Average Aerosol Optical Depth Research Articles

Rainwater characteristics and interaction with atmospheric particle matter transportation analyzed by remote sensing around Beijing

Air pollution in Beijing has attracted much more attentions, and multiple regulations have been enacted since 2013. Based on the close link between the atmospheric particle matter concentration and the deposited load in rainwater, 336 rainwater samplings with seven parameters (pH, NH4+-N, NO3−-N, P, S, Cu and Cd) at five-minute intervals in 2013 and 2014 were compared. The field monitoring and the temporal patterns analysis revealed a positive development of air quality. The lesser composition of coal in the energy consumption and the effective control of traffic emission were found. The average Aerosol Optical Depth (AOD) value around the sampling point during the 7 sampling rainfall events in 2014 was 2.855, which was higher than that in 2013 (1.807). It reflected the washing effect of rain on atmospheric particulates and highlighted the urban non-point source pollution effected by atmospheric deposition. AOD was demonstrated to perform well in reflecting regional air quality. A trajectory analysis conducted by HYSPLIT model in conjunction with the spatial distribution of AOD in the Beijing-Tian-Hebei (BTH) region depicted paths of air pollutants from long-range transport. The dominant trace was to the south of region. Cities around BTH were provided with different emission-reducing targets. Both Inner Mongolia and Henan province were suggested to control agricultural emissions. Shanxi, Shandong and cities around Bohai Bay should supervise the energy consuming industries. Furthermore, NO3−-N was introduced to be an indicator of effect of the regional joint prevention and control in the future.

Analysis of Aerosol Optical Depth Variation Characteristics for 10 years in Urumqi Based on MODIS_C006

Based on the MYD04_L2_C006 dataset gained from Moderate resolution imaging spectroradiometer MODIS Level 2 at 10 km resolution of NASA's EOS (Earth Observing System) from 2006 to 2015, the aerosol optical depths (AODs) for 10 years under the condition of mild above atmospheric pollution (including mild) were analyzed for Urumqi City. The results showed that the average AODs in Urumqi were unimodally distribution over the 10 years. From January to April, the AODs gradually increased. During a year, the annual peak of 0.37±0.19 was in April, and the lowest value of 0.22±0.20 was in October. Owing to the frequent occurrence of dust weather conditions in spring, the largest seasonal variation of AOD was in spring, as the air pollution in the entire area was very serious, followed by summer and winter, with the smallest in autumn. In addition, the urban area was the high AOD value area in Urumqi, as its air pollution was more severe than that in the suburbs. The average value of AOD during the 10 years was 0.293. The highest value of AOD was 0.33 in 2006, and the lowest value of 0.24 was in 2008, which declined to 23.3% of that in 2007. The annual change in AOD in Urumqi over the 10 years rose steadily. Compared with previous studies, the lowest and highest values were both higher to different degrees. Despite the deduction trend in 2015, air pollution in Urumqi was serious in the studied decade, and thus control of such needs to be strengthened.

Spatial analysis of aerosol optical depth obtained by air quality modelling and SEVIRI satellite observations over Portugal

The aim of this work is to explore the aerosol optical depth (AOD) data provided by different sources, namely satellite observations, air quality modelling and Aerosol Robotic Network (AERONET) observations, in order to improve characterisation of aerosol spatial distribution and to assess the contribution of the dust events on AOD over Portugal. Thus, the spatial and temporal variations of the AOD data were analysed for May 2011 taking into account the occurrence of mineral dust events. Satellite data obtained by Spinning Enhanced Visible and Infra-Red Imager (SEVIRI) instrument and air quality modelling results from the Comprehensive Air Quality Model (CAMx) over Portugal were explored. In this work, AOD values with a temporal resolution of 15 min were obtained from SEVIRI. Additionally, 3D data with 23 vertical layers for each aerosol constituent, size distribution and chemical composition were obtained from the modelling approach and used to compute the AOD at 550 nm by integrating of aerosol extinction coefficient (Qext) over the total atmospheric column. Despite the outbreaks of mineral dust occurring during this period, the overall contribution of organic carbon (OC), black carbon (BC) and sulphate (SO 4 ) compounds to the monthly average AOD values estimated by the model is very important achieving 95% mainly located in the northern part of the studied region. The model outputs and SEVIRI data were validated with AERONET observations presenting correlation coefficients of 0.71 and 0.68 respectively. Three different modes of aerosol were classified based on Ångström coefficient versus AOD and distinguishing between the dominance of fine and coarse aerosol fractions. The analysis implemented reveal that intercomparison of satellite observations with air quality modelling contribute to better understanding of the discrepancies presented in spatial pattern of AOD. • AOD data from satellite observations and air quality modelling were used. • Spatial and temporal variations of the AOD data were investigated. • Better correlation between modelling results and AERONET observations was obtained. • Satellite data and modelling results are in agreement but with different absolute values. • Analysis for observation points and for all domain provided different conclusions.

Aerosol optical, microphysical, chemical and radiative properties of high aerosol load cases over the Arctic based on AERONET measurements

Columnar mass concentrations of aerosol components over the Arctic are estimated using microphysical parameters derived from direct sun extinction and sky radiance measurements of Aerosol Robotic Network. Aerosol optical, microphysical, chemical and radiative properties show that Arctic aerosols are dominated by fine mode particles, especially for high aerosol load cases. The average aerosol optical depth (AOD) of the selected Arctic sites in the sampling period is approximately 0.08, with 75% composed of fine mode particles. The fine mode fraction mostly exceeds 0.9 when AOD greater than 0.4. The ammonium sulfate-like component (AS) contributes about 68% of total dry aerosol mass for high-AOD events. The estimated compositions and back trajectories show that the transported aerosol particles from biomass burning events have large amounts of black carbon (BC) and brown carbon, while those from pollution events are characterised by large AS fractions. The instantaneous radiative forcing at the top-of-atmosphere is higher for the more absorbing components, and varies greatly with surface albedo and solar zenith angle. A regression model of columnar composition and radiative forcing within the atmosphere (RFATM) for Arctic aerosol is established, showing that BC dominates a positive RFATM with a high warming efficiency.

Spatial distribution and temporal variation of aerosol optical depth and radiative effect in South China and its adjacent area

The spatio-temporal characteristics of aerosol loading over South China from 2001 to 2016 were investigated using aerosol optical depth (AOD) from the Moderate Resolution Imaging Spectroradiometer (MODIS) and NO2 from the Ozone Monitoring Instrument (OMI). AOD values were high in the central part and low in the southeast and northwest parts of South China. High AOD (larger than 0.7) were found in the Pearl River Delta, Nanning, and Hanoi (Vietnam). The seasonal average AOD was high in spring (approximately 0.7) and low in winter (approximately 0.4). Generally, an increasing trend of AOD was found from 2001 to 2004 and a decreasing trend from 2004 to 2016 in the continent due to the change in pollutant discharging, which was verified by annual NO2 data. Furthermore, the aerosol radiative effect (ARE) was calculated using the Mesoscale Atmospheric Global Irradiance Code (MAGIC) and MODIS AOD time series. The spatial distribution and temporal variation of ARE at surface showed similar patterns to AOD, with high values occurring in the Pearl River Delta (−39 W/m2), Hanoi (−36 W/m2), and Nanning (−30 W/m2). From 2001 to 2016, ARE at surface in South China decreased by approximately 4 W/m2 with the highest value (−24.75 W/m2) occurring in 2007.

Observations of the Interaction and Transport of Fine Mode Aerosols with Cloud and/or Fog in Northeast Asia from Aerosol Robotic Network (AERONET) and Satellite Remote Sensing.

Analysis of sun photometer measured and satellite retrieved aerosol optical depth (AOD) data has shown that major aerosol pollution events with very high fine mode AOD (>1.0 in mid-visible) in the China/Korea/Japan region are often observed to be associated with significant cloud cover. This makes remote sensing of these events difficult even for high temporal resolution sun photometer measurements. Possible physical mechanisms for these events that have high AOD include a combination of aerosol humidification, cloud processing, and meteorological co-variation with atmospheric stability and convergence. The new development of Aerosol Robotic network (AERONET) Version 3 Level 2 AOD with improved cloud screening algorithms now allow for unprecedented ability to monitor these extreme fine mode pollution events. Further, the Spectral Deconvolution Algorithm (SDA) applied to Level 1 data (L1; no cloud screening) provides an even more comprehensive assessment of fine mode AOD than L2 in current and previous data versions. Studying the 2012 winter-summer period, comparisons of AERONET L1 SDA daily average fine mode AOD data showed that Moderate Resolution Imaging Spectroradiometer (MODIS) satellite remote sensing of AOD often did not retrieve and/or identify some of the highest fine mode AOD events in this region. Also, compared to models that include data assimilation of satellite retrieved AOD, the L1 SDA fine mode AOD was significantly higher in magnitude, particularly for the highest AOD events that were often associated with significant cloudiness.

Spatiotemporal Variations in Atmospheric Aerosol Optical Depth along the Route of the 42nd Cruise of RV “Akademik Boris Petrov”

Results of expedition studies of the spatiotemporal variations in the aerosol optical depth (AOD) of the atmosphere during the 42nd cruise of research vessel (RV) “Akademik Boris Petrov” (January 18–March 28, 2017) along the route from Singapore to Kaliningrad (via the Suez Canal) are discussed. It is shown that the highest atmospheric turbidities were observed over the Arabian (due to the outflows of finemode aerosol from Hindustan) and Red (due to the outflows of dust aerosol) Seas, where the average AODs near 0.50 μm was 0.355 and 0.304, respectively. A considerable aerosol turbidity is also a characteristic of the atmosphere in the Strait of Malacca, burdened by aerosol outflows from the direction of Sumatra and western Malaysia (average AOD is 0.262). Different aerosol contents are noted in the neighboring equatorial regions of the Indian Ocean: the AOD in the middle of the ocean, more remote from the contents, was a factor of 1.5–2.0 larger than near Sumatra Island. All the tropical zone of the Indian Ocean is characterized by high values of the Angstrom selectivity exponent (1.08–1.18), indicating a predominant contribution of fine-mode aerosol to the AOD.

Proposal and evaluation of subordinate standard solar irradiance spectra for applications in solar energy systems

Reference solar irradiance spectra are needed to specify key parameters of solar technologies such as photovoltaic cell efficiency, in a comparable way. The IEC 60904-3 and ASTM G173 standards present such spectra for Direct Normal Irradiance (DNI) and Global Tilted Irradiance (GTI) on a 37° tilted sun-facing surface for one set of clear-sky conditions with an air mass of 1.5 and low aerosol content. The IEC/G173 standard spectra are the widely accepted references for these purposes. Hence, the authors support the future replacement of the outdated ISO 9845 spectra with the IEC spectra within the ongoing update of this ISO standard.The use of a single reference spectrum per component of irradiance is important for clarity when comparing and rating solar devices such as PV cells. However, at some locations the average spectra can differ strongly from those defined in the IEC/G173 standards due to widely different atmospheric conditions and collector tilt angles. Therefore, additional subordinate standard spectra for other atmospheric conditions and tilt angles are of interest for a rough comparison of product performance under representative field conditions, in addition to using the main standard spectrum for product certification under standard test conditions. This simplifies the product selection for solar power systems when a fully-detailed performance analysis is not feasible (e.g. small installations). Also, the effort for a detailed yield analyses can be reduced by decreasing the number of initial product options.After appropriate testing, this contribution suggests a number of additional spectra related to eight sets of atmospheric conditions and tilt angles that are currently considered within ASTM and ISO working groups. The additional spectra, called subordinate standard spectra, are motivated by significant spectral mismatches compared to the IEC/G173 spectra (up to 6.5%, for PV at 37° tilt and 10–15% for CPV). These mismatches correspond to potential accuracy improvements for a quick estimation of the average efficiency by applying the appropriate subordinate standard spectrum instead of the IEC/G173 spectra.The applicability of these spectra for PV performance analyses is confirmed at five test sites, for which subordinate spectra could be intuitively selected based on the average atmospheric aerosol optical depth (AOD) and precipitable water vapor at those locations.The development of subordinate standard spectra for DNI and concentrating solar power (CSP) and concentrating PV (CPV) is also considered. However, it is found that many more sets of atmospheric conditions would be required to allow the intuitive selection of DNI spectra for the five test sites, due in particular to the stronger effect of AOD on DNI compared to GTI.The matrix of subordinate GTI spectra described in this paper are recommended to appear as an option in the annex of future standards, in addition to the obligatory use of the main spectrum from the ASTM G173 and IEC 60904 standards.

Haze Optical Properties from Long-Term Ground-Based Remote Sensing over Beijing and Xuzhou, China

Aerosol haze pollution has had a significant impact on both global climate and the regional air quality of Eastern China, which has a high proportion of high level pollution days. Statistical analyses of aerosol optical properties and direct radiative forcing at two AERONET sites (Beijing and Xuzhou) were conducted from 2013 to 2016. Results indicate: (1) Haze pollution days accounted for 26% and 20% of days from 2013 to 2016 in Beijing and Xuzhou, respectively, with the highest proportions in winter; (2) The averaged aerosol optical depth (AOD) at 550 nm on haze days were about 3.7 and 1.6 times greater than those on clean days in Beijing and Xuzhou, respectively. At both sites, the maximum AOD occurred in summer; (3) Hazes were dominated by fine particles at both sites. However, as compared to Xuzhou, Beijing had larger coarse mode AOD and higher percentage of small α. This data, together with an analysis of size distribution, suggests that the hazes in Beijing were more susceptible to coarse dust particles than Xuzhou; (4) During hazes in Beijing, the single scattering albedo (SSA) is significantly higher when compared to clean conditions (0.874 vs. 0.843 in SSA440 nm), an increase much less evident in Xuzhou. The most noticeable differences in both SSA and the imaginary part of the complex refractive index between Beijing and Xuzhou were found in winter; (5) In Beijing, the haze radiative forcing produced an averaged cooling effect of −113.6 ± 63.7 W/m2 at the surface, whereas the averaged heating effect of 77.5 ± 49.7 W/m2 within the atmosphere was at least twice as strong as clean days. In Xuzhou, such a radiative forcing effect appeared to be much smaller and the difference between haze and clean days was insignificant. Derived from long-term observation, these findings are more significant for the improvement of our understanding of haze formation in China and the assessment of its impacts on radiative forcing of climate change than previous short-term case studies.

Observation of optical properties and sources of aerosols at Buddha's birthplace, Lumbini, Nepal: environmental implications.

For the first time, aerosol optical properties are measured over Lumbini, Nepal, with CIMEL sunphotometer of the Aerosol Robotic Network (AERONET) program. Lumbini is a sacred place as the birthplace of Lord Buddha, and thus a UNESCO world heritage site, located near the northern edge of the central Indo-Gangetic Plains (IGP) and before the Himalayan foothills (and Himalayas) to its north. Average aerosol optical depth (AOD) is found to be 0.64 ± 0.38 (0.06-3.28) over the sampling period (January 2013-December 2014), with the highest seasonal AOD during the post-monsoon season (0.72 ± 0.44). More than 80% of the daily averaged AOD values, during the monitoring period, are above 0.3, indicating polluted conditions in the region. The levels of aerosol load observed over Lumbini are comparable to those observed at several heavily polluted sites in the IGP. Based on the relationship between AOD and Ångstrom exponent (α), anthropogenic, biomass burning, and mixed aerosols are found to be the most prevalent aerosol types. The aerosol volume-size distribution is bi-modal during all four seasons with modes centered at 0.1-0.3 and 3-4μm. For both fine and coarse modes, the highest volumetric concentration of ~ 0.08μm-3μm-2 is observed during the post-monsoon and pre-monsoon seasons. As revealed by the single-scattering albedo (SSA), asymmetry parameter (AP), and refractive index (RI) analyses, aerosol loading over Lumbini is dominated by absorbing, urban-industrial, and biomass burning aerosols.

Long-Term Analysis of Aerosol Optical Depth over the Huaihai Economic Region (HER): Possible Causes and Implications

The quality of air is increasingly affecting regional climate and human activity. Collection 6 aerosol products retrieved from the Moderate-Resolution Imaging Spectroradiometer (MODIS) onboard the Terra satellite were validated based on CE-318 sun photometric data to analyze their applicability in the Huaihai economic region (HER) at the Xuzhou and Shouxian sites. The spatio-temporal variations of aerosol optical depth (AOD) were also analyzed over HER from 2000 to 2016, with analyses of the correlation with potential driving factors, including meteorology, vegetation and human factors. HER is an economic cooperation organization with multiple industrial structures, containing coal resource-based cities, a national transportation hub and agricultural and high-altitude areas, which shows regional differences in AOD. The results suggest that MODIS Terra AOD products show good agreement with ground observations, with correlation coefficients of above 0.84 in HER, and the main pollutants for high AOD values are fine particles (the mean Ångström exponent was 1.16). The average annual change in AOD varied with a weak growth trend over the past 17 years, while a transition in 2012 made the tendency change from upward to downward due to the extensive cooperation of cities in the joint prevention and control of the deterioration of the ecological environment. The largest monthly mean AOD value appeared in June, which resulted from significant agricultural residue burning. The spatial distribution of multi-year average AOD occurred with a banded high-value center, extending from the north-west to the south-east. The high aerosol loadings were located in resource-based cities, and industrially developed and south-eastern coastal areas, whereas the regions with relatively low AOD in HER were distributed in the southern agricultural and northern high-altitude areas. The AOD value in the western, northern, and eastern coastal areas of HER showed a significant increasing trend, while no area exhibited a decreasing trend. The average wind speed has the largest negative correlation with the AOD value in terms of the natural driving factors, and GDP (gross domestic product) was more positively correlated with AOD with respect to the human factors, in comparison with population density.

Generalization of Results of Atmospheric Aerosol Optical Depth Measurements on Spitsbergen Archipelago in 2011–2016

We discuss the results of spectral measurements of aerosol optical depth (AOD) of the atmosphere, carried out in warm periods of 2011–2016 in the Arctic settlement Barentsburg (Spitsbergen Archipelago). The statistical characteristics of seasonal and interannual variations in atmospheric AOD in the wavelength range of 0.34–2.14 μm are presented. The average AOD (0.5 μm) over the six-year period of observations had been 0.086, and, in particular, the fine mode AOD component had been 0.062 and the coarse mode AOD component had been 0.024. It is shown that the seasonal variations were best manifested in the decrease of modal (most probable) values of fine mode AOD component from 0.055 in spring to 0.025 in autumn. As compared to the preceding (pre-2011) period, we noted a closer convergence in the average AOD values between spring and summer, primarily due to a decrease in the content of fine mode aerosol in spring and its increase in summer. The summertime AOD growth is most likely due to episodic outflows of smoke aerosol from boreal zones of Eurasia and North America.

Impact of Diwali Festival on Aerosol Optical Properties over an Urban City, Ahmedabad (India)

An attempt has been made to investigate changes in the characteristics of aerosol optical properties induced during Diwali/ New Year celebrations of 2012 over an urban city, Ahmedabad (India). Ground based measurements of average Aerosol Optical Depth (AOD) were carried out using a Microtops-II at Physics Department, Gujarat University, Ahmedabad during 10th to 16th Nov, 2012. AOD on the day just after Diwali is found to increase significantly; around 75% compared to pre-Diwali days for visible wavelength, remained at higher level for one more day (next day of New Year). Turbidity factor (β) also changed considerably, to a high value of 0.34 on the next day of Diwali, indicating a significant increase in aerosol loading associated with Diwali/ New Year festivities. Normal anti-correlation between Angstrom exponent (α) and turbidity parameter (β) is not seen on days following Diwali. The AOD and β values in the evening of the next day of Diwali increased to about 164% and 171% respectively compared to pre-Diwali days evenings; such an increase associated with the festivities has not been reported at any part of the country. Spectral variation of AOD also shows a significant change in the pattern during following two days of celebrations compared to pre-Diwali days. It is seen to follow power law more closely. Significant increase in AOD and change in spectral pattern suggest an increase of fine/accumulation mode particles due to Diwali/ New Year festivities.

Standard aerosol optical depth index and application in the Middle East countries

Middle East is one of the most aerosol subjected region of the world. The records of aerosol optical depth (AOD) from 2003 to 2016 inclusive are investigated standard aerosol optical depth index (SAODI) providing verbal and numerical interpretations about the AOD records at Syria-Iraq northern, west Iraq, Iraq marshes, Iran Ahwaz and Ilam regions in addition to Saudi Arabia and United Arab Emirates. AOD records at different countries have different probability distribution functions (PDF), and the criteria of the average AOD plus 1, 2, 3 and 4 times the standard deviation of AOD record methodology as available in the literature does not provide a common footing for AOD assessment. Application of innovative trend analysis of the aerosol records indicated that although trends have rather small slopes, but in the Marshes, Ahwaz and Ilam locations there are decreasing trends, whereas in other locations the trends have increasing tendency.

Standard aerosol optical depth index and application in the Middle East countries

Middle East is one of the most aerosol subjected region of the world. The records of aerosol optical depth (AOD) from 2003 to 2016 inclusive are investigated standard aerosol optical depth index (SAODI) providing verbal and numerical interpretations about the AOD records at Syria-Iraq northern, west Iraq, Iraq marshes, Iran Ahwaz and Ilam regions in addition to Saudi Arabia and United Arab Emirates. AOD records at different countries have different probability distribution functions (PDF), and the criteria of the average AOD plus 1, 2, 3 and 4 times the standard deviation of AOD record methodology as available in the literature does not provide a common footing for AOD assessment. Application of innovative trend analysis of the aerosol records indicated that although trends have rather small slopes, but in the Marshes, Ahwaz and Ilam locations there are decreasing trends, whereas in other locations the trends have increasing tendency.

Modeling the radiative effects of biomass burning aerosols on carbon fluxes in the Amazon region.

Every year, a dense smoke haze covers a large portion of South America originating from fires in the Amazon Basin and central parts of Brazil during the dry biomass burning season between August and October. Over a large portion of South America, the average aerosol optical depth at 550 nm exceeds 1.0 during the fire season, while the background value during the rainy season is below 0.2. Biomass burning aerosol particles increase scattering and absorption of the incident solar radiation. The regional-scale aerosol layer reduces the amount of solar energy reaching the surface, cools the near-surface air, and increases the diffuse radiation fraction over a large disturbed area of the Amazon rainforest. These factors affect the energy and CO2 fluxes at the surface. In this work, we applied a fully integrated atmospheric model to assess the impact of biomass burning aerosols in CO2 fluxes in the Amazon region during 2010. We address the effects of the attenuation of global solar radiation and the enhancement of the diffuse solar radiation flux inside the vegetation canopy. Our results indicate that biomass burning aerosols led to increases of about 27% in the gross primary productivity of Amazonia and 10% in plant respiration as well as a decline in soil respiration of 3%. Consequently, in our model Amazonia became a net carbon sink; net ecosystem exchange during September 2010 dropped from +101 to -104 TgC when the aerosol effects are considered, mainly due to the aerosol diffuse radiation effect. For the forest biome, our results point to a dominance of the diffuse radiation effect on CO2 fluxes, reaching a balance of 50-50% between the diffuse and direct aerosol effects for high aerosol loads. For C3 grasses and savanna (cerrado), as expected, the contribution of the diffuse radiation effect is much lower, tending to zero with the increase in aerosol load. Taking all biomes together, our model shows the Amazon during the dry season, in the presence of high biomass burning aerosol loads, changing from being a source to being a sink of CO2 to the atmosphere.

Sensitivity of Aerosol Distribution and Climate Response to Stratospheric SO2 Injection Locations

AbstractInjection of SO2 into the stratosphere has been proposed as a method to, in part, counteract anthropogenic climate change. So far, most studies investigated injections at the equator or in a region in the tropics. Here we use Community Earth System Model version 1 Whole Atmosphere Community Climate Model (CESM1(WACCM)) to explore the impact of continuous single grid point SO2 injections at seven different latitudes and two altitudes in the stratosphere on aerosol distribution and climate. For each of the 14 locations, 3 different constant SO2 emission rates were tested to identify linearity in aerosol burden, aerosol optical depth, and climate effects. We found that injections at 15°N and 15°S and at 25 km altitude have equal or greater effect on radiation and surface temperature than injections at the equator. Nonequatorial injections transport SO2 and sulfate aerosols more efficiently into middle and high latitudes and result in particles of smaller effective radius and larger aerosol burden in middle and high latitudes. Injections at 15°S produce the largest increase in global average aerosol optical depth and increase the change in radiative forcing per Tg SO2/yr by about 15% compared to equatorial injections. High‐altitude injections at 15°N produce the largest reduction in global average temperature of 0.2° per Tg S/yr for the last 7 years of a 10 year experiment. Injections at higher altitude are generally more efficient at reducing surface temperature, with the exception of large equatorial injections of at least 12 Tg SO2/yr. These findings have important implications for designing a strategy to counteract global climate change.

Aerosol-type retrieval and uncertainty quantification from OMI data

Abstract. We discuss uncertainty quantification for aerosol-type selection in satellite-based atmospheric aerosol retrieval. The retrieval procedure uses precalculated aerosol microphysical models stored in look-up tables (LUTs) and top-of-atmosphere (TOA) spectral reflectance measurements to solve the aerosol characteristics. The forward model approximations cause systematic differences between the modelled and observed reflectance. Acknowledging this model discrepancy as a source of uncertainty allows us to produce more realistic uncertainty estimates and assists the selection of the most appropriate LUTs for each individual retrieval.This paper focuses on the aerosol microphysical model selection and characterisation of uncertainty in the retrieved aerosol type and aerosol optical depth (AOD). The concept of model evidence is used as a tool for model comparison. The method is based on Bayesian inference approach, in which all uncertainties are described as a posterior probability distribution. When there is no single best-matching aerosol microphysical model, we use a statistical technique based on Bayesian model averaging to combine AOD posterior probability densities of the best-fitting models to obtain an averaged AOD estimate. We also determine the shared evidence of the best-matching models of a certain main aerosol type in order to quantify how plausible it is that it represents the underlying atmospheric aerosol conditions.The developed method is applied to Ozone Monitoring Instrument (OMI) measurements using a multiwavelength approach for retrieving the aerosol type and AOD estimate with uncertainty quantification for cloud-free over-land pixels. Several larger pixel set areas were studied in order to investigate the robustness of the developed method. We evaluated the retrieved AOD by comparison with ground-based measurements at example sites. We found that the uncertainty of AOD expressed by posterior probability distribution reflects the difficulty in model selection. The posterior probability distribution can provide a comprehensive characterisation of the uncertainty in this kind of problem for aerosol-type selection. As a result, the proposed method can account for the model error and also include the model selection uncertainty in the total uncertainty budget.

The aerosol optical properties and PM2.5 components over the world's largest industrial zone in Tangshan, North China

To achieve an in-depth understanding of the aerosol optical properties in the highly-industrial region of Tangshan, we provided systematic aerosol optical properties analysis in this largest industrial zone for the first time. The aerosol optical datasets (2013.05–2015.04) and chemical component data of PM2.5 (2014–2015) obtained from the Tangshan site of the campaign on atmospheric aerosol research (CARE-China) network were analyzed. The results showed that the Tangshan region was seriously affected by fine-mode industrial aerosols all year, which would promote the accumulation of pollutants and influence the atmospheric circulation through changing the vertical temperature gradient. The annual average aerosol optical depth (AOD) and Ångstrӧm exponent (α) were 0.80±0.26 and 1.05±0.10, respectively. The aerosol optical properties revealed significant seasonal characteristics. The maximum seasonal average AOD (1.03±0.62) and α (1.12±0.19) accompanied the highest seasonal secondary inorganic aerosol concentrations (SIA: SO42−,NO3−,NH4+), 53.33μg/m3, occurred in summer, and this phenomenon was attributed to the photochemical reactions favored by the high temperature and humidity. During the spring, frequent dust events led to the maximum Ca2+ concentration of 6.57μg/m3 and the lowest seasonal α of 0.98±0.31. Coal was used for generating heat in winter, resulting in the highest levels of pollutant emissions (Cl−, Elemental carbon (EC) and organic carbon (OC)). The aerosol type classifications showed that the industrial aerosols were the main controls in the summer and fall, representing 56%–58% of the total aerosols. While for spring and winter, mixed aerosols represented 53%–54% of the total aerosols. Hygroscopic growth effect of aerosols existed all year, which could enhance the negative radiative forcing and eventually cool the earth-atmosphere system. The classification Wing for Tangshan data showed high AOD values (>0.70) were mainly clustered in the fine-mode growth wing and coarse-mode. Of these values, summer showed a dense AOD distribution and the highest magnitude of fine-mode AODs, with a corresponding fine particle fraction of η~90% and an Rf ~0.20μm. In addition, spring showed the most coarse-mode aerosols.

Comparisons of a Chemical Transport Model with a Four-Year (April to September) Analysis of Fine- and Coarse-Mode Aerosol Optical Depth Retrievals Over the Canadian Arctic

ABSTRACTWe compared April to September retrievals of total, fine-mode (sub-micron), and coarse-mode (super-micron) aerosol optical depth (AOD) from the Aerosol Robotic Network (AERONET) with simulations from a global three-dimensional chemical transport model, the Goddard Earth Observing System (GEOS-Chem), across five Arctic stations and a four-year sampling period. It was determined that the AOD histograms of both the retrievals and the simulations were better represented by a lognormal distribution and that the successful simulation of this empirical feature as well as its consequences (including a better model versus retrieval coefficient of determination in log-log AOD space) represented a general indicator of model evaluation success. Seasonal (monthly averaged) AOD retrievals were sensitive to the way in which the averaging was performed; this was ascribed to the presence of highly variable fine-mode smoke in the western Arctic. The retrieved and modelled station-by-station fine-mode AOD averages showed a peak in April/May that decreased over the summer, while the model underestimated the fine-mode AOD by an average of about 0.004 (∼6%). Both the retrievals and simulations showed seasonal coarse-mode AOD variations with a peak in April/May that was attributed to Asian and/or Saharan dust. The model's success in capturing such weak seasonal events helps to confirm the relevance of the separation of the fine and coarse modes and the general validity of model estimates in the Arctic.