events linked to atmospheric blocking, absorbing aerosols can further enhance temperature maxima and related heatwave intensity.

Capabilities and limitations of the single-particle extinction and scattering method for estimating the complex refractive index and size-distribution of spherical and non-spherical submicron particles

Nondestructive inline measurements of submicron-sized particles and gas bubbles are required in many fields of study, including environmental, industrial, and biomedical research. The single particle extinction and scattering (SPES) method is a promising technique for this purpose because of its sensitivity to both the complex refractive index and the size of individual particles. Herein an original modification of the SPES instrument that suppresses measurement bias and background noise is presented. In addition, relevant theory and a practical algorithm for data analysis are presented. The inverse scattering problem of estimating the physical characteristics of a particle from SPES data using Bayesian theory for model selection is formulated, and the performance of the algorithm using simulated data for spherical and non-spherical particles is evaluated. The results suggest that a cluster of SPES data points for an ensemble of particles contains sufficient information to retrieve the complex refractive index and volume equivalent size-distribution of the particles, regardless of particle shape. The Bayesian inversion method also allows incorporation of informative data from other analytical methods (e.g., electron microscope, upstream size-sorting device) through the model and parameter priors. As practical examples, the SPES method was used to constrain the complex refractive index and size distribution of water-insoluble aerosols contained in rainwater. The results suggest that the SPES method itself or its use in combination with other conventional analytical methods can quantify the complex refractive index and size distribution of almost any type of submicron particle (including gas bubbles) in environmental, industrial, and biomedical samples.

Lightning and Associated Convection Features in the Presence of Absorbing Aerosols Over Northern Alabama

AbstractMany of the previous aerosol‐convection/lightning enhancement studies are based on convective storms that occur in the presence of absorbing aerosols; such aerosols may impact deep convection through their microphysical and radiative effects. In this study, lightning flash rates (FRs) are analyzed together with aerosol optical depth (AOD) retrievals from the Moderate Resolution Imaging Spectroradiometer for summer storms during 2002–2015. Aerosol index retrievals from the Ozone Monitoring Instrument are used to separate nonabsorbing and absorbing aerosol cases. Statistical analyses are performed to test for significant sample differences and linear relationships between the AOD of case studies and the FRs, convective available potential energy, planetary boundary layer height, and low‐level vertical wind shear to identify if evidence of regulation of storms by absorbing aerosols exists. Overall, AOD and FR do not show any significant linear relationship. When just observing absorbing aerosols, however, AOD and FR show a stronger (but still very weak) positive correlation. The weak correlation may be related to the absorbing aerosols' impact on convective available potential energy, which has a moderate linear correlation to AOD particularly when the instability is low, which implies some kind of convection or convective environment modification by absorbing aerosols. Although the planetary boundary layer height tends to decrease with increasing amount of absorbing aerosols, it is found that low‐level vertical wind shear does not correlate with AOD for either absorbing or nonabsorbing aerosols. This result suggests little influence of the interaction between absorbing aerosols and turbulent mixing on storms.

Characterization of optically effective complex refractive index of black carbon composite aerosols

Black carbon aerosols, always existing as complicated mixtures of various species, have important impacts on the global and regional climate, whereas their effective aerosol complex refractive index (ACRI) is a must for modeling their radiative effects. Retrieval of optically effective ACRI from accurately calculated scattering and absorption properties of an internal-mixed aerosol model with BC and dust coated with sulfate, roughly representing ambient aged multicomponent BC particles, are performed. The sensitivities of retrieved optically effective ACRI to particle size distribution and composition ratio are explored, and the popular volume weighted average (VWA) method and Bruggeman effective medium (BEM) theory are also considered for comparison. The optically effective ACRIs depicts dramatic variation for monodisperse size parameter larger than π, as opposed to almost a constant for size parameter less than π, whereas the VWA and BEM can hardly provide acceptable effective ACRIs for coarse BC composite particles. The retrieved optically effective ACRI are sensitive to both size distribution and composition ratio, and becomes more sensitive to particle size distribution with aerosols containing more BC content. As BC volume content reaches ~4%, the VWA overestimates the imaginary part of effective ACRI with significant bias when compared with the optically effective ACRI, indicating overestimated aerosol absorption in most atmospheric environments in aerosol-climate models employing the VWA. We suggest that the optically effective ACRI, rather than the ACRI given by the VWA, should be applied to account for coarse BC-containing particles in the state-of-the-art aerosol-climate models.

A comparative evaluation of Aura-OMI and SKYNET near-UV single-scattering albedo products

Abstract. The aerosol single-scattering albedo (SSA) retrieved by the near-UV algorithm applied to the Aura Ozone Monitoring Instrument (OMI) measurements (OMAERUV) is compared with an independent inversion product derived from the sky radiometer network SKYNET – a ground-based radiation observation network with sites in Asia and Europe. The present work continues previous efforts to evaluate the consistency between the retrieved SSA from satellite and ground sensors. The automated spectral measurements of direct downwelling solar flux and sky radiances made by the SKYNET Sun-sky radiometer are used as input to an inversion algorithm that derives spectral aerosol optical depth (AOD) and single-scattering albedo (SSA) in the near-UV to near-IR spectral range. The availability of SKYNET SSA measurements in the ultraviolet region of the spectrum allows, for the first time, a direct comparison with OMI SSA retrievals eliminating the need of extrapolating the satellite retrievals to the visible wavelengths as is the case in the evaluation against the Aerosol Robotic Network (AERONET). An analysis of the collocated retrievals from over 25 SKYNET sites reveals that about 61 % (84 %) of OMI–SKYNET matchups agree within the absolute difference of ±0.03 (±0.05) for carbonaceous aerosols, 50 % (72 %) for dust aerosols, and 45 % (75 %) for urban–industrial aerosol types. Regionally, the agreement between the two inversion products is robust over several sites in Japan influenced by carbonaceous and urban–industrial aerosols; at the biomass burning site Phimai in Thailand; and the polluted urban site in New Delhi, India. The collocated dataset yields fewer matchups identified as dust aerosols mostly over the site Dunhuang with more than half of the matchup points confined to within ±0.03 limits. Altogether, the OMI–SKYNET retrievals agree within ±0.03 when SKYNET AOD (388 or 400 nm) is larger than 0.5 and the OMI UV Aerosol Index is larger than 0.2. The remaining uncertainties in both inversion products can be attributed to specific assumptions made in the retrieval algorithms, i.e., the uncertain calibration constant, assumption of spectral surface albedo and particle shape, and subpixel cloud contamination. The assumption of fixed and spectrally neutral surface albedo (0.1) in the SKYNET inversion appears to be unrealistic, leading to underestimated SSA, especially under lower aerosol load conditions. At higher AOD values for carbonaceous and dust aerosols, however, retrieved SSA values by the two independent inversion methods are generally consistent in spite of the differences in retrieval approaches.

Development of GK-2A AMI Aerosol Detection Algorithm in the East-Asia Region Using Himawari-8 AHI Data

The GEO-KOMPSAT 2A (GK-2A) satellite is a next-generation geostationary meteorological satellite with multi-wavelength channels. The GK-2A Advanced Meteorological Imager (AMI) aerosol detection production (ADP) algorithm can detect aerosol types (e.g., dust, haze, volcanic ash), using visible and infrared channels. This algorithm is not employed in existing meteorological satellites and can detect four different aerosol types in the daytime and three at night. Here, cloud and aerosol detection were performed using Advanced Himawari Imager (AHI) data with channels similar to those of simulation data. Comparing the results with the Communication, Ocean and Meteorological Satellite (COMS) aerosol index (AI) and AHI RGB aerosol data revealed generally similar dust classification and excellent daytime haze and nighttime aerosol detection performances. Regarding detection of haze, dust, and undefined aerosols generated across East Asia, a percent correct (PC) of 0.76, probability of detection (POD) of 68%, and false alarm rate (FAR) of 33% were obtained for GK-2A ADP of dust when validated with Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) Vertical Feature Mask (VFM) aerosol type data. Furthermore, the detection performance was ≥87% for haze and undefined aerosols, and the FAR was <30%. When Moderate Resolution Imaging Spectroradiometer (MODIS) L2 aerosol optical depth (AOD) data were used for validation (with and without AOD), the POD was 0.66 (ocean) and 0.87 (land), while the FAR was 0.29 (ocean) and 0.35 (land). Thus, aerosol detection performances of GK-2A ADP are similar to those of existing satellites and superior to satellites that use limited optical channels.

The role of aerosol layer height in quantifying aerosol absorption from ultraviolet satellite observations

Abstract. The purpose of this study is to demonstrate the role of aerosol layer height (ALH) in quantifying the single scattering albedo (SSA) from ultraviolet satellite observations for biomass burning aerosols. In the first experiment, we retrieve SSA by minimizing the near-ultraviolet (near-UV) absorbing aerosol index (UVAI) difference between observed values and those simulated by a radiative transfer model. With the recently released S-5P TROPOMI ALH product constraining forward simulations, a significant gap in the retrieved SSA (0.25) is found between radiative transfer simulations with spectral flat aerosols and those with strong spectrally dependent aerosols, implying that inappropriate assumptions regarding aerosol absorption spectral dependence may cause severe misinterpretations of the aerosol absorption. In the second part of this paper, we propose an alternative method to retrieve SSA based on a long-term record of co-located satellite and ground-based measurements using the support vector regression (SVR) approach. This empirical method is free from the uncertainties due to the imperfection of a priori assumptions on aerosol microphysics seen in the first experiment. We present the potential capabilities of SVR using several fire events that have occurred in recent years. For all cases, the difference between SVR-retrieved SSA and AERONET are generally within ±0.05, and over half of the samples are within ±0.03. The results are encouraging, although in the current phase the model tends to overestimate the SSA for relatively absorbing cases and fails to predict SSA for some extreme situations. The spatial contrast in SSA retrieved by radiative transfer simulations is significantly higher than that retrieved by SVR, and the latter better agrees with SSA from MERRA-2 reanalysis. In the future, more sophisticated feature selection procedures and kernel functions should be taken into consideration to improve the SVR model accuracy. Moreover, the high-resolution TROPOMI UVAI and co-located ALH products will guide us to more reliable training data sets and more powerful algorithms to quantify aerosol absorption from UVAI records.

One year of aerosol refractive index measurement from a coastal Antarctic site

Abstract. Though the environmental conditions of the Weddell Sea region and Dronning Maud Land are still relatively stable compared to the fast-changing Antarctic Peninsula, we may suspect pronounced effects of global climate change for the near future (Thompson et al., 2011). Reducing the uncertainties in climate change modeling requires a better understanding of the aerosol optical properties, and for this we need accurate data on the aerosol refractive index (RI). Due to the remoteness of Antarctica only very few RI data are available from this region (Hogan et al., 1979; Virkkula et al., 2006; Shepherd et al., 2018). We calculate the real refractive index of natural atmospheric aerosols from number size distribution measurements at the German coastal Antarctic station Neumayer III. Given the high average scattering albedo of 0.992 (Weller et al., 2013), we assumed that the imaginary part of the RI is zero. Our method uses the overlapping size range (particle diameter D between 120 and 340 nm) of a scanning mobility particle sizer (SMPS), which sizes the particles by their electrical mobility, and a laser aerosol spectrometer (LAS), which sizes the particles by their optical scattering signal at the 633 nm wavelength. Based on almost a complete year of measurement, the average effective refractive index (RIeff, as we call our retrieved RI because of the used assumptions) for the dry aerosol particles turned out to be 1.44 with a standard deviation of 0.08, in a good agreement with the RI value of 1.47, which we derived from the chemical composition of bulk aerosol sampling measurements. At Neumayer the aerosol shows a pronounced seasonal pattern in both number concentration and chemical composition. Despite this, the variability of the monthly averaged RIeff values remained between 1.40 and 1.50. Compared to the annual mean, two austral winter months (July and September) showed slightly but significantly increased values (1.50 and 1.47, respectively). The size dependency of the RIeff could be determined from time-averaged LAS and SMPS number size distributions measured between December 2017 and January 2018. Here we calculated RIeff for four different particle size ranges and observed a slight decrease from 1.47 (D range 116–168 nm) to 1.37 (D range 346–478 nm). We find no significant dependence of the derived RIeff values on the wind direction. Thus we conclude that RIeff is largely independent of the general weather situation, roughly classified as (i) advection of marine boundary layer air masses during easterly winds caused by passing cyclones in contrast to (ii) air mass transport from continental Antarctica under southern katabatic winds. Neumayer, the only relevant contamination source, is located 1.5 km north of the air chemistry observatory, where the measurements were performed. Given that northerly winds are almost absent, the potential impact of local contamination is minimized in general. Indeed our data show no impact of local contamination on RIeff. Just in one case a temporary high-contamination episode with diesel engines operating right next to the measurement site resulted in an unusual high RIeff of 1.59, probably caused by the high black carbon content of the exhaust fumes. To conclude, our study revealed largely constant RIeff values throughout the year without any sign of seasonality. Therefore, it seems reasonable to use a single, constant RIeff value of 1.44 for modeling optical properties of natural, coastal Antarctic sub-micrometer aerosol.

Carbonaceous aerosol emission reduction over Shandong province and the impact of air pollution control as observed from synthetic satellite data

Aerosol loading and trace gas emissions from agriculture crop residue burning has been one of the main factors leading to the deterioration of air quality especially during the harvest time in China. Here, we evaluate the contribution of atmospheric aerosol loading from biomass burning and its relationship to the air pollution control policy. We focused on Shandong province, China because it has the highest level of agriculture burning pollution in October. Satellite-based data related to biomass burning including aerosol optical depth (AOD) and fire pixel counts from Moderate Resolution Imaging Spectroradiometer (MODIS), carbon monoxide (CO) from Measurements Of Pollution In The Troposphere (MOPITT), aerosol index (AI) and tropospheric NO2 from Ozone Monitoring Instrument (OMI), and aerosol layer depths from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO). These metrics were synthetically utilized to evaluate spatial and temporal variation of atmospheric aerosols during the agriculture crop residue burning month from 2006 to 2017. Carbonaceous aerosol emission reduction and the impact of the air pollution control policy can be found in the CALIPSO aerosol types and MODIS fire pixel counts data. The AOD, NO2, CO, and AI over three different land covers (cropland, forest and impervious surfaces) were further obtained according to the land cover data set. The results indicated that there was an obvious decrease in all carbonaceous aerosol-related quantities after year 2012 when the local government began to enact and implement strict air pollution control policies, although a period (2–3 years) of transition after the policy implementation time was identified. We also found that CO is stable and low over the cropland versus forest and impervious surfaces; NO2 exhibits a continuously high value in the northern areas of the oil field. The impact of industry activity rather than straw burning should be also considered for NO2 and CO in certain areas.

AN AEROSOL TYPE CLASSIFICATION METHOD BASED ON REMOTE SENSING DATA IN GUANGDONG, CHINA

Abstract. This paper provides an aerosol classification method based on remote sensing data in Guangdong, China in year 2010 and 2011. Aerosol Optical Depth, Angstrom Exponent and Ultraviolet Aerosol Index, as important properties of aerosols, are introduced into classification. Data of these three aerosol properties are integrated to establish a 3-dimension dataset, and k-means clustering algorithm with Mahalanobis distance is used to find out four clusters of the dataset, which respectively represents four aerosol types of urban-industrial, dust, biomass burning and mixed type. Prior knowledge about the understanding of each aerosol type is involved to associate each cluster with aerosol type. Temporal variation of the aerosol properties shows similarities between these two years. The proportion of aerosol types in different cities of Guangdong Province is also calculated, and result shows that in most cities urban-industrial aerosols takes the largest proportion while the mixed type aerosols takes the second place. Classification results prove that k-means cluster algorithm with Mahalanobis distance is a brief and efficient method for aerosol classification.

Minimum principles in electromagnetic scattering by small aspherical particles: Extension to differential cross-sections

In this work we extend earlier findings that optically small and randomly oriented ovaloids extinguish more radiation than equal volume spheres [1], to differential cross-sections. Rather than working with the normalized phase function as is typically done in radiative transfer and in remote sensing, we compute absolute un-normalized differential scattering cross-sections Cscatt(θ) and show that for optically small to moderate size parameters, not only does the integrated extinction by randomly oriented spheroids Cext(θ) exceed that of equal volume spheres but it does so at each scattering angle (θ). Furthermore, at each θ, the effect is monotonic with the aspect ratio (AR) and its magnitude is appreciable for realistic refraction indices of terrestrial aerosols. Spherical shape optimality holds for absorption cross-sections as well. Absorption is not only volume-dependent but increases substantially with asphericity, rising by two orders of magnitude near resonant lines. We also compare the asymmetry parameter (g) and single scattering albedo (ω0) of randomly oriented spheroids to that of equal volume spheres and find that while the dependence of the ratios on the axis ratio is monotonic, change of either sign is possible, depending on the index of refraction. Ice in the microwave and quartz in the thermal IR are used to illustrate applications in atmospheric remote sensing.

A new parameterization scheme for the real part of the ambient urban aerosol refractive index

Abstract. The refractive index of ambient aerosols, which directly determines the aerosol optical properties, is widely used in atmospheric models and remote sensing. Traditionally, the real part of the refractive index (RRI) is parameterized by the measurement of ambient aerosol main inorganic components. In this paper, the characteristics of the ambient aerosol RRI are studied based on field measurements in East China. The results show that the measured ambient aerosol RRI varies significantly between 1.36 and 1.56. The direct aerosol radiative forcing is estimated to vary by 40 % when the RRI values were varied between 1.36 and 1.56. We find that the ambient aerosol RRI is highly correlated with the aerosol effective density (ρeff) rather than the main chemical components. However, the parameterization of the ambient aerosol RRI by ρeff is not available due to the lack of corresponding simultaneous field measurements. For the first time, the size-resolved ambient aerosol RRI and ρeff are measured simultaneously by our designed measurement system. A new parameterization scheme for the ambient aerosol RRI using ρeff is proposed for urban environments. The measured and parameterized RRI values agree well, with a correlation coefficient of 0.75 and slope of 0.99. Knowledge of the ambient aerosol RRI would improve our understanding of ambient aerosol radiative effects.

Consistent signal of aerosol indirect and semi-direct effect on water clouds in the oceanic regions adjacent to the Indian subcontinent

Abstract Quantifying aerosol-cloud interaction is one of the most challenging tasks in climate science. Previous studies in the oceanic regions adjacent to the Indian subcontinent providing evidence of aerosol impacts on cloud properties were mostly confined in the winter season. Here we analyze 15 years (March 2000–February 2015) of MODIS aerosol and cloud products and 10 years (October 2004–February 2015) of OMI absorbing aerosol index (AI) over the Arabian Sea (AS), Bay of Bengal (BoB) and South Indian Ocean (SIO) to examine the sensitivity of marine water cloud characteristics to changing aerosol loading. Amidst a monotonous increase in Reff (cloud effective radius) with an increase in LWP (liquid water path) in clean and polluted conditions (an indicator of the meteorological influence), a smaller Reff at higher AOD (aerosol optical depth) bins demonstrates that aerosol signal is detectable and consistent in LWP range of 150–350 g m−2. Furthermore, a faster rate of Reff growth with increasing LWP in clean condition and the pattern of change in cloud fraction (fc) with an increase in AOD that cannot be explained by meteorology alone also provide evidence of the consistency of aerosol indirect effect. Evidence of aerosol semi-direct (decrease of LWP with an increase in AI) is also conspicuous in the AS and BoB. Our results suggest improved representations of aerosol and cloud processes in the models are required to reduce uncertainty in regional climate forcing.

Monthly and seasonal variability of dust events over northern Saudi Arabia

AbstractThe monthly and seasonal variability and distribution of dust events over northern Saudi Arabia were studied using ground‐based measurements from 11 surface stations for the period of 1978–2010. Additionally, to study the synoptic climatology of the dust variability, the aerosol index (AI) data from the Total Ozone Mapping Spectrometer (TOMS) satellite and meteorological data from the National Center for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR) reanalysis datasets were used. The dust types observed at the surface stations are classified into two categories: weak dust and dust storms. A statistical study of the ground measurements demonstrated that weak dust category events are prevalent in the cold months, whereas storm category events are prevalent in the hot months. Additionally, the annual distribution distinguishes two periods for dust observations, before and after 1989, where the number of events in the first period is lower than the annual average but increases during the second period. The synoptic climate study indicated that two main atmospheric wind patterns, anticyclonic and northerly (shamal) patterns, accompany the dust events in the study area. The dust in winter and autumn is mainly affected by the anticyclonic pattern, while that in spring and summer is mainly affected by the shamal pattern. In addition, a synoptic study of selected cases confirmed the climate results and demonstrated the existence of two atmospheric patterns corresponding to winter and summer. Both patterns include troughs over the Red Sea and Arabian Gulf and a ridge or high‐pressure cell over the eastern Mediterranean region during the summer atmospheric pattern and over the mid‐Arabian Peninsula during the winter atmospheric pattern. The characteristic of non‐dust storm composition demonstrated that the storms exhibit pronounced synoptic systems (winter and summer) with the highest pressure/geopotential gradient near the Arabian Peninsula.

Effect of Aerosols on the Ice Cloud Properties Over the Tibetan Plateau

AbstractWith the highlight of environmental problems over the Tibetan Plateau (TP), aerosol pollution and the influence of this pollution on cloud properties are becoming a new area of research. Based on the aerosol index and cloud property parameters derived from satellite observations, in this study, the inconsistent effects of aerosols on ice cloud properties between daytime and nighttime over the TP are investigated. The results indicate that ice clouds are mainly distributed over the TP margin area, especially over the north slope, during both daytime and nighttime. The occurrence frequency of ice cloud is higher during the daytime than during the nighttime over the margin areas of the TP. Similarly, aerosols are mainly concentrated over the northern margin of the TP. A potential relationship may exist between the aerosol index and ice cloud properties. When the aerosol index increases from 0.05 to 0.17, the ice cloud droplet radius (ICDR) during the daytime decreases from 32.1 to 27.9 μm, while the ICDR during the nighttime remains almost constant (approximately 25 μm); furthermore, the ice water path (IWP) during the daytime decreases slightly due to the saturation effect, while the nocturnal IWP increases significantly. The changes in ice cloud optical depth (ICOD) during daytime and nighttime show significant and completely opposite trends. The removal of the influence of meteorological factors showed that aerosols have a more dominant influence than meteorological conditions on ice cloud properties (except for the nocturnal ICDR and IWP during the daytime).