Evolution of aerosol vertical distribution during particulate pollution events in Shanghai

A set of micro pulse lidar(MPL)systems operating at 532 nm was used for ground-based observation of aerosols in Shanghai in 2011.Three typical particulate pollution events(e.g.,haze)were examined to determine the evolution of aerosol vertical distribution and the planetary boundary layer(PBL)during these pollution episodes.The aerosol vertical extinction coefficient(VEC)at any given measured altitude was prominently larger during haze periods than that before or after the associated event.Aerosols originating from various source regions exerted forcing to some extent on aerosol loading and vertical layering,leading to different aerosol vertical distribution structures.Aerosol VECs were always maximized near the surface owing to the potential influence of local pollutant emissions.Several peaks in aerosol VECs were found at altitudes above 1 km during the dust-and bioburning-influenced haze events.Aerosol VECs decreased with increasing altitude during the local-polluted haze event,with a single maximum in the surface atmosphere.PM2.5 increased slowly while PBL and visibility decreased gradually in the early stages of haze events;subsequently,PM2.5 accumulated and was exacerbated until serious pollution bursts occurred in the middle and later stages.The results reveal that aerosols from different sources impact aerosol vertical distributions in the atmosphere and that the relationship between PBL and pollutant loadings may play an important role in the formation of pollution.     

气溶胶辐射效应对边界层结构及夹卷特征影响的观测分析

2017年12月22日至2018年1月18日利用无人机携带温、湿和颗粒物浓度探测仪对南京地区灰霾污染条件下大气边界层垂直结构开展加密观测。通过比较不同灰霾污染条件下温度、湿度和PM_2.5（直径小于2.5微米的颗粒物）浓度的垂直结构差异,结合地面热通量、2米空气温度、相对湿度、风速、风向及主要大气污染物（如臭氧和PM_2.5）浓度,定量评估了气溶胶辐射效应对边界层和夹卷过程的影响。分析表明,灰霾或气溶胶削弱到达地表太阳辐射,减小地表感热通量,延迟边界层发展,增加近地层大气稳定度,降低边界层高度,并加重灰霾污染。灰霾污染物在混合层顶处累积,导致PM_2.5浓度最大变化出现在边界层顶部而不是近地层。气溶胶辐射效应对夹卷特征及其特征参数有重要影响。灰霾浓度升高时,夹卷区厚度增加;无量纲化夹卷速度随对流理查逊数的变化不再符合负1次方幂函数关系,与大涡模拟结果一致。本研究进一步指出,为提高重霾污染条件下天气和空气质量数值预报水平,必须考虑气溶胶辐射效应对边界层和夹卷参数化的影响。     

A Case Study of CO<Subscript>2</Subscript>, CO and Particles Content Evolution in the Suburban Atmospheric Boundary Layer Using a 2-μm Doppler DIAL,a 1-μm Backscatter Lidar and an Array of In-situ Sensors

A network of remote and in-situ sensors was deployed in a Paris suburb in order to evaluate the mesoscale evolution of the daily cycle of CO₂ and related tracers in the atmospheric boundary layer (ABL) and its relation to ABL dynamics and nearby natural and anthropogenic sources and sinks. A 2-μm heterodyne Doppler differential absorption lidar, which combines measurements of, (1) structure of the atmosphere, (2) radial velocity, and (3) CO₂ differential absorption was a particularly unique element of the observational array. We analyse the differences in the diurnal cycle of CO, CO₂, lidar reflectivity (a proxy for aerosol content) and H₂O using the lidar, airborne measurements in the free troposphere and ground-based measurements made at two sites located few kilometres apart. We demonstrate that vertical mixing dominates the early morning drawdown of CO and aerosol content trapped in the former nocturnal layer but not the H₂O and CO₂ mixing ratio variations. Surface fluxes, vertical mixing and advection all contribute to the ABL CO₂ mixing ratio decrease during the morning transition, with the relative importance depending on the rate and timing of ABL rise. We also show evidence that when the ABL is stable, small-scale (0.1-km vertical and 1-km horizontal) gradients of CO₂ and CO are large. The results illustrate the complexity of inferring surface fluxes of CO₂ from atmospheric budgets in the stable boundary layer.     

北京秋季一次降雪前污染天气的激光雷达观测研究

以2009年11月5~8日北京地区发生的一次特殊天气形势下的重污染天气过程为例,研究分析本次污染特点和大气边界层结构特征以及此天气过程的大气温度和相对湿度结构特点。激光雷达是探测大气边界层及气溶胶的一个高效工具,利用ALS300激光雷达系统测量信号,应用Fernald方法反演大气消光系数,根据反演的气溶胶消光系数的最大突变,即最大递减率的高度来确定大气边界层的高度。利用其观测的退偏比分析大气污染物特性。利用微波辐射计数据,确定大气温度和湿度时空特征。研究结果表明:在本次污染天气下,大气具有很强的逆温结构,逆温最大可达近1 K(100 m)-1,500 m以上的大气相对湿度很低,在这种天气特征下的大气边界层高度在400 m左右,非常稳定。污染结束降雪开始前,大气逆温结构消失,大气湿度大幅度增加,接近饱和。根据lidar(light detection and ranging)退偏比的分析,本次污染天气是一次典型的烟尘类颗粒物的污染,污染具有区域性特点。PM2.5(空气动力学当量直径小于等于2.5μm的颗粒物)与AOT(Aerosol Optical Thickness)之间有明显的线性关系,相关系数达到0.72。该lidar系统能够反演出秋季降雪前本次污染天气背景下北京城区上空的大气污染特性和大气边界层高度。     

Convective Boundary-Layer Entrainment: Short Review and Progress using Doppler Lidar

The entrainment of air from the free atmosphere into the convective boundary layer is reviewed and further investigated using observations from a 2 μm Doppler lidar. It is possible to observe different individual processes entraining air into the turbulent layer, which develop with varying stability of the free atmosphere. These different processes are attended by different entrainment-zone thicknesses and entrainment velocities. Four classes of entrainment parametrizations, which describe relationships between the fundamental parameters of the process, are examined. Existing relationships between entrainment-zone thickness and entrainment velocity are basically confirmed using as scaling parameters boundary-layer height and convective velocity. An increase in the correlation coefficient between stability parameters based on the stratification of the free atmosphere and entrainment velocity (and entrainment-zone thickness respectively) up to 200% was possible using more suitable length and velocity scales.     

基于激光雷达资料的气溶胶辐射效应研究

利用新型激光雷达气溶胶探测资料及综合数值模式,以地形复杂的兰州市及周边地区冬季典型天气形势下的大气边界层为研究对象,通过理想试验模拟研究了城市气溶胶辐射效应与大气边界层的相互作用。结果表明:夜间,低空(50~600 m)气溶胶所在气层冷却效应明显,温度降低0.13~0.18℃,600 m高度以上,气溶胶浓度较低,其冷却效应较小,温度降低不足0.1℃;白天,受气溶胶短波辐射效应影响,边界层内增温明显,增温最大值位于低层脱地逆温层顶300 m高度附近,600 m以上由于气溶胶浓度减小,加热率亦降低,增温由0.2℃减至0.1℃。此外,气溶胶的存在使得所在层的风速降低。可见,激光雷达探测资料在边界层模式中有很好的应用价值,对于研究气溶胶辐射效应的大气边界层响应有重要意义。     

Airborne Lidar Observations of the Transition Zone Between the Convective Boundary Layer and Free Atmosphere During the International H<Subscript>2</Subscript>O Project (IHOP) in 2002

Airborne, light detection and ranging (lidar) backscatter observations of the convective boundary layer from the International H₂O Project (IHOP) in 2002 are analysed to study the structure of the transition zone; the backscatter gradient between the convective boundary layer and free atmosphere. A new mathematical algorithm is developed and used to extract high-resolution (15 m) transition-zone boundaries from 6,500 km (flight legs) of airborne observations. The cospectra of transition-zone boundaries and its thickness indicate that thickness changes occur from boundaries moving in opposite directions (vertically) at small wavelengths (<1 km), while at longer wavelengths (>1 km) both boundaries move coherently, with the lower boundary changing altitude more rapidly. Daily probability distributions of the transition-zone thickness are positively skewed with a mode of 60 m. The structure of the transition zone shows no dependence on the “overall” Richardson number, unlike the entrainment zone. This study provides the first quantitative characterization of the structure of the instantaneous transition zone, a contribution towards an improved understanding of convective boundary-layer entrainment.     

Vertical distributions of aerosol optical properties during haze and floating dust weather in Shanghai

A comparative study on the vertical distributions of aerosol optical properties during haze and floating dust weather in Shanghai was conducted based on the data obtained from a micro pulse lidar.There was a distinct difference in layer thickness and extinction coefficient under the two types of weather conditions.Aerosols were concentrated below 1 km and the aerosol extinction coefficients ranged from 0.25 to 1.50km^-1 on haze days.In contrast,aerosols with smaller extinction coefficients(0.20 0.35 km^-1) accumulated mainly from the surface to 2 km on floating dust days.The seasonal variations of extinction and aerosol optical depth（AOD） for both haze and floating dust cases were similar greatest in winter,smaller in spring,and smallest in autumn.More than 85%of the aerosols appeared in the atmosphere below 1 km during severe haze and floating dust weather.The diurnal variation of the extinction coefficient of haze exhibited a bimodal shape with two peaks in the morning or at noon,and at nightfall,respectively.The aerosol extinction coefficient gradually increased throughout the day during floating dust weather.Case studies showed that haze aerosols were generated from the surface and then lifted up,but floating dust aerosols were transported vertically from higher altitude to the surface.The AOD during floating dust weather was higher than that during haze.The boundary layer was more stable during haze than during floating dust weather.     

Aerosol climatology for the planetary boundary layer derived from regular lidar measurements

Regular aerosol extinction and backscatter measurements using a UV Raman Lidar have been performed for almost 3 years in Hamburg in the frame of the German Lidar Network. A set of 92 aerosol extinction and 164 aerosol backscatter profiles has been used for statistical investigations. Mean values and variances of the aerosol extinction and backscatter in the boundary layer have been calculated. Large fluctuations during the whole year have been found. The measured aerosol extinction over Hamburg shows a seasonal cycle with highest values in early fall and a second less prominent peak in spring.An analysis of the data using back trajectories showed a dependence of the aerosol extinction on the origin of the air mass. The residence time of the air mass over industrialized areas was found to be an important parameter for the measured aerosol extinction at Hamburg. However, only a small part of the total variability could be explained by the air mass origin.For 75 cases of aerosol extinction measurements under cloud-free conditions, the aerosol backscatter profile and therefore, the lidar ratio as a function of altitude could be determined. Winter measurements of the lidar ratio are often close to model results for maritime aerosol, the summer measurements are close to the model results for urban or continental aerosols.The high quality of the data has been proven by intercomparisons with other lidar systems and with star photometer measurements of the aerosol optical depth during the Lindenberg Aerosol Characterization Experiment (LACE'98) field campaign.     

Determination of the convective boundary-layer height with laser remote sensing

Different methods to determine the height of the convective boundary layer from lidar measurements are described and compared. The differences in either aerosol backscatter or in humidity between the boundary layer and the free troposphere are used, and either the variance or the gradient profile of the parameter under study is evaluated. On average the different methods are in very good agreement. Temporal resolution of the gradient methods is very high, on the order of seconds, but often there is an ambiguity in the choice of the “relevant” minimum in the gradient that corresponds to the boundary-layer height. This is avoided by combining the variance and the gradient methods, using the result of the variance analysis as an indicator for the region where the minimum of the gradient is sought. The combined method is useful for automated determination of the boundary-layer height at least under convective conditions. Aerosol backscatter is found to be as good an indicator for boundary-layer air as humidity, so a relatively simple backscatter lidar is sufficient for determination of the boundary-layer height.     

ACE‐2 multiple angle micro‐pulse lidar observations from Las Galletas, Tenerife, Canary Islands

Multiple‐angle micro‐pulse lidar (MPL) observations were made at Las Galletas on Tenerife, Canary Islands during the Aerosol Characterization Experiment‐2 (ACE‐2) conducted June–July, 1997. A principal objective of the MPL observations was to characterize the temporal/spatial distributions of aerosols in the region, particularly to identify and profile elevated Saharan dust layers which occur intermittently during the June–July time period. Vertical and slant angle measurements taken 16 and 17 July characterize such an occurrence, providing aerosol backscatter, extinction, and optical depth profiles of the dust layer between 1 and 5 km above mean sea level (MSL). Additionally, horizontal measurements taken in Las Galletas throughout the 6‐week period provide a time profile of the varying aerosol extinction at the surface. This profile exhibits the alternating periods of clean maritime air and pollution outbreaks that typified the region. Horizontal measurements also provide some evidence suggesting the possible influx of Saharan dust from the free troposphere to the surface. This paper presents estimates of aerosol optical properties retrieved from the multi‐angle MPL measurements in addition to an outline of the methodologies employed to obtain these results.     

Optical and radiative properties of aerosols during a severe haze episode over the North China Plain in December 2016

The optical and radiative properties of aerosols during a severe haze episode from 15 to 22 December 2016 over Beijing, Shijiazhuang, and Jiaozuo in the North China Plain were analyzed based on the ground-based and satellite data, meteorological observations, and atmospheric environmental monitoring data. The aerosol optical depth at 500 nm was < 0.30 and increased to > 1.4 as the haze pollution developed. The Ångström exponent was > 0.80 for most of the study period. The daily single-scattering albedo was > 0.85 over all of the North China Plain on the most polluted days and was > 0.97 on some particular days. The volumes of fine and coarse mode particles during the haze event were approximately 0.05–0.21 and 0.01–0.43 μm³, respectively—that is, larger than those in the time without haze. The daily absorption aerosol optical depth was about 0.01–0.11 in Beijing, 0.01–0.13 in Shijiazhuang, and 0.01–0.04 in Jiaozuo, and the average absorption Ångström exponent varied between 0.6 and 2.0. The aerosol radiative forcing at the bottom of the atmosphere varied from –23 to –227,–34 to –199, and –29 to –191 W m^–2 for the whole haze period, while the aerosol radiative forcing at the top of the atmosphere varied from –4 to –98, –10 to –51, and –21 to –143 W m^–2 in Beijing, Shijiazhuang, and Jiaozuo, respectively. Satellite observations showed that smoke, polluted dust, and polluted continental components of aerosols may aggravate air pollution during haze episodes. The analysis of the potential source contribution function and concentration-weighted trajectory showed that the contribution from local emissions and pollutants transport from upstream areas were 190–450 and 100–410 μg m^–3, respectively.     

Smoke haze over the European part of Russia in the summer of 2016: A link to wildfires in Siberia and atmospheric circulation anomalies

The mechanism of smoke haze formation over the European part of Russia (EPR) in the summer of 2016 is analyzed using satellite measurements, ground-based observations, reanalysis data, and trajectory modeling. The analysis reveals that smoke in the atmosphere over EPR with the aerosol optical depth increase up to 3 was caused by the long-range transport of combustion products from Siberian wildfires. The increase in the concentration of smoke aerosol in the atmosphere over EPR was accompanied by the increase in the concentration of carbon monoxide in the air. The long-range atmospheric transport of the products of pyrogenic emission from east to west for a distance up to 5000 km with the speed of 5–6 m/s predominantly occurred in the lower troposphere. The peculiarities of tropospheric circutation over Northern Eurasia in July 2016 are identified which were responsible for the transport of combustion products in the troposphere for thousands of kilometers in the direction opposite to the westerlies that prevail in the mid-latitudes.     

Characteristics of Boundary Layer Structure during a Persistent Haze Event in the Central Liaoning City Cluster,Northeast China

The characteristics of boundary layer structure during a persistent regional haze event over the central Liaoning city cluster of Northeast China from 16 to 21 December 2016 were investigated based on the measurements of particulate matter (PM) concentration and the meteorological data within the atmospheric boundary layer (ABL). During the observational period, the maximum hourly mean PM2.5 and PM10 concentrations in Shenyang, Anshan, Fushun, and Benxi ranged from 276 to 355 μg m^–3 and from 378 to 442 μg m^–3, respectively, and the lowest hourly mean atmospheric visibility (VIS) in different cities ranged from 0.14 to 0.64 km. The central Liaoning city cluster was located in the front of a slowly moving high pressure and was mainly controlled by southerly winds. Wind speed (WS) within the ABL (< 2 km) decreased significantly and WS at 10-m height mostly remained below 2 m s^–1 during the hazy episodes, which was favorable for the accumulation of air pollutants. A potential temperature inversion layer existed throughout the entire ABL during the earlier hazy episode [from 0500 Local Time (LT) 18 December to 1100 LT 19 December], and then a potential temperature inversion layer developed with the bottom gradually decreased from 900 m to 300 m. Such a stable atmospheric stratification further weakened pollutant dispersion. The atmospheric boundary layer height (ABLH) estimated based on potential temperature profiles was mostly lower than 400 m and varied oppositely with PM_2.5 in Shenyang. In summary, weak winds due to calm synoptic conditions, strong thermal inversion layer, and shallow atmospheric boundary layer contributed to the formation and development of this haze event. The backward trajectory analysis revealed the sources of air masses and explained the different characteristics of the haze episodes in the four cities.     

The aerosol radiative effect on a severe haze episode in the Yangtze River Delta

Due to increased aerosol emissions and unfavorable weather conditions, severe haze events have occurred frequently in China in the last 10 years. In addition, the interaction between the boundary layer and the aerosol radiative effect may be another important factor in haze formation. To better understand the effect of this interaction, the aerosol radiative effect on a severe haze episode that took place in December 2013 was investigated by using two WRF-Chem model simulations with different aerosol configurations. The results showed that the maximal reduction of regional average surface shortwave radiation, latent heat, and sensible heat during this event were 88, 12, and 37 W m^–2, respectively. The planetary boundary layer height, daytime temperature, and wind speed dropped by 276 m, 1°C, and 0.33 m s^–1, respectively. The ventilation coefficient dropped by 8%–24% for in the central and northwestern Yangtze River Delta (YRD). The upper level of the atmosphere was warmed and the lower level was cooled, which stabilized the stratification. In a word, the dispersion ability of the atmosphere was weakened due to the aerosol radiative feedback. Additional results showed that the PM_2.5 concentration in the central and northwestern YRD increased by 6–18 μg m^–3, which is less than 15% of the average PM_2.5 concentration during the severely polluted period in this area. The vertical profile showed that the PM_2.5 and PM₁₀ concentrations increased below 950 hPa, with a maximum increase of 7 and 8 μg m^–3, respectively. Concentrations reduced between 950 and 800 hPa, however, with a maximum reduction of 3.5 and 4.5 μg m^–3, respectively. Generally, the aerosol radiative effect aggravated the level of pollution, but the effect was limited, and this haze event was mainly caused by the stagnant meteorological conditions. The interaction between the boundary layer and the aerosol radiative effect may have been less important than the large-scale static weather conditions for the formation of this haze episode.     

Detection of Sahara dust intrusions during mixed advection patterns over south-east Italy: A case study

Measurements from July 4 to July 8, 2005 by a high resolution visible radiometer, a Raman lidar, a ground particulate matter sampler, and ground meteorological sensors have been combined in synergy to infer the intrusion over south-east Italy, of air masses from north-west Sahara, the Atlantic Ocean, and the continental Europe. It is shown that backscatter coefficient, depolarization-ratio, and lidar ratio vertical profiles represent the best tools to detect the intrusion of long range transported air masses and to monitor their effects on the vertical distribution of aerosol optical and microphysical properties. High resolution radiometers are instead important tools to monitor changes on columnar aerosol properties and size distributions.Backscatter coefficient, depolarization-ratio, and lidar ratio vertical profiles have revealed that aerosol optical and microphysical properties significantly changed with time and space during African dust outbreaks: the intrusion of dust particles that at first occurred above 2 km of altitude extending up to 6 km, affected the all aerosol load down to ground within few hours. Aerosol size distributions showed during dust events a clear bimodality with an accumulation mode maximum at 0.24 µm and a coarse mode maximum at 0.94 μm. Conversely, we have found that during the advection of air masses from the Atlantic and continental Europe, aerosol particles were mainly located below 2 km, their optical and microphysical properties were affected by smaller changes in time and space, and were characterized by depolarization ratios rather close to those due to a pure molecular atmosphere. In this case bimodal size distributions with an accumulation mode showing two sub-modes at 0.16 μm and 0.24 μm, respectively and a coarse mode centred at 0.94 μm have also been observed.     

An experimental investigation of Arctic haze at Alert,N.W.T., March 1985

Abstract

Arctic haze has been attributed to industrial pollution released at mid‐latitudes. Our current understanding has been pieced together from routine meteorological data, ground‐based air chemistry observations and limited aircraft measurements. This study investigates the relationship between synoptic boundary‐layer meteorology and the composition of the near‐surface atmosphere during the polar sunrise at Alert, N.W.T. A secondary objective is to characterize the influence of local activity on the atmospheric composition at a site for a new baseline monitoring station and at a location where aerosol chemistry and grab‐flask samples for CO₂ have been made for many years. Detailed measurements of the vertical distribution of aerosols were obtained from an upward‐looking lidar to complement the ground‐based measurements. Meteorological profiles of the near‐surface boundary layer were obtained from both free‐flying and tethered balloons. Near‐surface measurements were made of aerosol physical and chemical properties, O₃, NO₂, NO/NO_x, Peroxyacetylnitrate (PAN) and hydrocarbons.

The study period was characterized by prolonged periods with strong surface inversions, which were broken up occasionally by intrusions of cold air into the warmer air aloft. Lidar observations showed that ice crystals often accompanied aerosols and were responsible for reducing visibility below 30–40 km. There was a strong correlation between aerosol mass in the diameter size range 0.15 to 1.5 μm and total SO₄ ⁼ . PAN found at concentrations of about 200 ppt(v) was the main carrier of atmospheric nitrogen. Aerosol trace elements were divisible into anthropogenic soil, mixed soil/anthropogenic sea salt and halogens. Vertical transport in the surface boundary layer, as regulated by the strength of the surface radiation inversion, may play an important role in influencing the chemical composition of the air at the ground. The location of the new baseline monitoring laboratory was found to be generally windier and warmer than the lower altitude weather station, and the influence of local activity was found to be minimal.     

Observational Evidence And Modelling Of An Internal Hydraulic Jump At The Atmospheric Boundary-Layer Top During A Tramontane Event

A hydraulic jump has been observed with the airborne backscatter lidar LEANDRE 1 (Lidar embarqué pour l'Etude des Aérosols, des Nuages, des interactions Dynamique-Rayonnement etdu cycle de l'Eau) at the top of the atmospheric boundary layer (ABL) during a Tramontane event in the framework of the Pyréneés experiment. An analytical fluid mechanics model is used to interpret lidar observations in connection with in situ measurements andto study the sensitivity of the hydraulic jump triggering to the boundary conditions. This model, which generalizes the reduced-gravity shallow-water theory for two-dimensional stratified flows over a topograpy, is diagnostic (i.e., the reduced gravity g' = g _{v v} is prescribed) and uses boundary conditions defined in terms of Riemann invariants. Using inflow and outflow boundary conditions as well as the reduced gravity prescribed from in situ measurements, the model is able to diagnose the presence of a hydraulic jump at the location suspected from the lidar observations. The wind speed, ABL height and Froude number derived from the model are in good agreement with the observations (within about 20–30%).     

Microphysical and chemical characteristics of cloud droplet residuals and interstitial particles in continental stratocumulus clouds

During June and July 2003 the Sources and Origins of Atmospheric Cloud Droplets experiment (SOACED) was carried out on a mountain-top site in central Sweden. The main objective of the experiment was to characterise the microphysical and chemical properties of cloud droplet residuals and interstitial aerosol particles in continental clouds and to understand the processes controlling cloud properties at this location.Interstitial and residual aerosol size distributions, cloud liquid water content and species- and size-resolved aerosol mass concentrations are the main variables employed to address questions pertaining to the cloud droplet number concentration and scavenging efficiency during a stratocumulus cloud event observed on July 28, 2003. In this cloud event, about 56% of the aerosol mass was associated with organic species, whilst SO₄ accounted for 23% and NH₄ for 14%. NO₃ and Cl made up about 7% of the total mass.The partitioning of the aerosol particles between cloud droplets and interstitial air has been studied in terms of their microphysical properties. The scavenging efficiency, defined as the fraction of particles activated into cloud elements compared to the total amount of particles, was investigated as a function of size. The scavenging efficiency curves displayed different shapes during the cloud event, from an S-shaped curve, with low scavenging efficiency in the Aitken mode and larger scavenging efficiency in the accumulation mode, to more unusual shapes where Aitken-mode particles were either solely activated or activated in addition to accumulation-mode particles.This study suggests that alterations of the aerosol chemical composition occurred during the measurement period, changing the hygroscopic nature of the CCN and decreasing their activation diameter. It is also hypothesized that entrainment of drier air aloft may have introduced inhomogeneities in the supersaturation field and modified the S-shaped scavenging curves.     

Extinction versus backscatter relationships for lidar applications at 351 nm: maritime and desert aerosol simulations and comparison with observations

Functional relationships linking at λ₀=351 nm aerosol extinction α_λ0^aer and backscatter coefficient β_λ0^aer of maritime and desert type aerosols are determined to allow for inversion of the single-wavelength lidar signals. Such relationships are derived as mean behavior of 20,000 extinction versus backscatter computations, performed for aerosol size distributions and compositions whose describing parameters are randomly chosen within the naturally observed variability. For desert-type aerosols, the effect of the particle non-sphericity is considered and it is shown that the extinction to backscatter ratio of non-spherical dust particles can be up to 60% larger than the values obtained for spherical particles. Aerosol extinction and backscatter coefficient profiles obtained inverting the single-wavelength lidar signal with the modeled relationships are then compared to the same profiles measured by a combined elastic-Raman lidar operating at 351 nm. Analytical back trajectories and satellite images are used to characterize advection patterns during lidar measurements and to properly choose the modeled functional relationship. A good accordance between the two techniques is found for advection patterns over the lidar site typical of maritime and dust conditions. Maximum differences between the model-based α_λ0^aer and β_λ0^aer vertical profiles and the corresponding ones measured by the combined elastic-Raman lidar technique are of 30% and 40% in maritime and desert dust conditions, respectively. The comparison of elastic-Raman lidar measurements and model-based results also reveals that particle non-sphericity must be taken into account when mineral dust-type aerosols are directly advected over the measurement site.