Spatial Distribution Of Aerosols Research Articles

Intra-Seasonal and Annual Variation of Aerosols and Their Radiative Impact in the Sahelian Zone of Burkina Faso

This paper deals with the characterization of aerosols in the Sahelian zone, particularly in Burkina Faso based on MODIS observations and in situ measurements of the AERONET network on the Ouagadougou site (12.2&#176N, 1.4&#176W). Thus, a seasonal spatial distribution of aerosols made over the period from 2001 to 2016 gives a very great variability of aerosols in Burkina Faso, whose maxima are encountered in Spring, characterized by winds from the North East. This seasonality of aerosols is also shown by the annual cycles of optical, radiative and microphysical parameters measured by AERONET between 1999 and 2006. Moreover, an analysis of these parameters shows the prevalence of mineral dusts characterized by low values of the Angstrom coefficient (α440-870 0.9) and the cooling noticed in the bottom of atmosphere (BOA) and at the top of the atmosphere (TOA). Also, the climatology of the volume size distribution of aerosols shows a very great variability of particles in terms of size influenced by the thin and coarse pattern where most sizes are between 1 and 10 μm.

An Improved High‐Spatial‐Resolution Aerosol Retrieval Algorithm for MODIS Images Over Land

AbstractMODerate resolution Imaging Spectroradiometer (MODIS) data can play an important role in aerosol retrieval at the global scale due to their short revisit period and long‐term observations. The operational MODIS aerosol optical depth (AOD) products are severely limited in air quality studies at the city or local scales due to their coarse spatial resolutions. Therefore, an improved aerosol retrieval algorithm for MODIS images at 1‐km spatial resolution is proposed in this paper. This algorithm is based on the high‐resolution aerosol retrieval algorithm with a priori land surface reflectance database support (HARLS), which was developed over bright urban areas and was subsequently modified and validated over land. For this study, an eight‐day surface reflectance database and a seasonal aerosol‐type database over land are constructed using the MODIS surface reflectance and aerosol products. Four typical regions (in Europe, North America, Beijing‐Tianjin‐Hebei, and the Sahara) with different underlying surfaces and aerosol types are selected to perform the aerosol retrieval experiments. The AOD retrievals are validated against the AERosol RObotic NETwork (AERONET) version 2 level 2.0 AOD measurements and compared with the operational MODIS AOD product at 3‐km resolution (MOD04_3K). The results show that AOD retrievals adequately match with AERONET AOD measurements, with 79.56%, 72.69%, 74.71%, and 61.01% of the collections falling within the MOD04_3K expected error over land [±(0.05 + 20%)] for each of the above‐listed regions, respectively. The Improved HARLS (I‐HARLS) algorithm performs well overall under different surface conditions, but the data quality gradually decreases with the increase in surface reflectance. Moreover, the I‐HARLS algorithm is robust with less bias and can provide more detailed aerosol spatial distributions than those provided by the MOD04_3K AOD product. These results suggest that the I‐HARLS algorithm can be used for air‐pollution‐ and climate‐related studies at medium or small scales.

Aerosol as a potential factor to control the increasing torrential rain events in urban areas over the last decades

Abstract. This study examines the role played by aerosol in torrential rain that occurred in the Seoul area, which is a conurbation area where urbanization has been rapid in the last few decades, using cloud-system-resolving model (CSRM) simulations. The model results show that the spatial variability in aerosol concentrations causes the inhomogeneity of the spatial distribution of evaporative cooling and the intensity of associated outflow around the surface. This inhomogeneity generates a strong convergence field in which torrential rain forms. With the increases in the variability in aerosol concentrations, the occurrence of torrential rain increases. This study finds that the effects of the increases in the variability play a much more important role in the increases in torrential rain than the much-studied effects of the increases in aerosol loading. Results in this study demonstrate that for a better understanding of extreme weather events such as torrential rain in urban areas, not only changing aerosol loading but also changing aerosol spatial distribution since industrialization should be considered in aerosol–precipitation interactions.

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.

Diurnal spatial distributions of aerosol optical and cloud micro-macrophysics properties in Africa based on MODIS observations

The diurnal spatial distribution of both natural and anthropogenic aerosols, as well as liquid and ice cloud micro-macrophysics have been evaluated over Africa using Terra and Aqua MODIS collection 6 products. The variability of aerosol optical depth (AOD), Ångström exponent (AE), liquid and ice cloud microphysics (Liquid cloud effective radius LCER, Ice cloud effective radius ICER) and cloud macrophysics (Liquid cloud optical thickness LCOT, Liquid cloud water path LCWP, Ice cloud optical thickness ICOT, Ice cloud water path ICWP) parameters were investigated from the morning to afternoon over Africa from 2010 to 2014. In both the morning (Terra) and afternoon (Aqua) heavy pollution (AOD ≥ 0.6) occurs in the coastal and central areas (between 120 N-170 N and 100 E-150 E) of West of Africa (WA), Central of Africa (CA) (0.50 S-70S and 100 E-250 E),. Moderate pollution (0.3 < AOD < 0.6) often occurs in West and North of Africa (between 50 N-270 N and 160 W-50E), and clean environmental (AOD < 0.3) conditions are common in South of Africa (SA), East of Africa (EA) and some regions in North of Africa (NA). The West-North of Africa (WNA) and Central-South of Africa (CSA) regions are dominated by dust (AE < 0.7) and biomass burning (AE > 1.2) aerosols. The mixture of dust and biomass burning aerosols (0.7 < AE < 1.2) are found at the coastal areas in West of Africa (CoWA) and Central of Africa (CA) (50 N-80N and 100 E-340 E), particularly in the morning and afternoon respectively. The LCER often decrease from the morning to the afternoon in all seasons, but an increase occur from the morning to the afternoon in CSA (50 S-220 S) in DJF, both CA (20 S-50N) and CoWA in JJA and SON. The ICER increase from the morning to afternoon in all seasons over Africa and decreases in South of Africa (50 S-200 S) in DJF. The LCOT increases from the morning to afternoon in NA and SA while a decrease occur in CA in all seasons. The LCWP increase in many regions of Africa in all seasons while a decrease occurs in CoWA during JJA. The ICOT and ICWP show a remarkable increase from the morning to afternoon in regions dominated by biomass burning (CSA) compared to regions dominated by dust (WNA) aerosols in DJF, MAM and SON. Dust aerosols are mainly distributed in WNA by northerly and westerly winds in both January and April, southerly and southwesterly winds in July, and southerly and southwesterly winds in October, while biomass burning aerosols are mainly distributed in CSA by the northerly and northeasterly winds in January, easterly winds in April, July and October. The diurnal variability of cloud parameters is associated with both convective processes and cloud types. The knowledge of interactions between natural and anthropogenic aerosols with liquid and ice cloud microphysics parameters could contribute to improve aerosol and cloud remote sensing retrieval.

Sensitivity of the radiative forcing by stratospheric sulfur geoengineering to the amount and strategy of the SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;injection studied with the LMDZ-S3A model

Abstract. The enhancement of the stratospheric sulfate aerosol layer has been proposed as a method of geoengineering to abate global warming. Previous modelling studies found that stratospheric aerosol geoengineering (SAG) could effectively compensate for the warming by greenhouse gases on the global scale, but also that the achievable cooling effect per sulfur mass unit, i.e. the forcing efficiency, decreases with increasing injection rate. In this study we use the atmospheric general circulation model LMDZ with the sectional aerosol module S3A to determine how the forcing efficiency depends on the injected amount of SO2, the injection height, and the spatio-temporal pattern of injection. We find that the forcing efficiency may decrease more drastically for larger SO2 injections than previously estimated. As a result, the net instantaneous radiative forcing does not exceed the limit of –2 W m−2 for continuous equatorial SO2 injections and it decreases (in absolute value) for injection rates larger than 20 Tg S yr−1. In contrast to other studies, the net radiative forcing in our experiments is fairly constant with injection height (in a range 17 to 23 km) for a given amount of SO2 injected. Also, spreading the SO2 injections between 30∘ S and 30∘ N or injecting only seasonally from varying latitudes does not result in a significantly larger (i.e. more negative) radiative forcing. Other key characteristics of our simulations include a consequent stratospheric heating, caused by the absorption of solar and infrared radiation by the aerosol, and changes in stratospheric dynamics, with a collapse of the quasi-biennial oscillation at larger injection rates, which has impacts on the resulting spatial aerosol distribution, size, and optical properties. But it has to be noted that the complexity and uncertainty of stratospheric processes cause considerable disagreement among different modelling studies of stratospheric aerosol geoengineering. This may be addressed through detailed model intercomparison activities, as observations to constrain the simulations of stratospheric aerosol geoengineering are not available and analogues (such as volcanic eruptions) are imperfect.

Meteorological and Land Surface Properties Impacting Sea Breeze Extent and Aerosol Distribution in a Dry Environment

AbstractThe properties of sea breeze circulations are influenced by a variety of meteorological and geophysical factors that interact with one another. These circulations can redistribute aerosol particles and pollution and therefore can play an important role in local air quality, as well as impact remote sensing. In this study, we select 11 factors that have the potential to impact either the sea breeze circulation properties and/or the spatial distribution of aerosols. Simulations are run to identify which of the 11 factors have the largest influence on the sea breeze properties and aerosol concentrations and to subsequently understand the mean response of these variables to the selected factors. All simulations are designed to be representative of conditions in coastal sub tropical environments and are thus relatively dry, as such they do not support deep convection associated with the sea breeze front. For this dry sea breeze regime, we find that the background wind speed was the most influential factor for the sea breeze propagation, with the soil saturation fraction also being important. For the spatial aerosol distribution, the most important factors were the soil moisture, sea‐air temperature difference, and the initial boundary layer height. The importance of these factors seems to be strongly tied to the development of the surface‐based mixed layer both ahead of and behind the sea breeze front. This study highlights potential avenues for further research regarding sea breeze dynamics and the impact of sea breeze circulations on pollution dispersion and remote sensing algorithms.

Scanning elastic lidar observations of aerosol transport in New York City

In this study, spatial distribution of aerosols in New York City is observed using a scanning eyesafe 532 nm elastic-backscatter micro-pulse lidar system. Observations show dynamics of the boundary layer and inhomogeneous distribution and transport of aerosols. The data acquired are complemented with simultaneous measurements of particulate matter and wind speed and direction. Furthermore, the system observations are validated by comparing them with a colocated multi-wavelength lidar.

Advanced Forecasting Approach to Improve Uncertainty of Solar Irradiance Associated with Aerosol Direct Effects

Numerical Weather Prediction (NWP) models such as the Weather Research and Forecasting (WRF) model are essential for forecasting one-day-ahead solar irradiance. In order to evaluate the performance of the WRF in forecasting solar irradiance over the Korean Peninsula, we compared WRF prediction data from 2008 to 2010 corresponding to weather observation data (OBS) from the Korean Meteorological Administration (KMA). The WRF model showed poor performance at polluted regions such as Seoul and Suwon where the relative Root Mean Square Error (rRMSE) is over 30%. Predictions by the WRF model alone had a large amount of potential error because of the lack of actual aerosol radiative feedbacks. For the purpose of reducing this error induced by atmospheric particles, i.e., aerosols, the WRF model was coupled with the Community Multiscale Air Quality (CMAQ) model. The coupled system makes it possible to estimate the radiative feedbacks of aerosols on the solar irradiance. As a result, the solar irradiance estimated by the coupled system showed a strong dependence on both the aerosol spatial distributions and the associated optical properties. In the NF (No Feedback) case, which refers to the WRF-only stimulated system without aerosol feedbacks, the GHI was overestimated by 50-200 W m -2 compared with OBS derived values at each site. In the YF (Yes Feedback) case, in contrast, which refers to the WRF-CMAQ two-way coupled system, the rRMSE was significantly improved by 3.1-3.7% at Suwon and Seoul where the Particulate Matter (PM) concentrations, specifically, those related to the PM 10 size fraction, were over 100 μg m -3 . Thus, the coupled system showed promise for acquiring more accurate solar irradiance forecasts.

Spatial variation of Aerosol Optical Depth and Solar Irradiance over Delhi -NCR during Summer season

Present study shows the spatial variation of Aerosol Optical Depth (AOD), solar irradiance and their association at the urban and rural sites in Delhi and National Capital Region (NCR) during the summer season of the year 2015. Summer-time AOD data from the NASA’s Terra satellite MODIS sensor has been used to study the spatial distribution of aerosols over Delhi and its surrounding rural area. The ground data for the direct and global solar irradiances was collected over this region at urban and rural locations in Delhi and NCR using a Fieldspec Spectro-radiometer. HYSPLIT model has been used for the air mass trajectory analysis. The AOD values were observed to be higher over Delhi compared to the relatively lower AOD in rural area of NCR. The NCR site observed higher average solar irradiances than Delhi during the summer season. This may be because of the higher aerosol concentration in Delhi as compared to its outskirts. Also, this region is affected by the severe dust storm events during the summer season which further increases the aerosol load in the atmosphere. HYSPLIT results show the influence of western Thar Desert air masses on the Delhi-NCR. Windblown as well anthropogenic aerosols play a major role in scattering and absorption of the incoming solar radiation and hence, in governing the micro-climatology of the region.

Satellite observations of the aerosol direct radiative effect during a pollution episode in Nanjing, China, using high-resolution HJ-1 CCD imagery

ABSTRACTThe charge-coupled device imageries from the Chinese environmental satellites (HJ-1) with 30 m spatial resolution are used in this study to investigate the shortwave aerosol direct radiative effect (ADRE) during a pollution episode in March 2013 over Nanjing City. The Deep Blue algorithm is used to retrieve the aerosol optical depth (AOD) and the results are then validated using ground-based data. The mean values of the AOD in Nanjing at 550 nm are 0.41, 0.54, and 0.79 on 3, 4, and 7 March 2013, respectively, with fitting degree 0.796. The spatial aerosol distribution indicates that the pollution is induced by local emission, accumulated as a result of suitable weather conditions, and then spread by the wind. The ADRE results at the surface and at the top of the atmosphere (TOA) are then calculated using radiative transfer model. The mean values and standard deviations of the ADRE on 3, 4, and 7 March are −75.6 ± 21.3, −98.8 ± 16.6, and −138.1 ± 26.0 W m−2 at the surface and −3.7 ± 3.8, −5.2 ± 4.6, and −8.1 ± 5.9 W m−2 at the TOA, respectively. The results show that although aerosols can cause warming at the TOA over a highly reflectivity surface, the total radiative effect corresponds to the cooling of both the Earth-atmosphere system and the surface at the expense of heating the atmosphere.

Observations of aerosol color ratio and depolarization ratio over Wuhan

The aerosol color ratio, depolarization ratio and aerosol optical depth (AOD) were measured by a two-wavelength-depolarization lidar at Wuhan, China during the period from May 2015–July 2016. The annual average AOD at Wuhan was about 0.33 during the period 2015–2016. The seasonal average AOD is small (0.26 ± 0.25) during the winter (December–February) season and large (0.4 ± 0.1) during the summer (June–August) season. The monthly average color ratio is small (0.23 ± 0.09) in January and large (0.76 ± 0.21) in August with an annual average value 0.54. The maximum monthly mean depolarization ratio (0.2 ± 0.07) occurred in the month of October, while the minimum (0.06 ± 0.02) occurred in the month of September, and the annual mean depolarization ratio was about 0.17. An analysis of temporal variations of color ratio and depolarization ratio suggests the presence of coarse and non-spherical particles during the autumn. The aerosol color ratio between 0.3 and 2.0 km was large (0.65), suggesting a large number of coarse particles in this range. The vertical distribution of the depolarization ratio is uniform. Finally, the spatial aerosol distribution under different weather conditions and its relationship with the color ratio is investigated in detail. The color ratio value of 0.74 could be used as a threshold for distinguishing polluted weather from clean weather. The aerosol optical and physical properties are investigated to provide a comprehensive understanding of aerosol radiative forcing and environmental problems in this region.

Multiband infrared inversion for low-concentration methane monitoring in a confined dust-polluted atmosphere

A multiband infrared diagnostic (MBID) method for methane emission monitoring in limited underground environments was presented considering the strong optical background of gas/solid attenuation. Based on spatial distribution of aerosols and complex refractive index of dust particles, forward calculations were carried out with/without methane to obtain the spectral transmittance through the participating atmosphere in a mine roadway. Considering the concurrent attenuation and absorption behavior of dust and gases, four infrared wavebands were selected to retrieve the methane concentration combined with a stochastic particle swarm optimization (SPSO) algorithm. Inversion results prove that the presented MBID method is robust and effective in identifying methane at concentrations of 0.1% or even lower with inversed relative error within 10%. Further analyses illustrate that the four selected wavebands are indispensable, and the MBID method is still valid with transmission signal disturbance in a conventional dust-polluted atmosphere under mechanized mining condition. However, the effective detection distance should be limited within 50 m to ensure inversed relative error less than 5% at 1% methane concentration.

Estimates of the aerosol indirect effect over the Baltic Sea region derived from 12 years of MODIS observations

Abstract. Retrieved from the Moderate Resolution Imaging Spectroradiometer (MODIS) on-board the Aqua satellite, 12 years (2003–2014) of aerosol and cloud properties were used to statistically quantify aerosol–cloud interaction (ACI) over the Baltic Sea region, including the relatively clean Fennoscandia and the more polluted central–eastern Europe. These areas allowed us to study the effects of different aerosol types and concentrations on macro- and microphysical properties of clouds: cloud effective radius (CER), cloud fraction (CF), cloud optical thickness (COT), cloud liquid water path (LWP) and cloud-top height (CTH). Aerosol properties used are aerosol optical depth (AOD), Ångström exponent (AE) and aerosol index (AI). The study was limited to low-level water clouds in the summer. The vertical distributions of the relationships between cloud properties and aerosols show an effect of aerosols on low-level water clouds. CF, COT, LWP and CTH tend to increase with aerosol loading, indicating changes in the cloud structure, while the effective radius of cloud droplets decreases. The ACI is larger at relatively low cloud-top levels, between 900 and 700 hPa. Most of the studied cloud variables were unaffected by the lower-tropospheric stability (LTS), except for the cloud fraction. The spatial distribution of aerosol and cloud parameters and ACI, here defined as the change in CER as a function of aerosol concentration for a fixed LWP, shows positive and statistically significant ACI over the Baltic Sea and Fennoscandia, with the former having the largest values. Small negative ACI values are observed in central–eastern Europe, suggesting that large aerosol concentrations saturate the ACI.

Aerosol distribution over Brazil with ECHAM-HAM and CAM5-MAM3 simulations and its comparison with ground-based and satellite data

The accurate representation of the impacts of natural and anthropogenic aerosols in the climate system presents a challenge in General Circulation Models. This paper analyzes the performance of the aerosol component of two Atmospheric General Circulation Models (AGCM): the Europe Centre Hamburg Model - Hamburg Aerosol Model (ECHAM-HAM), and the Community Atmosphere Model - Modal Aerosol Model (CAM5-MAM3) and their comparison with aerosol observations. We analyzed the spatial distribution of aerosols over Brazil represented in terms of the aerosol optical depth (AOD) simulated by these models. The model results are compared to measurements from Aerosol Robotic Network (AERONET) ground station, and satellite observations provided by the Moderate Resolution Imaging Spectroradiometer (MODIS). While both the models provide AODs at 550 nm, only HAM provides the Angström exponent that is compared with AERONET measurements. The comparison between the model simulations and the satellite observations of AOD show that the models can reproduce the spatial and temporal distributions, however models underestimate AOD for the four cities and for almost every South American continent during all seasons. During the dry season, characterized by intense biomass burning, CAM5-MAM3 shows inconsistent, but comparatively better results that ECHAM-HAM, with negative biases over Northern and Northeastern regions of Brazil. The Angström parameter is reasonably reproduced by ECHAM-HAM, except for Cuiabá, indicating that the particle size distribution is correctly represented in most cities.

Preface to Special Issue - Long-range transported air pollutants in East Asia ― Observation, measurements, and model analysis

ABSTRACT Rapid economic growth in East Asia causes consumption of vast amount of fossil fuel for energy source. Emission of acid forming gases such as NOx and SO2 as well as particulate matter affects the environment of the source country, surrounding countries, and moreover, whole northern hemisphere by long-range trans-boundary air pollution. Among these atmospheric pollutants, aerosols attract special attention these days, because there are many unclear points; for example, aerosol spatial distribution, formation mechanisms in the atmosphere, quantitative evaluation of environmental impacts, and so on. Therefore, scientifically supported data of trans-boundary air pollutants observed by well-arranged ground-based observations in this region as well as reliable and detailed model studies are very important. In order to analyze trans-boundary air pollution within and from East Asia observations on isolated islands and/or on the summit of mountains utilizing a network among East Asian and Southeast Asian countries are urgently needed. Emission source analysis with such as statistical analyses and estimation of the contribution of long-range transported aerosols and locally emitted aerosols are also very important.

Regional Impact of Biomass Burning in Southeast Asia on Atmospheric Aerosols during the 2013 Seven South-East Asian Studies Project

ABSTRACTA nested air quality prediction modeling system (NAQPMS) with an online tracer-tagged module was utilized to investigate the regional impact of biomass burning (BB) on aerosols and source–receptor relationships in Southeast Asia during March–April 2013. NAQPMS could reproduce the three-dimensional spatial distribution of aerosols. Both monthly and episodic analyses indicated that BB significantly contributed to surface and column aerosol concentrations in Southeast Asia along two long-range transport pathways. In the first pathway, aerosols from BB were blown northward from the Indochina peninsula to southwestern provinces in China. The mean contributions of BB decreased from 70%–80% in the source regions to 10%–40% in southwestern China. Myanmar was the largest exporter. In the second pathway, PM2.5 emitted by BB was uplifted into the mid-altitudes (2000 m) in the Indochina peninsula and transported eastward to the western Pacific at altitudes of 2500–4000 m, passing the South China Sea, southern China and western Pacific. In downwind regions, BB contributed 30%–60% of aerosols at altitudes of 2000–4000 m and 10%–30% below 2000 m. A simple estimation based on source–receptor relationships showed that BB emissions were likely overestimated by 35%–50% in the Fire Inventory from National Center for Atmospheric Research (v1.5).

Aircraft Measurements of the Aerosol Spatial Distribution and Relation with Clouds over Eastern China

ABSTRACTTo investigate the spectral and spatial distribution characteristics of aerosol particles over eastern China, this study conducted a set of aircraft measurements during August 12–28, 2014, over Anhui province, China. The aerosol number concentration and size distributions from five flights as well as the cloud and meteorological parameters were analyzed. In Anhui province, the average number concentration of aerosol particles in the size range of 0.1–3.0 µm was 481 ± 199 cm–3, and accumulation mode particles accounted for more than 95% of the total aerosol particles. Most of the aerosol particles were concentrated in the layer below 1000 m, where the number concentration decreased with the altitude, except in the presence of thermal inversion layers (TILs). The TILs prevented the vertical transport of aerosol particles, and led to a higher number concentration in the boundary layer. A large fraction of aerosol particles was removed when clouds were present, and the removed in-cloud aerosols lead to an increase in cloud droplet concentrations for the size range of 3.5–10.0 µm. Our results are valuable for understanding the spatial distribution of aerosol particles and their interactions with clouds.

Evaluating secondary inorganic aerosols in three dimensions

Abstract. The spatial distribution of aerosols and their chemical composition dictates whether aerosols have a cooling or a warming effect on the climate system. Hence, properly modeling the three-dimensional distribution of aerosols is a crucial step for coherent climate simulations. Since surface measurement networks only give 2-D data, and most satellites supply integrated column information, it is thus important to integrate aircraft measurements in climate model evaluations. In this study, the vertical distribution of secondary inorganic aerosol (i.e., sulfate, ammonium, and nitrate) is evaluated against a collection of 14 AMS flight campaigns and surface measurements from 2000 to 2010 in the USA and Europe. GISS ModelE2 is used with multiple aerosol microphysics (MATRIX, OMA) and thermodynamic (ISORROPIA II, EQSAM) configurations. Our results show that the MATRIX microphysical scheme improves the model performance for sulfate, but that there is a systematic underestimation of ammonium and nitrate over the USA and Europe in all model configurations. In terms of gaseous precursors, nitric acid concentrations are largely underestimated at the surface while overestimated in the higher levels of the model. Heterogeneous reactions on dust surfaces are an important sink for nitric acid, even high in the troposphere. At high altitudes, nitrate formation is calculated to be ammonia limited. The underestimation of ammonium and nitrate in polluted regions is most likely caused by a too simplified treatment of the NH3 ∕ NH4+ partitioning which affects the HNO3 ∕ NO3− partitioning.

Seasonal monitoring and estimation of regional aerosol distribution over Po valley, northern Italy, using a high-resolution MAIAC product

In this work, the new 1 km-resolved Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm is employed to characterize seasonal PM10 – AOD correlations over northern Italy. The accuracy of the new dataset is assessed compared to the widely used Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 5.1 Aerosol Optical Depth (AOD) data, retrieved at 0.55 μm with spatial resolution of 10 km (MYD04_L2). We focused on evaluating the ability of these two products to characterize both temporal and spatial distributions of aerosols within urban and suburban areas. Ground PM10 measurements were obtained from 73 of the Italian Regional Agency for Environmental Protection (ARPA) monitoring stations, spread across northern Italy, during a three-year period from 2010 to 2012. The Po Valley area (northern Italy) was chosen as the study domain because of its severe urban air pollution, resulting from it having the highest population and industrial manufacturing density in the country, being located in a valley where two surrounding mountain chains favor the stagnation of pollutants. We found that the global correlations between the bin-averaged PM10 and AOD are R2 = 0.83 and R2 = 0.44 for MYD04_L2 and for MAIAC, respectively, suggesting a greater sensitivity of the high-resolution product to small-scale deviations. However, the introduction of Relative Humidity (RH) and Planetary Boundary Layer (PBL) depth corrections allowed for a significant improvement to the bin-averaged PM – AOD correlation, which led to a similar performance: R2 = 0.96 for MODIS and R2 = 0.95 for MAIAC. Furthermore, the introduction of the PBL information in the corrected AOD values was found to be crucial in order to capture the clear seasonal cycle shown by measured PM10 values. The study allowed us to define four seasonal linear correlations that estimate PM10 concentrations satisfactorily from the remotely sensed MAIAC AOD retrieval. Overall, the results show that the high resolution provided by MAIAC retrieval data is much more relevant than the 10 km MODIS data to characterize PM10 in this region of Italy which has a pretty limited geographical domain but a broad variety of land usages and consequent particulate concentrations.