Layer Aerosol Optical Depth Research Articles

Characterization of Tajogaite volcanic plumes detected over the Iberian Peninsula from a set of satellite and ground-based remote sensing instrumentation

Three volcanic plumes were detected during the Tajogaite volcano eruptive activity (Canary Islands, Spain, September–December 2021) over the Iberian Peninsula. The spatiotemporal evolution of these events is characterised by combining passive satellite remote sensing and ground-based lidar and sun-photometer systems. The inversion algorithm GRASP is used with a suite of ground-based remote sensing instruments such as lidar/ceilometer and sun-photometer from eight sites at different locations throughout the Iberian Peninsula. Satellite observations showed that the volcanic ash plumes remained nearby the Canary Islands covering a mean area of 120 ± 202 km2 during the whole period of eruptive activity and that sulphur dioxide plumes reached the Iberian Peninsula. Remote sensing observations showed that the three events were mainly composed of sulphates, which were transported from the volcano into the free troposphere. The high backscatter-related Ångström exponents for wavelengths 532–1064 nm (1.17 ± 0.20 to 1.40 ± 0.24) and low particle depolarization ratios (0.08 ± 0.02 to 0.09 ± 0.02), measured by the multi-wavelength Raman lidar, hinted at the presence of spherical small particles. The layer aerosol optical depth at 532 nm (AODL532) obtained from lidar measurements contributed between 49% and 82% to the AERONET total column AOD at 532 nm in event II (11–13 October). According to the GRASP retrievals, the layer aerosol optical depth at 440 nm (AODL440) was higher in all sites during event II with values between 0.097 (Badajoz) and 0.233 (Guadiana-UGR) and lower in event III (19–21 October) varying between 0.003 (Granada) and 0.026 (Évora). Compared with the GRASP retrievals of total column AOD at 440 nm, the AODL440 had contributions between 21% and 52% during event II. In the event I (25–28 September), the mean volume concentrations (VC) varied between 5 ± 4 μm3cm−3 (El-Arenosillo/Huelva) and 17 ± 10 μm3cm−3 (Guadiana-UGR), while in event II this variation was from 11 ± 7 μm3cm−3 (Badajoz) to 27 ± 10 μm3cm−3 (Guadiana-UGR). Due to the impact of volcanic events on atmospheric and economic fields, such as radiative forcing and airspace security, a proper characterization is required. This work undertakes it using advanced instrumentation and methods.

A global evaluation of daily to seasonal aerosol and water vapor relationships using a combination of AERONET and NAAPS reanalysis data

Abstract. The co-transport of aerosol particles and water vapor has long been noted in the literature, with a myriad of implications such as air mass characterization, radiative transfer, and data assimilation. Here, the relationship between aerosol optical depth (AOD) and precipitable water vapor (PW) is evaluated to our knowledge for the first time globally, at daily to seasonal levels using approximately 20 years of NASA Aerosol Robotic Network (AERONET) observational data and the 16-year Navy Aerosol Analysis Prediction System (NAAPS) reanalysis v1.0 (NAAPS-RA) model fields. The combination of AERONET observations with small uncertainties and the reanalysis fields with global coverage is used to provide a best estimate of the seasonal AOD and PW relationships, including an evaluation of correlations, slope, and PW probability distributions for identification of statistically significant differences in PW for high-AOD events. The relationships produced from the AERONET and NAAPS-RA datasets were compared against each other and showed consistency, indicating that the NAAPS-RA provides a realistic representation of the AOD and PW relationship. The analysis includes layer AOD and PW relationships for proxies of the planetary boundary layer and the lower, middle, and upper free troposphere. The dominant AOD and PW relationship is positive, supported by both AERONET and model evaluation, which varies in strength by season and location. These relationships were found to be statistically significant and present across the globe, observed on an event-by-event level. Evaluations at individual AERONET sites implicate synoptic-scale transport as a contributing factor in these relationships at daily levels. Negative AOD and PW relationships were identified and predominantly associated with regional dry-season timescales in which biomass burning is the predominant aerosol type. This is not an indication of dry-air association with smoke for an individual event but is a reflection of the overall dry conditions leading to more biomass burning and higher associated AOD values. Stronger correlations between AOD and PW are found when evaluating the data by vertical layers, including the boundary layer and the lower, middle, and upper free troposphere (corresponding to typical water vapor channels), with the largest correlations observed in the free troposphere – indicative of aerosol and water vapor transport events. By evaluating the variability between PW and relative humidity in the NAAPS-RA, hygroscopic growth was found to be a dominant term to (1) amplify positive AOD–PW relationships, particularly in the midlatitudes; (2) diminish negative relationships in dominant biomass burning regions; and (3) lead to statistically insignificant changes in PW for high-AOD events for maritime regions. The importance of hygroscopic growth in these relationships indicates that PW is a useful tracer for AOD or light extinction but not necessarily as strongly for aerosol mass. Synoptic-scale African dust events are an exception where PW is a strong tracer for aerosol transport shown by strong relationships even with hygroscopic effects. Given these results, PW can be exploited in coupled aerosol and meteorology data assimilation for AOD, and the collocation of aerosol and water vapor should be carefully taken into account when conducting particulate matter (PM) retrievals from space and in evaluating radiative impacts of aerosol, with the season and location in mind.

Obtaining vertical distribution of PM2.5 from CALIOP data and machine learning algorithms

Aerosol optical depth (AOD) has been widely used to estimate the near-surface PM2.5 (fine particulate matter with particle size less than 2.5 μm). However, the total-column AOD obtained by passive remote sensing instruments can neither distinguish the contribution of AOD in various altitude layers nor obtain the vertical PM2.5 concentration. In this study, we compared several AOD-PM2.5 models including Extra Trees (ET), Random Forest (RF), Deep Neural Network (DNN), and Gradient Boosting Regression Tree (GBRT), and analyzed the corresponding results using AOD of different altitudes and auxiliary data from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). The results indicate that the ET model performs best in terms of the model effectiveness and feature interpretation on the training dataset. We conclude that the feature importance of the bottom layer AOD is higher than that of the upper and total column AOD. The results showed that regional differences existed in the optimal height of the AOD-PM2.5 correlation in study area. The results of cross-validation indicate that ET manages the most appealing overall performance with an R2 (RMSE) of 0.85 (17.77 μg/m3). Regarding the 729 sites involved in this study, 73% had R2 > 0.7, and the region or season with higher AOD feature importance achieves better model performance. The results of the AOD-PM2.5 model in each layer were corrected using the AOD weight, to obtained the PM2.5 vertical concentrations from 2015 to 2019. The results highlight that the high PM2.5 concentration area is primarily near the ground and decreases with height. Additionally, the PM2.5 vertical concentration in Beijing-Tianjin-Hebei (−1.80 μg/m3, P < 0.001), Central China (−1.62 μg/m3, P < 0.001), and Pearl River Delta (−0.66 μg/m3, P < 0.001) show an apparent downward trend. We believe that the vertical distribution analysis of PM2.5 can provide meaningful information for studying air pollution.

An interpretable self-adaptive deep neural network for estimating daily spatially-continuous PM2.5 concentrations across China

Accurate estimation of daily spatially-continuous PM2.5 (fine particulate matter) concentration is a prerequisite to address environmental public health issues, and satellite-based aerosol optical depth (AOD) products have been widely used to estimate PM2.5 concentrations using statistical-based or machine learning-based models. However, statistical-based models oversimplify the AOD-PM2.5 relationships, whereas complex machine learning technologies ignore the spatiotemporal heterogeneity of the predictors and demonstrate shortage in interpretation. Besides, large AOD data gaps resulting in PM2.5 estimation biases have been seldom imputed in previous studies, especially at national scales. To fill the above research gaps, this study attempts to present a feasible methodology to estimate daily spatially-continuous PM2.5 concentrations in China. The AOD data gaps across China were first imputed via a random forest (RF) model. Then, an interpretable self-adaptive deep neural network (SADNN) model, incorporating AOD, meteorological and other auxiliary predictors, was developed to estimate daily spatially-continuous PM2.5 concentrations from 2017 to 2018. Five-fold sample (site)-based cross-validation results showed a high accuracy of the SADNN model, with coefficient of determination and root mean square error values equal to 0.86 (0.84) and 13.07 (14.30) μg/m3, respectively, outperforming the standard DNN and the RF model. Furthermore, the SADNN model identified the spatiotemporal patterns of predictor importance, and demonstrated that the boundary layer height, elevation and AOD were the most important predictors both spatially and temporally. And the predictor importance in the Qinghai-Tibet Plateau was different from that in the rest of China. These results enhance our understanding of AOD-PM2.5 relationships and elucidate the estimated PM2.5 datasets with complete coverage are applicable for related air pollution studies and epidemiological cohort studies. Moreover, considering the effective nonlinear model capability and interpretability, the SADNN model is beneficial for not only PM2.5 estimation but also other earth data and scenarios.

Comparison of MODIS- and CALIPSO-Derived Temporal Aerosol Optical Depth over Yellow River Basin (China) from 2007 to 2015

In this study, Collection 6.1 (C6.1) of different aerosol optical depth (AOD) products of different spatial resolutions were used from the aqua moderate resolution imaging spectroradiometer (MODIS) including dark target (DT), deep blue (DB), deep blue (DB), and DT-DB (DTB). These products were compared with cloud-aerosol lidar, and infrared pathfinder satellite observation (CALIPSO) AOD retrievals over the Yellow River Basin (YERB), China from 2003 to 2017. The YERB was divided into three sub-regions, namely YERB1 (the mountainous terrain in the upper reaches of the YERB), YERB2 (the Loess Plateau region in the middle reaches of the YERB), and YERB3 (the plain region downstream of the YERB). Errors and agreement between MODIS and CALIPSO data were reported using Pearson’s correlation (R) and relative mean bias (RMB). Results showed that the CALIPSO whole layers AOD (AODS) were better matched with MODIS AOD than the CALIPSO lowest layer AOD (AOD1). The time series of AOD shows higher values in spring and summer, and a small difference in AOD products was observed in autumn. The overall average value of CALIPSO AOD and MODIS AOD both fitted the order: YERB3 > YERB2 > YERB1. The CALIPSO AOD retrievals have the best consistency with the DTB10K and the lowest consistency with DT3K. Overall, the regional distributions of the CALIPSO AOD and MODIS AOD are significantly different over the YERB, and the difference is closely related to the season, region, and topography. This study can help researchers understand the difference of aerosol temporal and spatial distribution utilizing different satellite products over YERB, and also can provide data and technical support for the government in atmospheric environmental governance over YERB.

Variation of the vertical distribution of Nabro volcano aerosol layers in the stratosphere observed by LIDAR

We present results of the vertical distribution variation of volcanic aerosol layers in the upper troposphere and lower stratosphere. The data were taken with our multiwavelength aerosol Raman lidar at Gwangju (35.10° N, 126.53° E), Korea. The volcanic ash particles and gases were released around 12 June 2011 during the eruption of the Nabro volcano (13.37° N, 41.7° E) in Eritrea, east Africa. Forward trajectory computations show that the volcanic aerosols were advected from North Africa to East Asia. The first measurement of the aerosol layer over Korea was on 19 June 2011. The aerosol layers appeared between 15 km and 17 km height asl (above sea level). The maximum value of the aerosol layer of the particle backscatter coefficient (1.5 ± 0.3 Mm−1 sr−1) and the linear particle depolarization ratio at 532 nm (2.2%) were observed at 16.4 km height asl. We continuously probed the upper troposphere and lower stratosphere for this volcanic aerosol layer during the following 6 months, until December 2011. The volcanic aerosol layer showed a single-peak of the particle backscatter coefficient and a comparably narrow vertical thickness at our observation site at the beginning of our observation period (i.e. comparably soon after the initial eruption period). After that initial period the vertical distribution of the plume changed. Multiple peaks and a comparably broad geometrical thickness developed with progressing observation time. The vertical thickness of the volcanic aerosol layer expanded up to 10 km by 3 August 2011. The linear particle depolarization ratios were larger in the lower part of the aerosol layer than the upper part of the aerosol layer. We observed a strong variation of the AOD (aerosol optical depth) in the first two months of our lidar observations. After these two months the AOD gradually decreased with time from September to December 20111 and the maximum particle backscatter coefficients consistently decreased. The corresponding e-folding decay time of the layer AOD was 117 days.

Nabro aerosol evolution observed jointly by lidars at a mid-latitude site and CALIPSO

Evolution of the Nabro volcanic aerosols from initially-localized plumes to a decaying hemispherically-covered layer was jointly observed by ground-based lidars at Wuhan (30.5°N, 114.4°E), China and CALIPSO. During the aerosol plume formation period, from the Nabro eruption to early July 2011, the lidar backscatter ratio related to the Nabro aerosols above Wuhan varied strongly both in vertical structure and intensity, suggesting that the Nabro aerosol distribution was horizontally inhomogeneous. The stratospheric aerosol optical depth (AOD) from CALIPSO shows that the Nabro plume first circled around the Asian monsoon anticyclone and then gradually fulfilled the whole anticyclone area with a net aerosol enhancement, which may reflect a gas-particle conversion (from sulfur dioxide gas) and/or particle injection from the upper troposphere. During the horizontal dispersion period, from early July to mid-August 2011, the stratospheric AOD over Wuhan declined rapidly since the Nabro particles were transported throughout the northern hemisphere. A nearly horizontally-uniform volcanic aerosol layer was formed. During the local cleansing period, from mid-August to the end of 2011, the Nabro aerosol layer over Wuhan had a single-peak structure and decayed uniformly. The corresponding e-folding decay time for the layer AOD is ∼130 days. The lidar measurements at Wuhan gave a small depolarization ratio and large backscatter-related Ångström exponent for the Nabro aerosols on 8 July, suggesting that the majority of these aerosols were spherical and small. The effective radius and total mass for the Nabro aerosol particles were estimated to be ∼0.26 μm and ∼0.32 Tg respectively.

Looking through the haze: evaluating the CALIPSO level 2 aerosol optical depth using airborne high spectral resolution lidar data

Abstract. The Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument onboard the Cloud–Aerosol Lidar and Pathfinder Satellite Observations (CALIPSO) spacecraft has provided over 8 yr of nearly continuous vertical profiling of Earth's atmosphere. In this paper we investigate the V3.01 and V3.02 CALIOP 532 nm aerosol layer optical depth (AOD) product (i.e the AOD of individual layers) and the column AOD product (i.e., the sum AOD of the complete column) using an extensive database of coincident measurements. The CALIOP AOD measurements and AOD uncertainty estimates are compared with collocated AOD measurements collected with the NASA High Spectral Resolution Lidar (HSRL) in the North American and Caribbean regions. In addition, the CALIOP aerosol lidar ratios are investigated using the HSRL measurements. In general, compared with the HSRL values, the CALIOP layer AOD are biased high by less than 50% for AOD &lt; 0.3 with higher errors for higher AOD. Less than 60% of the HSRL AOD measurements are encompassed within the CALIOP layer 1 SD uncertainty range (around the CALIOP layer AOD), so an error estimate is created to encompass 68% of the HSRL data. Using this new metric, the CALIOP layer AOD error is estimated using the HSRL layer AOD as ±0.035 ± 0.05 · (HSRL layer AOD) at night and ±0.05 ± 0.05 · (HSRL layer AOD) during the daytime. Furthermore, the CALIOP layer AOD error is found to correlate with aerosol loading as well as aerosol subtype, with the AODs in marine and dust layers agreeing most closely with the HSRL values. The lidar ratios used by CALIOP for polluted dust, polluted continental, and biomass burning layers are larger than the values measured by the HSRL in the CALIOP layers, and therefore the AODs for these types retrieved by CALIOP were generally too large. We estimated the CALIOP column AOD error can be expressed as ±0.05 ± 0.07 · (HSRL column AOD) at night and ±0.08 ± 0.1 · (HSRL column AOD) during the daytime. Multiple sources of error contribute to both positive and negative errors in the CALIOP column AOD, including multiple layers in the column of different aerosol types, lidar ratio errors, cloud misclassification, and undetected aerosol layers. The undetected layers were further investigated and we found that the layer detection algorithm works well at night, although undetected aerosols in the free troposphere introduce a mean underestimate of 0.02 in the column AOD in the data set examined. The decreased signal-to-noise ratio (SNR) during the daytime led to poorer performance of the layer detection. This caused the daytime CALIOP column AOD to be less accurate than during the nighttime, because CALIOP frequently does not detect optically thin aerosol layers with AOD &lt; 0.1. Given that the median vertical extent of aerosol detected within any column was 1.6 km during the nighttime and 1.5 km during the daytime, we can estimate the minimum extinction detection threshold to be 0.012 km−1 at night and 0.067 km−1 during the daytime in a layer median sense. This extensive validation of level 2 CALIOP AOD products extends previous validation studies to nighttime lighting conditions and provides independent measurements of the lidar ratio; thus, allowing the assessment of the effect on the CALIOP AOD of using inappropriate lidar ratio values in the extinction retrieval.

Satellite Retrievals of Aerosol Optical Depth over a Subtropical Urban Area: The Role of Stratification and Surface Reflectance

ABSTRACTWe explore the relationship between satellite retrievals of aerosol optical depth (AOD) and surface aerosol mass concentrations over a subtropical urban area, namely, Santiago, Chile (33.5°S, 70.6°W, 500 m.a.s.l.). We compare 11 years of AOD from the MODerate resolution Imaging Spectroradiometer (MODIS) with in situ particulate matter mass concentrations (PM). MODIS AOD reaches its maximum in summer and minimum in winter, the opposite of the annual cycle of surface PM. To improve our understanding of the relevant governing processes, we use a simple model that estimates the boundary layer (BL) AOD based on measured PM, relative humidity and BL height (BLH) as well as best estimates of aerosol composition, size distribution, and optical properties. Model results indicate that a weak annual AOD cycle is due to the opposite annual cycles in BLH and PM, which is largely supported by the Aerosol Robotic NETwork (AERONET) data collected in 2001 and 2002 in Santiago. We identify a possible bias linked to the operational estimate of surface reflectance that may lead to a spurious summer maximum in MODIS AOD over Santiago. This misfit in surface reflectance appears to affect not only Santiago but also a significant area of the semi-arid Southern South America. Sensitivity experiments with the simple model indicate an underestimate of simulated AOD as compared to AERONET data. This underestimate points to the possible role of residual aerosol layers in the AOD measured at the surface (not included in the simple model). Cirrus clouds appear not to play a significant role in explaining the MODIS AOD seasonality. The need for improved characterizations of aerosol properties and their temporal and spatial distribution in cities such as Santiago is emphasized.

Relationship between wind speed and sea salt aerosol production: A new approach

A relationship between ocean surface wind speed and sea salt aerosol production is established through a study carried out using the aerosol flux continuity equation by introducing satellite data on aerosols and ocean surface wind speed into it. This mathematical approach eliminates interferences from background aerosols and aerosol variations due to advection and convergences/divergences in wind field and correctly estimates the rate of sea salt aerosol production by winds. To avoid oceanic biogenic sources and transport from other oceanic and land regions, the study is done over ocean sites substantially remote from the continents with low chlorophyll concentrations (<0.06mg/m3) and restricting to aerosols in the bottom layer (<0.5km altitude) of the atmosphere. The surface wind speed is found to correlate better with the estimated aerosol production rate (RWS=0.99, p<0.0001) than with the bottom layer aerosol optical depth (RWB=0.97, p<0.0001). Aerosol production is observed at wind speeds even below 4m/s and the production rate is found to follow a linear relationship with ocean surface wind speed with a slope 0.0053 and an intercept 0.0163 for low as well as high winds.

Aerosol, Ozone and Water Vapour Distributions Observed over Land and Sea Regions during INDOEX Field Phases

Extensive land- and ocean-based experiments involving active and passive remote sensing of aerosols and minor constituents have been conducted simultaneously over Pune and over the Indian Ocean and Arabian Sea during the INDian Ocean EXperiment (INDOEX) field phases with an aim to study the interface between the polluted continental air-mass and pristine marine air-mass, and the underlying circulat ion and Inter Tropical Convergence Zone (ITCZ) phenomena, which mixes the polluted air mass from Northern hemisphere with the pristine air mass from the Southern hemisphere. In this communicat ion, we report some important findings of the special observational programs which were carried out during the First Field Phase (FFP) i.e. fro m 17 February to 31 March 1998 and Intense Field Phase (IFP) i.e. fro m 1 January to 31 March 1999 of the INDOEX. These programs involve synchronous measurements of aerosol, ozone and precipitable water vapor using ground-based lidar, sunphotometer / spectroradiometer and ozone / water vapor monitor at the Indian Institute of Tropical Meteorology (IITM ), Pune, India, and aerosol characterization over the Indian Ocean and Arabian Sea utilizing a mult i-channel solar radio meter installed onboard Ocean Research Vessel (ORV) Sagar Kanya ship. Besides significant deviations between aerosol features over the land and oceanic regions, the lidar derived aerosol colu mn content (ACC) in the boundary layer, and radio meter derived height-integrated aerosol optical depth (AOD) show significant trend and characteristic changes in aerosol size distribution over land during both FFP and IFP. Moreover, the IFP measurements indicate higher aerosol ext inction (particularly in the initial period) as co mpared to those of FFP. On some experimental days, the land AODs appear to be of the magnitude similar to those of marine AODs observed close to the Indian subcontinent and adjoining South Asian regions. The variations in AOD over land also show an association with those observed over ocean, particularly near the coast. Besides an increasing trend in total colu mn ozone throughout the IFP, it exhib its a specific relat ionship with those observed in AOD and water vapor over land. Co mparison between the boundary layer and total column AODs reveals that the observed increasing trend in AOD is not confined to the boundary layer alone but extends to higher altitudes with significant contribution fro m the former to the latter.

Analysis of aerosol characteristics and their relationships with meteorological parameters over Anhui province in China

The characteristics of Aerosol Optical Depth (AOD) and Ångström exponent are analyzed and AODs are compared with CE318 measurements from 2001 to 2009 over Anhui province in China, with an evaluation of potential contributing factors, including regional burning emissions and meteorology. The result indicates that MODIS AODs (Collection 5, C005) are in good agreement with those from CE318 with correlation coefficient of above 0.80 and RMS of below 0.21 in Anhui. Most of MODIS cases fall in the range of ∆τ=±0.05±0.20τ at four AERONET sites. Aerosol optical properties show a notable spatial–temporal distribution feature in Anhui. The values of lower AOD and higher Ångström exponent both appear at southern mountain area. We document seasonal variations in patterns of AOD and Ångström exponent in Anhui, for which peak values respectively occur in June and August. Time series plots for AOD and fire pixel counts are produced to provide a better understanding of influence of burning emission for aerosol. Agricultural residue burning occurs seriously in June, which leads to appearance of AOD peak values. The results between AODs and meteorological parameters show good relationships at Fuyang. Wind speed and wind direction can strongly influence aerosol optical depth and its size, meanwhile, relative humidity and visibility respectively have positive and negative correlation with AODs. Mixed layer depths and AODs have similar monthly variation trend. We use HYSPLIT model for trajectory analysis to reveal the origins of air masses for different levels AODs and Ångström exponent, with the aim of discriminating contributions of different sources.

Saharan dust infrared optical depth and altitude retrieved from AIRS: a focus over North Atlantic – comparison to MODIS and CALIPSO

Abstract. Monthly mean infrared (10 μm) dust layer aerosol optical depth (AOD) and mean altitude are simultaneously retrieved over the tropics (30° S–30° N) from almost seven years of Atmospheric Infrared Sounder (AIRS) observations covering the period January 2003 to September 2009. The method developed relies on the construction of look-up-tables computed for a large selection of atmospheric situations and follows two main steps: first, determination of the observed atmospheric thermodynamic situation and, second, determination of the dust properties. A very good agreement is found between AIRS-retrieved AODs and visible optical depths from the Moderate resolution Imaging Spectroradiometer (MODIS/Aqua) during the main (summer) dust season, in particular for three regions of the tropical North Atlantic and one region of the north-western Indian Ocean. Outside this season, differences are mostly due to the sensitivity of MODIS to aerosol species other than dust and to the more specific sensitivity of AIRS to the dust coarse mode. AIRS-retrieved dust layer mean altitudes are compared to the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP/CALIPSO) aerosol mean layer altitude for the period June 2006 to June 2009. Results for a region of the north tropical Atlantic downwind of the Sahara show a good agreement between the two products (σ≈360 m). Differences observed in the peak-to-trough seasonal amplitude, smaller from AIRS, are principally attributed to the large difference in spatial sampling of the two instruments. They also come from the intrinsic limit in sensitivity of the passive infrared sounders for low altitudes. These results demonstrate the capability of high resolution infrared sounders to measure not only dust aerosol AOD but also the mean dust layer altitude.

Clear‐column closure studies of aerosols and water vapor aboard the NCAR C‐130 during ACE‐Asia, 2001

Column closure studies are a tool to assess whether in situ and remote measurements of aerosol optical properties on a given aircraft are mutually consistent. In this paper we describe aerosol and water vapor column closure studies on the basis of instrumentation flown aboard the NCAR C‐130 aircraft in the ACE‐Asia field experiment in March–May 2001. For in situ observations, aerosol particles were sampled through a newly designed low‐turbulence inlet (LTI). In 28 profiles extending to altitudes of up to 8 km, the in situ observations of scattering and absorption were compared to measurements with the six‐channel NASA Ames Airborne Tracking Sun photometer (AATS‐6). The comparison of Sun photometer and in situ‐derived layer aerosol optical depth (AOD) at 550 nm showed agreement (closure) within the measurement uncertainties in 25 out of 28 case studies. The average difference in layer AOD derived from the two methods was 0.03, corresponding to an average difference of 11.5%. The uncertainties in AATS‐6‐derived layer AOD ranged between 5 and 59% (with a mean of 22%), and for the first time included an estimate for the uncertainty in layer AOD caused by possible horizontal variability in AOD encountered in the vertical profile. The average uncertainty in AATS‐6‐derived layer AOD due to possible horizontal variability alone was 19%. The uncertainties in in situ‐derived layer AOD were between 10 and 55% (with a mean of 19%). Stratification of the extinction closure data by ambient relative humidity (RH) revealed that in situ‐derived aerosol extinction at low ambient relative humidity (&lt;20% RH) tended to be slightly less than Sun photometer‐derived aerosol extinction, while in situ‐derived aerosol extinction at higher relative humidity was slightly greater than the Sun photometer‐derived values. Stratification of the extinction closure data by the fine mode fraction of scattering indicated a modest enhancement of coarse mode extinction in the combined LTI/plumbing system. Analogous closure studies for layer water vapor and water vapor density showed that AATS‐6 measured these quantities with very high accuracy, with correlation coefficients of 0.989 and 0.955 (rms differences of 10% and 33%), respectively.

Column closure studies of lower tropospheric aerosol and water vapor during ACE‐Asia using airborne Sun photometer and airborne in situ and ship‐based lidar measurements

We assess the consistency (closure) between solar beam attenuation by aerosols and water vapor measured by airborne Sun photometry and derived from airborne in situ and ship‐based lidar measurements during the April 2001 Asian Pacific Regional Aerosol Characterization Experiment (ACE‐Asia). The airborne data presented here were obtained aboard the Twin Otter aircraft. Comparing aerosol extinction σep(550 nm) from four different techniques shows good agreement for the vertical distribution of aerosol layers. However, the level of agreement in absolute magnitude of the derived aerosol extinction varied among the aerosol layers sampled. The σep(550 nm) computed from airborne in situ size distribution and composition measurements shows good agreement with airborne Sun photometry in the marine boundary layer but is considerably lower in layers dominated by dust if the particles are assumed to be spherical. The σep(550 nm) from airborne in situ scattering and absorption measurements are about ∼13% lower than those obtained from airborne Sun photometry during 14 vertical profiles. Combining lidar and the airborne Sun photometer measurements reveals the prevalence of dust layers at altitudes up to 10 km with layer aerosol optical depth (from 3.5 to 10 km altitude) of ∼0.1 to 0.2 (500 nm) and extinction‐to‐backscatter ratios of 59–71 sr (523 nm). The airborne Sun photometer aboard the Twin Otter reveals a relatively dry atmosphere during ACE‐Asia with all water vapor columns &lt;1.5 cm and water vapor densities ρw &lt; 12 g/m3. Comparing layer water vapor amounts and ρw from the airborne Sun photometer to the same quantities measured with aircraft in situ sensors leads to a high correlation (r2 = 0.96), but the Sun photometer tends to underestimate ρw by 7%.

Clear-sky closure studies of lower tropospheric aerosol and water vapor during ACE-2 using airborne sunphotometer, airborne in-situ, space-borne, and ground-based measurements

We report on clear-sky column closure experiments (CLEARCOLUMN) performed in the Canary Islands during the second Aerosol Characterization Experiment (ACE-2) in June/July 1997. We present CLEARCOLUMN results obtained by combining airborne sunphotometer and in-situ (optical particle counter, nephelometer, and absorption photometer) measurements taken aboard the Pelican aircraft, space-borne NOAA/AVHRR data and ground-based lidar and sunphotometer measurements. During both days discussed here, vertical profiles flown in cloud-free air masses revealed 3 distinctly different layers: a marine boundary layer (MBL) with varying pollution levels, an elevated dust layer, and a very clean layer between the MBL and the dust layer. A key result of this study is the achievement of closure between extinction or layer aerosol optical depth (AOD) computed from continuous in-situ aerosol size-distributions and composition and those measured with the airborne sunphotometer. In the dust, the agreement in layer AOD (λ=380−1060 nm) is 3−8%. In the MBL there is a tendency for the in-situ results to be slightly lower than the sunphotometer measurements (10−17% at λ=525 nm), but these differences are within the combined error bars of the measurements and computations.

Comparisons between Stratospheric Aerosol and Gas Experiment II and microwave limb sounder ozone measurements and aliasing of SAGE II ozone trends in the lower stratosphere

SAGE II ozone measurements are compared with coincident microwave limb sounder (MLS) measurements over the period September 1991 to December 1993. Between 1.5 and 10 mbar the MLS ozone values are approximately 5% larger than the Stratospheric Aerosol and Gas Experiment (SAGE) II values. These differences are remarkably systematic in space and time. At 1 mbar the mean differences are zero and the mean differences oscillate with level at lower pressures. A month of comparisons against Halogen Occultation Experiment ozone measurements suggests that the differences at pressures less than 1.5 mbar are a feature of the MLS measurements. There are also differences between SAGE II sunrise and sunset measurements at 1 mbar which may be associated with the diurnal tide. At pressures greater than 10 mbar the comparisons indicate that the SAGE II ozone retrievals are being biased by the large aerosol concentrations resulting from the Mount Pinatubo eruption. For a fixed aerosol extinction the SAGE II/MLS difference (ppm) is larger at higher altitudes. It also depends nonlinearly on the aerosol extinction at pressures greater than 20 mbar. These effects are probably caused by the interpolation of the SAGE II aerosol extinction to 0.6 μm and by the evolution of the aerosol size distribution. For UARS layer aerosol optical depths less than 2 × 10−3 at 1.02 μm, the aerosol effect on the SAGE II ozone retrievals is inferred to be 3 × 1010 cm−3/10−3 aerosol layer optical depth at pressures greater than 20 mbar. This is equivalent to approximately 3% of the aerosol extinction at 0.6 μm being interpreted as ozone. At low aerosol concentrations and between 10 and 31 mbar, MLS ozone values are found to be approximately 5% larger than SAGE II ozone values (in agreement with the higher‐altitude differences). Atmospheric aerosol concentrations prior to the Mount Pinatubo eruption were large enough, particularly in the tropics after Ruiz in 1985, that long‐term trends in SAGE II ozone in the lower stratosphere are inferred to be biased downward. As a result, the SAGE II column ozone trends in the tropics over the period 1984–1991 need to be increased by approximately 0.2%/yr. This effect can account for a large fraction of SAGE II total ozone mapping spectrometer (TOMS) column ozone trend differences over this period. Good agreement is found between the TOMS and the SAGE II column ozone trends if the period of comparison is restricted to times of low aerosol concentrations.