Optical Properties Of Atmospheric Particles Research Articles

Aeolus L2A aerosol optical properties product: standard correct algorithm and Mie correct algorithm

Abstract. Aeolus carries the Atmospheric LAser Doppler INstrument (ALADIN), the first high-spectral-resolution lidar (HSRL) in space. Although ALADIN is optimized to measure winds, its two measurement channels can also be used to derive optical properties of atmospheric particles, including a direct retrieval of the lidar ratio. This paper presents the standard correct algorithm and the Mie correct algorithm, the two main algorithms of the optical properties product called the Level-2A product, as they are implemented in version 3.12 of the processor, corresponding to the data labelled Baseline 12. The theoretical basis is the same as in Flamant et al. (2008). Here, we also show the in-orbit performance of these algorithms. We also explain the adaptation of the calibration method, which is needed to cope with unforeseen variations of the instrument radiometric performance due to the in-orbit strain of the primary mirror under varying thermal conditions. Then we discuss the limitations of the algorithms and future improvements. We demonstrate that the L2A product provides valuable information about airborne particles; in particular, we demonstrate the capacity to retrieve a useful lidar ratio from Aeolus observations. This is illustrated using Saharan dust aerosol observed in June 2020.

Optical properties of atmospheric particles over an urban site in Mexico City and a peri-urban site in Queretaro

Optical properties of atmospheric particles at Mexico City (UNAM) and Queretaro (JQRO) were measured with a Photoacoustic Extinctiometer (PAX) at 870 nm. The Mexico City Metropolitan Area has around 21 million inhabitants and Queretaro Metropolitan Area has little more than a million. Observations of meteorological parameters (relative humidity, solar radiation, and wind speed) were used to identify the rainy and dry seasons and explain the daily and seasonal behaviors of particles optical properties. The measurements were made from November 1, 2014 to July 31, 2016. At UNAM, the mean values of the scattering coefficient (Bscat) in cold dry, warm dry, and rainy seasons were 35.8, 27.1, and 31.3 Mm−1, respectively; while at JQRO were 10.9, 11.9, and 15.0 Mm−1. The average values of the absorption coefficient (Babs) at UNAM during the cold dry, warm dry, and rainy seasons were 14.5, 12.7, and 12.7 Mm−1, respectively; whereas at JQRO were 4.9, 4.7, and 3.9 Mm−1. Both absorption and scattering coefficients showed similar diurnal behaviors, but at UNAM they are three times higher than JQRO. Concentrations of criteria gases (O3, NO, NO2 and NOx) were also measured. At UNAM no difference was observed between the seasonal values for the single scattering albedo (SSA); while in JQRO, the rainy season had the highest seasonal value, being 13% higher than in the dry seasons. The Mass Scattering Cross-Section (MSC) values at UNAM were close to 2 m2/g; on the other hand, at JQRO the MSC values were lower than 1 m2/g. The results suggest a seasonal variability in the aerosol optical properties in both sites, which should be verified with more long-term studies.

Modelling aerosol optical properties over urban environment (New Delhi) constrained with balloon observation

Vertical variation in aerosol optical properties [e.g. Single Scattering Albedo (SSA) and aerosol extinction coefficient] over a polluted environment is extremely important for better understanding of columnar radiative characteristics. The present case study over a typical polluted environment (New Delhi) discusses the vertical profile (ground to 700 m) of modelled optical properties of atmospheric particles at different altitudes. Here, we used the aerosol physico-chemical data generated in the tethered balloon-based observation conducted at CSIR-NPL, New Delhi (28° 38′ 10″ N, 77° 10′ 17″ E) from 21st −27th February 2014. Based on the microscopic observations of individual particles, we developed the aerosol model shapes (coated spheres) for simulating their optical properties. Total three cases were considered for simulating the aerosol optics at varying altitude; Case A: External mixture of coated dust and coated sulfate particles; Case B: External mixture of coated dust, coated sulfate, coated OC (Organic Carbon) and coated EC (Elemental Carbon) (with assumption that 6% EC at ground level); Case C: External mixture of coated dust, coated sulfate, coated OC and coated EC (with assumption that 10% EC at ground level). At 550 nm wavelength, the value of SSA has been found to be highest (i.e. 0.985) at 200 m altitude for Case A while Case B (i.e. 0.9523) and Case C (i.e. 0.9291) show highest values at 500 m altitude. SSA was found to be maximum at 500 m altitude for both Case B and Case C due to the presence of lowest graphitic counts at that particular altitude. Case B and Case C exhibit similar pattern but differ in magnitude, this is due to two EC extremities at ground level i.e. minima (6%) and maxima (10%). The percentage deviation in SSA from ground level to 700 m was found to be highest for Case C (i.e. 5.95%) followed by Case B (i.e. 4.19%) and Case A (i.e. 1.4%). Modelled aerosol optical properties within boundary layer may improve our understanding about the thermodynamics of lower atmosphere.

On the application of principal component analysis to the calculation of the bulk integral optical properties for radiation parameterizations in climate models.

Rigorous electromagnetic computations required for the calculation of high-resolution monochromatic bulk integral optical properties of irregular atmospheric particles are onerous in memory and in time requirements. Here, it is shown that from a set of 145 monochromatic bulk integral ice optical properties, it is possible to reduce the set to eight hinge wavelengths by using the method of principal component analysis (PCA) regression. From the eight hinge wavelengths, the full set can be reconstructed to within root mean square errors of ≪1%. To obtain optimal reconstruction, the training set must cover as wide a range of parameter space as possible. Rigorous electromagnetic methods can now be routinely applied to represent accurately the integral optical properties of atmospheric particles in climate models.

基于主动差分吸收光谱系统获取大气颗粒物光学特性研究

基于主动差分吸收光谱系统获取大气颗粒物光学特性研究

Optical scattering simulation of ice particles with surface roughness modeled using the Edwards-Wilkinson equation

Constructing an appropriate particle morphology model is essential for realistic simulation of optical properties of atmospheric particles. This paper presents a model for generating surface roughness based on a combination of methods from discrete differential geometry combined with a stochastic partial differential equation for surface evolution introduced by Edwards and Wilkinson. Scattering of light by roughened particles is simulated using the Invariant Imbedding T-Matrix (II-TM) method. The effects of surface roughness on the single-scattering properties, namely, the phase matrix, asymmetry factor, and extinction efficiency, are investigated for a single wavelength in the visible range and for a range of size parameters up to x=50. Three different smooth shapes are considered: spherical, spheroidal, and hexagonal, the latter two in just the “compact particle” case of unit aspect ratio. It is shown that roughness has negligible effects on the optical scattering properties for size parameters less than 20. For size parameters ranging from 20 to 50, the phase matrix elements are more sensitive to the surface roughness than are two important integral optical properties, the extinction efficiency and asymmetry factor. As has been seen in studies using other forms of roughening, the phase function is progressively smoothed as roughness increases. The effect on extinction efficiency is to increase it, and on asymmetry factor is to decrease it. Each of these effects is relatively modest in the size range considered, but the trend of results suggests that greater effects will be seen for size parameters larger than ones considered here.

Regional and global modeling of aerosol optical properties with a size, composition, and mixing state resolved particle microphysics model

Abstract. There exist large uncertainties in the present modeling of physical, chemical, and optical properties of atmospheric particles. We have recently incorporated an advanced particle microphysics (APM) model into a global chemistry transport model (GEOS-Chem) and a regional weather forecasting and chemistry model (WRF-Chem). Here we develop a scheme for calculating regional and global aerosol optical depth (AOD) from detailed aerosol information resolved by the APM model. According to GEOS-Chem-APM simulations, in most parts of the globe, the mass of secondary species resides mainly within secondary particles (60–90%), but in certain regions a large fraction (up to 50–80%) can become coated on various primary particles. Secondary species coated on black carbon and primary organic carbon particles significantly increase the size and hygroscopicity of these particles and thus impact their optical properties. The GEOS-Chem-APM model captures the global spatial distributions of AOD derived from AERONET, MODIS, and MISR measurements, generally within a factor of ~2. Our analysis indicates that modeled annual mean AODs at all sky and clear sky conditions differ by ~20% globally averaged and by >50% in some regions. The time series of WRF-Chem-APM predicted AOD over the northeastern United States in June 2008 have been compared to those from seven AERONET sites. Overall, the model mostly captures the absolute values as well as the variations of AOD at the AERONET sites (including dramatic changes associated with the crossing of high AOD plumes). Both GEOS-Chem and WRF-Chem simulations indicate that AOD over the northeastern US is dominated by secondary particles and have large spatiotemporal variations.

Insights into secondary organic aerosol formed via aqueous-phase reactions of phenolic compounds based on high resolution mass spectrometry

Abstract. Recent work has shown that aqueous-phase reactions of phenolic compounds – phenol (C6H6O), guaiacol (C7H8O2), and syringol (C8H10O3) – can form secondary organic aerosol (SOA) at high yields. Here we examine the chemical characteristics of this SOA and its formation mechanisms using a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-AMS), an Ion Chromatography system (IC), and a Total Organic Carbon (TOC) analyzer. The phenolic SOA are highly oxygenated with oxygen-to-carbon (O/C) ratios in the range of 0.80–1.06 and carbon oxidation states (=2×O/C-H/C) between −0.14 and +0.47. The organic mass-to-carbon (OM/OC) ratios determined by the HR-AMS (=2.21–2.55) agree well with values determined based on the SOA mass measured gravimetrically and the OC mass from the TOC analyzer. Both the O/C and OM/OC ratios of the phenolic SOA are similar to the values observed for ambient low-volatility oxygenated/secondary OA (LV-OOA). Oxalate is a minor, but ubiquitous, component of the SOA formed from all three phenolic precursors, accounting for 1.4−5.2% of the SOA mass, with generally higher yields in experiments with H2O2 added as an OH source compared to without. The AMS spectra show evidence for the formation of syringol and guaiacol dimers and higher oligomers via C-C and C-O coupling of phenoxyl radicals, which are formed through oxidation pathways such as abstraction of the phenolic hydrogen atom or OH addition to the aromatic ring. This latter pathway leads to hydroxylation of the aromatic ring, which is one mechanism that increases the degree of oxidation of the SOA products. Compared to direct photochemical reactions of the phenols, OH-initiated reactions favor the formation of smaller oxidation products but less dimers or higher oligomers. Two unique and prominent ions in the syringol and guaiacol SOA spectra, m/z 306 (C16H18O6+) and m/z 246 (C14H14O4+), respectively, are observed in ambient aerosols significantly influenced by wood combustion and fog processing. Our results indicate that cloud and fog processing of phenolic compounds, especially in areas with active biomass burning, might represent an important pathway for the formation of low-volatility and highly oxygenated organic species, which would remain in the particle phase after fog/cloud evaporation and affect the chemical and optical properties of atmospheric particles.

Simultaneous Inverse Determination of All Optical Properties of Atmospheric Particles: Improved Results by Using a Realistic Phase Function Approximation

Abstract Through mathematical inversion of photometric data all optical properties of atmospheric particles necessary for radiative transfer calculations are derived simultaneously. These optical properties are the volume phase function, the volume extinction, and the volume absorption coefficient. Additionally, the apparent complex refractive index and the apparent volume fraction of soot within the particles are calculated from the absorption-to-extinction ratio. The phase function of the particles is approximated by a combination of two Henyey–Greenstein functions, one of them governing the forward and the other the backward scattering. This approximation contains only three constants, one for weighting of the two Henyey–Greenstein functions, and two asymmetry parameters. It describes very well the phase functions known from measurements on airborne particles in the entire range of scattering angles from 0° to 180°. Thus the new results can be used not only for climate modeling but also for remote sens...

Wintertime optical properties of atmospheric particles and weather

Atmospheric particles from the boundary layer were collected on Nuclepore filters at the summit of a mountain near Frankfurt am Main (Germany). The optical properties (absorption, scattering and extinction coefficients, the asymmetry parameter of the phase function of scattered light and the complex refractive index) and the soot content of the particles were determined using a photometric method and a subsequent mathematical inversion. The dependence of the optical properties and the soot content on meteorological parameters measured at the sampling site and on large and mesoscale motions of the atmosphere was investigated. The major findings are: (I) The observed optical properties of the particles are correlated with the air temperature measured at the sampling site. (2) The optical properties change during the passage of a cold front. (3) The optical properties depend on the existence of a stable stratified layer in the lower troposphere. (4) The optical properties depend on the local weather situation and on the history of the air masses arriving at the sampling site.

Optical properties of atmospheric particles: complete parameter sets obtained through polar photometry and an improved inversion technique

Complete sets of optical parameters of dry particles sampled on a Nuclepore filter are derived through interpretation of photometric data with an improved inversion technique. The parameters are the volume-extinction and absorption coefficients, the single-scattering albedo, the asymmetry parameter of the volume scattering function, the apparent complex refractive index, and the apparent soot content. They may serve as input data for solar radiation-budget considerations. Results from preliminary measurements taken in Central Europe and Italy show an extreme variability of the optical parameters. Both large regional and temporal variabilities have been observed caused by the fluctuating midlatitude weather systems and human activities.