Mean Aerosol Extinction Coefficient Research Articles

Changes in stratospheric aerosol extinction coefficient after the 2018 Ambae eruption as seen by OMPS-LP and MAECHAM5-HAM

Abstract. Stratospheric aerosols are an important component of the climate system. They not only change the radiative budget of the Earth but also play an essential role in ozone depletion. These impacts are particularly noticeable after volcanic eruptions when SO2 injected with the eruption reaches the stratosphere, oxidizes, and forms stratospheric aerosol. There have been several studies in which a volcanic eruption plume and the associated radiative forcing were analyzed using climate models and/or data from satellite measurements. However, few have compared vertically and temporally resolved volcanic plumes using both measured and modeled data. In this paper, we compared changes in the stratospheric aerosol loading after the 2018 Ambae eruption observed by satellite remote sensing measurements and simulated by a global aerosol model. We use vertical profiles of the aerosol extinction coefficient at 869 nm retrieved at the Institute of Environmental Physics (IUP) in Bremen from OMPS-LP (Ozone Mapping and Profiling Suite – Limb Profiler) observations. Here, we present the retrieval algorithm and a comparison of the obtained profiles with those from SAGE III/ISS (Stratospheric Aerosol and Gas Experiment III on board the International Space Station). The observed differences are within 25 % for most latitude bins, which indicates a reasonable quality of the retrieved limb aerosol extinction product. The volcanic plume evolution is investigated using both monthly mean aerosol extinction coefficients and 10 d averaged data. The measurement results were compared with the model output from MAECHAM5-HAM (ECHAM for short). In order to simulate the eruption accurately, we use SO2 injection estimates from OMPS and OMI (Ozone Monitoring Instrument) for the first phase of eruption and the TROPOspheric Monitoring Instrument (TROPOMI) for the second phase. Generally, the agreement between the vertical and geographical distribution of the aerosol extinction coefficient from OMPS-LP and ECHAM is quite remarkable, in particular, for the second phase. We attribute the good consistency between the model and the measurements to the precise estimation of injected SO2 mass and height, as well as to the nudging to ECMWF ERA5 reanalysis data. Additionally, we compared the radiative forcing (RF) caused by the increase in the aerosol loading in the stratosphere after the eruption. After accounting for the uncertainties from different RF calculation methods, the RFs from ECHAM and OMPS-LP agree quite well. We estimate the tropical (20∘ N to 20∘ S) RF from the second Ambae eruption to be about −0.13 W m−2.

Comparative observation of aerosol vertical profiles in urban and suburban areas: Impacts of local and regional transport

Ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) instruments were used to carry out observation of aerosol in the urban and suburban areas of Shanghai from October 17 to November 21, 2019. Fudan University (FDU) site is a typical urban environment, surrounded by residential areas, commercial areas and arterial roads, while Dianshan Lake (DSL) site is a suburban environment with high vegetation coverage and no pollutant emission sources. The aerosol retrieved by MAX-DOAS was in good correlation with the observation of sun photometer and the PM2.5 concentration of the corresponding site, which demonstrates that the aerosol retrieved by MAX-DOAS is reliable and feasible. Comparing the mean aerosol extinction coefficient (AEC) profiles during the observation period between urban and suburban areas, it was found that the occurrence of high aerosol concentration at FDU was nearly 3 h later than that of DSL at suburban site. And the aerosol at DSL was concentrated at an altitude of 0.3– 0.5 km, with a mean peak value of 0.486 km−1, which was slightly higher than the peak AEC of 0.453 km−1 at FDU of 0.2– 0.4 km. The difference in aerosol characteristics between the two sites may be due to the fact that the influences of aerosol transport and boundary layer dynamics are different between the two sites. The backward trajectories analysis also presents that there were mutual transports of aerosol between urban and suburban areas, which affect the optical properties of the aerosol in these two sites. In a case of aerosol pollution, we visualized the transport pathway of aerosol from the western part of the North China Plain to Shanghai using AEC profiles and backward trajectories, providing the evidence that the local aerosol pollution in Shanghai was affected by long-distance transport.

Global aerosol vertical structure analysis by clustering gridded CALIOP aerosol profiles with fuzzy k-means

Aerosol vertical structures are critical to understanding distribution and source-sink patterns of aerosol on a large scale. In this study, we carried out spatial clustering analysis for 10-years long CALIOP aerosol profiles with a fuzzy k-means (FKM) method. Raw and normalized data sets were both classified into three representative clusters. Raw aerosol profiles of original data described both aerosol density and structure patterns, which were classified into polluted cluster, medium cluster and clean cluster with visual inspection. The mean aerosol extinction coefficient values in near surface from large to small respectively belonged to polluted cluster, medium cluster and clean cluster. As altitude increased, mean aerosol extinction coefficients of polluted cluster were in rapid decline trend from surface to upper atmosphere. In comparison, there was a slower decrease speed for the aerosol extinction coefficient values of mean aerosol profiles of the other two clusters. Aerosol profiles clusters using normalized data could be used to describe aerosol vertical structure patterns. Normalized aerosol profiles were classified into boundary-layer concentrated cluster (boundary cluster), vertically even distributed cluster (v-even cluster) and surface-layer concentrated cluster (surface cluster). The boundary cluster was stable in the low atmosphere with a decline trend upwards, which was spatially corresponds to strong anthropogenic emission and dust regions. The mean normalized extinction coefficient values of v-even cluster were relatively stable in a large vertical range (about 4 km) at regions with relatively weak wind fields. The coefficients of surface cluster were mainly distributed in the near surface with mostly in coastlines and low aerosol optical depth (AOD) regions. The cluster analysis of CALIOP aerosol profiles provided general patterns for global distributions of aerosol profile density and structure.

Assessment of dominating aerosol properties and their long-term trend in the Pan-Third Pole region: A study with 10-year multi-sensor measurements

Abstract The Pan-Third Pole (PTP), stretching from Eastern Asia to Middle-central Europe, has experienced unprecedented accelerated warming and even retreat of glaciers. Absorbing aerosols reduce snow and ice albedo and radiative forcing, consequently enhancing a great melting of snow cover and ice sheet in the PTP. Employing the 10-year (2007–2016) space-based active and passive measurements, this study investigated the distribution, optical properties and decadal trends for dominating aerosols at a seasonal scale in the PTP divided into six sub-regions. Results showed that the sub-regions of PTP were mainly dominated by dust, polluted dust and elevated smoke. The Taklimakan Desert (TD) and the Iranian Plateau (IP) were dominated by mineral dust, accounting for 96% and 86% of the total aerosol extinction while the Central Europe (CE), Indo-China (IC) and Anatolia Plateau (AP) were dominated by the mixture of the dominating aerosol types. The mean aerosol extinction coefficient (MAEC) showed an obvious variability depending on the sub-regions and a tendency of decreasing with an increase in the topographic height. The strongest extinction layer (>0.1 km−1) mainly occurred below 4 km and the weak extinction layers (>0.001 km−1) were mainly distributed between 5 km and 8 km, indicating pronounced vertical transport in the region. The decadal trends of columnar aerosol optical depth (AOD) showed a relation with the contributions of the dominating aerosol types. For example, significant upward or downward trends of total aerosol loading in the IC region were driven by elevated smoke while the AOD trends of total aerosol loading for the CE, the AP and the IP were driven by the dominating aerosol types. The Tibetan Plateau (TP), the cleanest region in the PTP, has been regularly exposed to polluted air masses with significant amounts of absorbing aerosols. Therefore, understanding the dominating aerosol types, properties and decadal trends in the PTP region will contribute considerably to assessing their effects on radiative forcing, climate change, and even snowmelt and glacier retreat.

Validation of OMI HCHO Products Using MAX-DOAS observations from 2010 to 2016 in Xianghe, Beijing: Investigation of the Effects of Aerosols on Satellite Products

Formaldehyde (HCHO) is one of the most abundant hydrocarbons in the atmosphere. Its absorption features in the 320–360 nm range allow its concentration in the atmosphere to be retrieved from space. There are two versions of HCHO datasets derived from the Ozone Monitoring Instrument (OMI)—one provided by the Royal Belgian Institute for Space Aeronomy (BIRA-IASB) and one provided by the National Aeronautics and Space Administration (NASA)—referred to as OMI-BIRA and OMI-NASA, respectively. We conducted daily comparisons of OMI-BIRA and multi-axis differential optical absorption spectrometry (MAX-DOAS), OMI-NASA and MAX-DOAS, and OMI-BIRA and OMI-NASA and monthly comparisons of OMI-BIRA and MAX-DOAS and OMI-NASA and MAX-DOAS. Daily comparisons showed a strong impact of effective cloud fraction (eCF), and correlations were better for eCF < 0.1 than for eCF < 0.3. By contrast, the monthly and multi-year monthly mean values yielded correlations of R2 = 0.60 and R2 = 0.95, respectively, for OMI-BIRA and MAX-DOAS, and R2 = 0.45 and R2 = 0.78 for OMI-NASA and MAX-DOAS, respectively. Therefore, use of the monthly mean HCHO datasets is strongly recommended. We conducted a sensitivity test for HCHO air mass factor (AMF) calculations with respect to the HCHO profile, the aerosol extinction coefficient (AEC), the HCHO profile–AEC combination, the aerosol optical depth (AOD), and the single scattering albedo (SSA) to explicitly account for the aerosol optical effects on the HCHO AMF. We found that the combination of AEC and HCHO profiles can account for 23–39% of the HCHO AMF variation. Furthermore, a high load of absorptive aerosols can exert a considerable effect (−53%) on the AMF. Finally, we used the HCHO monthly mean profiles from Goddard Earth Observing System coupled to Chemistry (GEOS-Chem), seasonal mean AECs from Cloud-Aerosol LIDAR with Orthogonal Polarization (CALIOP) and monthly climatologies of AOD and SSA from the OMAERUV (OMI level-2 near UV aerosol data product) dataset at Xianghe station to determine the aerosol correction. The results reveal that aerosols can account for +6.37% to +20.7% of the HCHO monthly change. However, the changes are greatest in winter and are weaker in summer and autumn, indicating that the aerosol correction is more applicable under high-AAOD conditions and that there may be other reasons for the significant underestimation between satellite and MAX-DOAS observations.

Global trends of columnar and vertically distributed properties of aerosols with emphasis on dust, polluted dust and smoke - inferences from 10-year long CALIOP observations

In this paper, the decadal datasets available from the space-borne lidar, Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) onboard Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) are analyzed in order to understand the spatial and vertical distributions of aerosols at a global scale. Decadal (September 2006–August 2016) Level 3 night time Standard All-sky data products, created using quality controlled Level 2 datasets are utilized in the study. Columnar aerosol properties are discussed in conjugation with vertical properties of the three major aerosol types i.e. dust, polluted dust and smoke. It is found that the global mean aerosol optical depth (AOD) amounts to 0.10, with AOD values of 0.13 and 0.09 for land and ocean respectively, during the last decade. Mean aerosol extinction coefficients are also reported for four altitude bins i.e. 0–1 km, 1–2 km, 2–3 km and 3–4 km above mean sea level (a.m.s.l.). It is observed that dust and smoke particles have stronger vertical transport compared to polluted dust particles, which are generally found to be confined within the lower altitudes. In order to observe the changes in aerosol loading over different parts of the globe, de-seasonalized trends of AOD and vertical extinction coefficients are computed using linear regression technique. Globally, the total columnar particle load shows a significant negative trend during the last decade. In particular, trends of total particle loading (AOD and extinction) are decreasing over North America, South America, East China and Australia. However, the smoke emissions (columnar and vertically distributed) have increased over India, Russia, North America, Australia and Central Africa. This is of concern in the context of rising temperatures accompanied with increasing wildfires throughout the globe. Additionally, enhanced levels of polluted dust over the developing regions of Africa and Asia are also revealed.

The surface aerosol optical properties in the urban area of Nanjing, west Yangtze River Delta, China

Abstract. Observational studies of aerosol optical properties are useful for reducing uncertainties in estimations of aerosol radiative forcing and forecasting visibility. In this study, the observed near-surface aerosol optical properties in urban Nanjing are analysed from March 2014 to February 2016. Results show that near-surface urban aerosols in Nanjing are mainly from local emissions and the surrounding regions. They have lower loadings but are more scattering than aerosols in most cities in China. The annual mean aerosol extinction coefficient (EC), single-scattering albedo (SSA) and asymmetry parameter (ASP) at 550 nm are 381.96 Mm−1, 0.9 and 0.57, respectively. The aerosol absorption coefficient (AAC) is about 1 order of magnitude smaller than its scattering coefficient (SC). However, the absorbing aerosol has a larger Ångström exponent (AAE) value, 1.58 at 470∕660 nm, about 0.2 larger than the scattering aerosols (SAE). All the aerosol optical properties follow a near-unimodal pattern, and their values are mostly concentrated around their averages, accounting for more than 60 % of the total samplings. Additionally, they have substantial seasonality and diurnal variations. High levels of SC and AAC all appear in winter due to higher aerosol and trace gas emissions. AAE (ASP) is the smallest (largest) in summer, possibly because of high relative humidity (RH) which also causes considerably larger SC and smaller SAE, although intensive gas-to-particle transformation could produce a large number of finer scattering aerosols in this season. Seasonality of EC is different from the columnar aerosol optical depth. Larger AACs appear during the rush hours of the day while SC and back-scattering coefficient (Bsp) only peak in the early morning. Aerosols are fresher in the daytime than at night-time, leading to their larger Ångström exponent and smaller ASP. Different temporal variations between AAC and SC cause the aerosols to be more absorbing (smaller SSA) in autumn, winter and around rush hours. ASP has a good quasi-log-normal growth trend with increasing SC when RH is below 60 %. The correlation between AAC and SC at the site is close but a little smaller than that in suburban Nanjing in spring. Atmospheric visibility decreases exponentially with increasing EC or SC, more sharply in spring and summer, and it could be further deteriorated with increasing SSA and ASP.

Transport, vertical structure and radiative properties of dust events in southeast China determined from ground and space sensors

Two dust events were detected over the Yangtze Delta region of China during March 14–17 and April 25–26 in 2009 where such dust events are uncommon. The transport behavior, spatio-temporal evolution, vertical structure, direct radiative effects, as well as induced heating rates, are investigated using a combination of ground-based and satellite-based measurements, a back-trajectory analysis, an aerosol model and a radiative transfer model. Back-trajectories, wind fields and aerosol model analyses show that the first dust originated in northern/northwestern China and the second generated in the Taklimakan desert in northwest China, and traveled across the Hexi corridor and Loess Plateau to the Yangtze Delta region (the so-called “dust corridor”). The mean lidar extinction-to-backscatter ratio (LR) during the two dust events was 38.7 ± 10.4 sr and 42.7 ± 15.2 sr, respectively. The mean aerosol depolarization ratio (δa) for the first dust event was 0.16 ± 0.07, with a maximum value of 0.32. For the second, the mean δa was around 0.19 ± 0.06, with a maximum value of 0.29. Aerosol extinction coefficient and δa profiles for the two events were similar: two aerosol layers consisting of dust aerosols and a mixture of dust and anthropogenic pollution aerosols. The topmost aerosol layer is above 3.5 km. The maximum mean aerosol extinction coefficients were 0.5 km−1 and 0.54 km−1 at about 0.7 km and 1.1 km, respectively. Significant effects of cooling at the surface and heating in the atmosphere were found during these dust events. Diurnal mean shortwave radiative forcings (efficiencies) at the surface, the top-of-the-atmosphere and within the atmosphere were −36.8 (−80.0), −13.6 (−29.6) and 23.2 (50.4) W m−2, respectively, during the first dust event, and −48.2 (−70.9), −21.4 (−31.5) and 26.8 (39.4) W m−2, respectively, during the second dust event. Maximum heating rates occurred at 0.7 km during the first dust event and at 1.1 km during the second dust event, with a maximum value of 2.74 K day−1 for each case. This significant atmospheric heating induced by elevated dust aerosol layers can affect convection and stability in the lower troposphere.

Synergy between CALIOP and MODIS instruments for aerosol monitoring: application to the Po Valley

Abstract. In this study aerosol optical properties are studied over the Po Valley from June 2006 to February 2009 using Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations/Cloud-Aerosol LIdar with Orthogonal Polarization (CALIPSO/CALIOP) and Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Aqua and Terra. The choice of the Po valley has been driven by the numerous occurrences of pollutant events leading to a mean MODIS-derived aerosol optical depth (AOD) of 0.27 (±0.17) at 550 nm over a large area of ~120 000 km2. AOD derived from MODIS, AERONET and CALIOP have been compared. The comparison with AERONET sun-photometers has highlighted an overestimation of AOD from MODIS radiometers of 0.047 for Aqua and 0.088 for Terra. A systematic underestimation of AOD derived from CALIOP Level-2 products has been observed in comparison to Aqua (0.060) and Terra (0.075) MODIS values. Considering those discrepancies a synergistic approach combining CALIOP level-1 data and MODIS AOD has been developed for the first time over land to retrieve the equivalent extinction-to-backscatter ratio at 532 nm (LR). MODIS-derived AOD were indeed used to constrain CALIOP profiles inversion. A significant number of CALIOP level-1 vertical profiles have been averaged (~200 individual laser shots) in the Po Valley, leading to a signal-to-noise ratio (SNR) higher than 10 in the planetary boundary layer (PBL), which is sufficient to invert the mean lidar profiles. The mean LR (together with the associated variabilities) over the Po Valley retrieved from the coupling between CALIOP/MODIS-Aqua and CALIOP/MODIS-Terra are ~78±22 sr and ~86±27 sr, respectively. The total uncertainty on LR retrieval has been assessed to be ~12 sr using a Monte Carlo approach. The mean LR determined from a look-up table through a selection algorithm in CALIOP level 2 operational products (~63±8 sr) show a good agreement for daytime inversion (70±11 sr for Aqua and 74±14 sr for Terra). These values appear close to what is expected for pollution aerosols in an urban area. Contrarily large differences are observed when considering nighttime CALIOP profiles inverted with daytime AOD from MODIS (63±7 sr for CALIOP level-2 compared with 89±28 sr for CALIOP/Aqua and 103±32 sr for CALIOP/Terra synergies). They can be explained by a significant evolution of AOD between lidar and radiometer passing times. In most of cases, the mean aerosol extinction coefficient in the PBL significantly differs between the level-2 operational products and the result CALIPSO/MODIS synergy results. Mean differences of 0.10 km−1 (~50%) and 0.13 km−1 (~60%) have indeed been calculated using MODIS-Aqua/CALIOP and MODIS-Terra/CALIOP coupling studies, respectively. Such differences may be due to the identification of the aerosol model by the operational algorithm and thus to the choice of the LR.

Enhancement of Aerosol Concentration in Korea due to the Northeast Asian Forest Fire in May 2003

Enhancement of aerosol optical thickness (AOT) and surface aerosol mass concentration in Korea for an active forest fire episode in Northeast Asia were estimated by Community Multi-scale Air Quality (CMAQ) model. MODIS/TERRA remote detects of fires in Northeast Asia for May 2003 gave a constraint for estimation of wildfire emissions with an NDVI distribution for recent five years. The simulated wildfire plumes and enhancement of AOT were evaluated and well resolved by comparing multiple satellite observations such as MODIS, TOMS, and others. Scatter plots of observed daily mean aerosol extinction coefficient versus PM 10 concentration in ground level in Korea showed distinctively different trends based on the ambient relative humidity.

Influence of surface albedo on the change in the atmospheric radiation balance due to aerosols

Using an initial value radiative-convective atmospheric model we show that atmospheric aerosols can produce either heating or cooling, depending on the surface albedo. The model, a modification of that of Manabe, Strickler and Weatherald, consists of 9 points vertically and includes effects of CO 2, H 2O, O 3, low-lying aerosols and three layers of clouds. Both solar and terrestrial fluxes are calculated. Absolute humidity is assumed to be constant. For input parameters characteristic of 35°N latitude in April and surface short wave albedos, ω s , of 0.07 and 0.3, the model predicts cooling due to aerosols. However, when ω s is increased to 0.6, heating is predicted. A plot of surface temperature change, ΔT s , vs mean aerosol extinction coefficient in the visible per kilometer, σ vis, (for ω s = 0.3) has a minimum at σ vis ≈ 3. Similar plots for ω s = 0.07 and 0.6 show a leveling-off of ΔT s with increasing σ vis .