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Nitrogen dioxide (NO2) absorption correction of the sulfur dioxide (SO2) camera was demonstrated for the first time. The key to improving the measurement accuracy is to combine a differential optical absorption spectroscopy (DOAS) instrument with the SO2 camera for the real-time NO2 absorption correction and aerosol scattering correction. This method performs NO2 absorption correction by the correlation between the NO2 column density measurement of the DOAS and the NO2 optical depth of the corresponding channel from the SO2 camera at a narrow wavelength window around 310 and 310 nm. The error of correction method is estimated through comparison with only using the second channel of the traditional SO2 camera to correct for aerosol scattering and it can be reduced by 11.3% after NO2 absorption corrections. We validate the correction method through experiments and demonstrate it to be of greatly improved accuracy. The result shows that the ultraviolet (UV) SO2 camera system with NO2 absorption corrections appears to have great application prospects as a technology for visualized real-time monitoring of SO2 emissions. [ABSTRACT FROM AUTHOR]

A practical Atmospheric Correction algorithm for inLand and Nearshore Coastal waters (ACLANC) is proposed in this study. The ACLANC algorithm uses interpolated aerosol optical depth (AOD) products (AOD $_{\mathrm {interp}}$) from nearby land surfaces and simulates the corresponding aerosol reflectance spectrum using a combination of the continental model in the satellite signal in the solar spectrum-vector (6SV) radiative transfer code and an approximate aerosol model (r85f20) in the Sea-viewing Wide Field-of view Sensor (SeaWiFS) Data Analysis System (SeaDAS). Validations with worldwide in situ measurements show that the ACLANC-derived remote-sensing reflectance ($R_{\mathrm {rs}}$) for nine Moderate Resolution Imaging Spectroradiometer (MODIS) bands agreed well with the in situ datasets, where the mean $R^{2}$ was 0.77 ± 0.09 and the mean unbiased percent difference was 28.7% ± 9.8%. ACLANC outperformed the existing atmospheric correction algorithms in not only the accuracy of the $R_{\mathrm {rs}}$ retrievals but also data coverage. Vicarious calibration over the ACLANC algorithm showed minor improvement in the derived $R_{\mathrm {rs}}$ products. Error budget analysis revealed that the uncertainties in AODinterp represent >50% of the errors for ACLANC and that this proportion increases with decreasing AOD. Further efforts can also be applied to improve the aerosol models, especially for turbid aerosol environments, where the fixed aerosol model in SeaDAS contributes up to 30% of the error budget. The ACLANC algorithm can potentially be implemented in ocean color missions other than MODIS to obtain $R_{\mathrm {rs}}$ with high accuracy and wide coverage for global inland and nearshore coastal waters. [ABSTRACT FROM AUTHOR]

From August to October 2019, the aerosol backscattering and deformation of the Earth's crust were monitored in the tunnels of the Baksan Neutrino Observatory above the Elbrus volcano chamber. An omnidirectional modulation of the seasonal crust compression with a multiple excess of the average rate (~4 μm/day) and with a change in the sign of decrease in the lidar-signal scattering on aerosols to anomalous growth in the "hot" tunnel is found. It is established that the modulation is induced by the rapid cooling of the external atmosphere. The delays of the accelerated compression of the strainmeter arm (up to 14 μm/day) and the beginning of growth of the backscattering lidar signal relative to the beginning of the temperature decrease (~2 and ~7 days, respectively) are measured. The mechanism of the phenomena discovered is discussed. [ABSTRACT FROM AUTHOR]

Due to the polarization effects of the Earth's surface reflection and atmospheric particles' scattering, high-precision retrieval of atmospheric parameters from near-infrared satellite data requires accurate vector atmospheric radiative transfer simulations. This paper presents a near-infrared vector radiative transfer model based on the doubling and adding method. This new model utilizes approximate calculations of the atmospheric transmittance, reflection, and solar scattering radiance for a finitely thin atmospheric layer. To verify its accuracy, the results for four typical scenarios (single molecular layer with Rayleigh scattering, single aerosol layer scattering, multi-layer Rayleigh scattering, and true atmospheric with multi-layer molecular absorption, Rayleigh and aerosol scattering) were compared with benchmarks from a well-known model. The comparison revealed an excellent agreement between the results and the reference data, with accuracy within a few thousandths. Besides, to fulfill the retrieval algorithm, a numerical differentiation-based Jacobian calculation method is developed for the atmospheric and surface parameters. This is coupled with the adding and doubling process for the radiative transfer calculation. The Jacobian matrix produced by the new algorithm is evaluated by comparison with that from the perturbation method. The relative Jacobian matrix deviations between the two methods are within a few thousandths for carbon dioxide and less than 1.0×10−3% for aerosol optical depth. The two methods are consistent for surface albedo, with a deviation below 2.03×10−4%. All validation results suggest that the accuracy of the proposed radiative transfer model is suitable for inversion applications. This model exhibits the potential for simulating near-infrared measurements of greenhouse gas monitoring instruments. [ABSTRACT FROM AUTHOR]

An efficient spectrally resolved radiative model is used to calculate surface downwelling longwave (DLW) radiation (0  ∼  2500 cm − 1 ) under clear sky (cloud free) conditions at the ground level. The wavenumber spectral resolution of the model is 0.01 cm − 1 and the atmosphere is represented by 18 non-uniform plane-parallel layers with pressure in each layer determined on a pressure-based coordinate system. The model utilizes the most up-to-date (2016) HITRAN molecular spectral data for 7 atmospheric gases: H 2 O, CO 2 , O 3 , CH 4 , N 2 O, O 2 and N 2 . The MT_CKD model is used to calculate water vapor and CO 2 continuum absorption coefficients. Longwave absorption and scattering coefficients for aerosols are modeled using Mie theory. For the non-scattering atmosphere (aerosol free), the surface DLW agrees within 2.91% with mean values from the InterComparison of Radiation Codes in Climate Models (ICRCCM) program, with spectral deviations below 0.035 W cm m − 2 . For a scattering atmosphere with typical aerosol loading, the DLW calculated by the proposed model agrees within 3.08% relative error when compared to measured values at 7 climatologically diverse SURFRAD stations. This relative error is smaller than a calibrated parametric model regressed from data for those same 7 stations, and within the uncertainty (+/− 5 W m − 2 ) of pyrgeometers commonly used for meteorological and climatological applications. The DLW increases by 1.86  ∼  6.57 W m − 2 when compared with aerosol-free conditions, and this increment decreases with increased water vapor content due to overlap with water vapor bands. As expected, the water vapor content at the layers closest to the surface contributes the most to the surface DLW, especially in the spectral region 0  ∼  700 cm − 1 . Additional water vapor content (mostly from the lowest 1 km of the atmosphere) contributes to the spectral range of 400  ∼  650 cm − 1 . Low altitude aerosols ( ∼  3.46 km or less) contribute to the surface value of DLW mostly in the spectral range 750  ∼  1400 cm − 1 . [ABSTRACT FROM AUTHOR]

This study analyzes the aerosol optical properties (scattering, absorption coefficients, single scattering albedo), supported by chemical composition measurements, in the marine boundary layer of the East Mediterranean – Middle East (EMME) region, aiming to explore the spatio-temporal variability, aerosol mixing state, sources and dominant types. The measurements were taken during the AQABA (Air Quality and climate change in the Arabian Basin) cruise campaign (1st July – September 1, 2017) from the south of France to Kuwait and back. Both scattering and absorption coefficients for PM 1 and PM 10 particles maximized in the southern Red Sea, due to continental outflow from East Africa, and in the Arabian/Persian Gulf due to enhanced anthropogenic/industrial emissions, leading to 3-fold sulfate concentrations compared to the other regions. The East Mediterranean exhibited moderate aerosol loading, with high scattering Ångström Exponent (SAE) values (1.98, 1.30 for PM 1 and PM 10 , respectively), which increased in the Suez Canal and the Arabian Gulf due to impact from combustion sources. The aerosol over the Gulf of Aden and the West Arabian Sea was dominated by coarse particles (SAE<1), with higher Ca2+ concentrations (dust influence). The absorption Ångström Exponent (AAE) remained close to 1, indicative of black carbon (BC) from fossil fuel combustion, while it increased in regions dominated by dust (Gulf of Aden, Arabian Sea). Contrasting aerosol types were identified between the sub-regions using the SAE vs. AAE classification scheme. The " BC-dominated " type prevailed over the East Mediterranean and Suez Canal, while dust mixtures dominated in the Gulf of Aden and the Arabian Sea. Over the Arabian Gulf, mixing of anthropogenic pollution with marine aerosols formed a type with fine particles and low spectral dependence of absorption (AAE<1). Spectral SSA significantly varied along the ship cruise, while it was used for determination of dust influence in the marine environment. [Display omitted] • Investigation of aerosol optical, chemical properties in the marine environments around the EMME region. • Aerosol scattering/absorption maximized in the Persian Gulf due to anthropogenic/industrial aerosol. • Contrasting aerosol-type dominance between the marine subregions. • Spectral SSA enables to detect the dust imprint in the marine boundary layer. • Significant influence of the particle size on aerosol properties and dominant types. [ABSTRACT FROM AUTHOR]

Abstract: Large retrieval errors in column‐weighted CO2 mixing ratio (XCO2) over deserts are evident in the Orbiting Carbon Observatory 2 version 7 L2 products. We argue that these errors are caused by the surface albedo being close to a critical surface albedo (αc). Over a surface with albedo close to αc, increasing the aerosol optical depth (AOD) does not change the continuum radiance. The spectral signature caused by changing the AOD is identical to that caused by changing the absorbing gas column. The degeneracy in the retrievals of AOD and XCO2 results in a loss of degrees of freedom and information content. We employ a two‐stream‐exact single scattering radiative transfer model to study the physical mechanism of XCO2 retrieval error over a surface with albedo close to αc. Based on retrieval tests over surfaces with different albedos, we conclude that over a surface with albedo close to αc, the XCO2 retrieval suffers from a significant loss of accuracy. We recommend a bias correction approach that has significantly improved the XCO2 retrieval from the California Laboratory for Atmospheric Remote Sensing data in the presence of aerosol loading. [ABSTRACT FROM AUTHOR]

Global solar radiation is the driving force in hydrological cycle especially for evapotranspiration (ET) and is quite infrequently measured. This has led to the reliance on indirect techniques of estimation for data scarce regions. This study presents an improved technique that uses information from a numerical weather prediction (NWP) model (National Centre for Atmospheric Research NCAR's Mesoscale Meteorological model version 5 MM5), for the determination of a cloud cover index (CI), a major factor in the attenuation of the incident solar radiation. The cloud cover index (CI) together with the atmospheric transmission factor ( K T ) and output from a global clear sky solar radiation were then used for the estimation of global solar radiation for the Brue catchment located in the southwest of England. The results clearly show an improvement in the estimated global solar radiation in comparison to the prevailing approaches. [ABSTRACT FROM AUTHOR]

This paper focuses on the assessment of atmospheric aerosol optical properties at the surface and in atmospheric column during both desert dust and dust-free conditions over Granada, South-eastern Iberian Peninsula. Indeed, the spectral dependence of aerosol absorption and scattering properties is analyzed in detail. The analyzed period ranges from June 2008 to December 2010. On dusty days, the mean scattering Angström exponent value obtained in the atmospheric column (SAE col ) (0.5 ± 0.3) was lower than the observed at the surface level (SAE is ) (1.3 ± 0.6), indicating higher contribution of coarse particles at high atmospheric level than at ground level during the analyzed dust events. In addition, it is noticed that the absorption Angström exponent in the atmospheric column (AAE col ) with mean value of 1.5 ± 0.2 and at the surface (AAE is ) with mean value of 1.3 ± 0.2 obtained during dusty situations are indicative of mixture of desert dust and black carbon particles as dominant absorbers both in the atmospheric column and at the surface during dust intrusions over Granada. On the other hand, a non-parametric test (Kolmogorov–Smirnov) revealed that no significant statistical difference was found for AAE is between desert dust and free-dust conditions. This result may be due to the important contribution of urban absorbing aerosol (e.g. Black carbon) at ground level in the study location. Therefore, these parameters (AAE col and AAE is ) are not very useful to detect desert dust events without the use of other information (e.g., aerosol size) over urban area like Granada. A dust extreme event was analyzed in order to retrieve optical parameters during situation dominated by desert dust. The values of SAE col and SAE is obtained during this extreme event were in agreement with the values showed above for the period 2008–2010, although the differences between dust-free and dust conditions are more noticeable in this special event. [ABSTRACT FROM AUTHOR]

Abstract: In this paper the detection of components of volcanic eruption has been carried out investigating, in appropriate bands of the electromagnetic spectrum (6.25, 8.7, 10.8, 12μm), the values of the brightness temperature. The analysis has been performed in the Thermal Infrared Region (TIR) studying both the absorption–emission and scattering phenomena related to the interactions between electromagnetic radiation and volcanic emissions. The results have been achieved by means of a combined use of numerical simulations, devoted to examining the behaviour of the atmosphere gases and volcanic components, and remotely sensed satellite images. The proposed methodologies allow an estimate of the amount of gaseous and solid components, of the size of the emitted particles, of the height of the volcanic plume and of the distance of the volcanic components from the crater. The processed images come from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) sensor on board the geo-stationary satellite Meteosat Second Generation (MSG) and take into consideration an eruption of the Etna volcano as a study case (1st of April 2012, 04:30 and 05:30UTC). The procedures are general and may therefore be extended to any other similar case. [Copyright &y& Elsevier]

Abstract: Measurements of aged aerosol scattering in the Eastern Mediterranean at ambient and low relative humidity (RH) are combined with measurements of its size composition distribution and its aerosol water content to gain insights about its hygroscopic and optical properties. The particle water uptake above 60% RH can be explained by the water absorption by the inorganic particulate matter in this sulphate rich environment. However, the particles were supersaturated aqueous solutions at low RH and this water cannot be explained just by the water associated with their inorganic components in a metastable state. We estimate that approximately 20% of this water may be contributed by the aged organic particulate matter. A semi-empirical equation is derived allowing the estimation of the aged aerosol Volume Growth Factor (VGF) from the ratios of scattering at ambient and low RH. The predictions of the equation are in good agreement with the measurement dataset obtained during the study. [Copyright &y& Elsevier]

An aerosol modulation transfer function (MTF) model is developed to assess the impact of aerosol scattering on passive long-range imaging sensors. The methodology extends from previous work to explicitly address imaging scenarios with a nonuniform distribution of scattering characteristics over the propagation path and incorporates the moderate resolution transfer code database of aerosol cross-section and phase function characteristics in order to provide an empirical foundation for realistic quantitative MTF assessments. The resulting model is compared with both predictions from a Monte-Carlo scattering simulation and a scene-derived MTF estimate from an empirical image, with reasonable agreement in both cases. Application to long-range imaging situations at both visible and infrared wavelengths indicates that the magnitude and functional form of the aerosol MTF differ significantly from other contributors to the composite system MTF. Furthermore, the image-quality impact is largely radiometric in the sense that the contrast reduction is approximately independent of spatial frequency, and image blur is practically negligible. [ABSTRACT FROM AUTHOR]

Abstract: In this paper we apply an entropy analysis to measured scattering matrices from the Amsterdam light scattering database. We select examples of mineral aerosols from the database and use them to demonstrate differences in polarization behavior between the particle clouds using a new coherency matrix formulation. These differences are further investigated by analyzing the polarized component of the matrices via two new eigenvector parameters, which can be mapped conveniently onto the surface of a sphere, analogous to the Poincaré sphere used for wave states. We conclude by considering the potential for discriminating different aerosols on the basis of their entropy/eigenvector signatures by solving the contrast optimization problem for clouds with different scattering matrices by using a novel generalized coherency eigenvalue formulation. [Copyright &y& Elsevier]

Abstract: Observation and appreciation of scenic landscape features, airport runway visibility, and the earth''s radiation balance are all dependent on the radiative properties of the atmosphere, which in turn are dependent on the scattering and absorption characteristics of ambient aerosols. Atmospheric scattering and, to a lesser degree, absorption characteristics are highly dependent on the amount of water vapor absorbed by aerosols under ambient relative humidity (RH) conditions. Water vapor absorptive properties of inorganic aerosols have been extensively measured and modeled; however, hygroscopic properties of organic aerosols are less understood, especially as they occur in the ambient atmosphere. Therefore, an aerosol characterization study was conceived and implemented at Yosemite National Park, which is highly impacted by carbonaceous aerosols. The overall objective of the study was to characterize the physical, chemical, and optical properties of a carbon-dominated aerosol, including the ratio of total organic matter weight to organic carbon, organic mass scattering efficiencies, and the hygroscopic characteristics of a carbon-laden ambient aerosol. The study was conducted during July, August, and the first week of September at Turtleback Dome on the south rim of the entrance to Yosemite Valley. The ratio of the scattering coefficient at some RH divided by the scattering coefficient at some minimum RH () was measured over a wide range of RHs. f(85&#60;RH&#60;90) decreased from about 2.0 to &#60;1.2 as the organic carbon mass (OMC/(NH4)2(SO4)) ratio increased from a low of 0.57 to 11.15, implying that the f(RH) associated with organics is small, possibly on the order of 1.1 or less. Furthermore, modeling f(RH) as a function of RH suggested that ambient organic mass aerosols may be weakly hygroscopic with an f(RH) at RH=85–90% of about 1.1. [Copyright &y& Elsevier]

Abstract: The spatial distribution of the vector of the Stokes parameters characterizing the radiance intensity and the radiance polarization describes the radiation field in the atmosphere. A simplified treatment of light as the scalar has only restricted application. A few studies compared previously results of the vector and scalar radiative transfer models and showed that scalar models are in error by up to 10% for many cases. Though several observational conditions were exploited, an effect of polarization on modeling of UV radiance has not been investigated yet for twilight. The paper presents a preliminary study of modeled UV radiance during twilight taking into account polarization. The intensity and the degree of linear polarization of the scattered UV radiance for two cases of the ground-based observations are discussed. In the first case, radiation incoming from the zenith for the solar zenith angles (SZA) from 90° to 98° is under investigation. Radiation in the solar principal plane for the beginning of twilight (SZA=90.1°) was calculated in the second case. The study showed that the UV radiation field in the twilight atmosphere can be handled correctly only using the vector theory. The errors of scalar radiative transfer strongly depend on wavelength, line of an observation and solar position. The revealed distortion of the zenith radiance caused by using of the scalar approximation reaches maximum of 15% at 340nm for the solar zenith angle (SZA) equal to 98°. The shorter wavelengths have the smaller errors, about 5% at 305nm for SZA=98°, due to the larger part of the single scattered radiance. The error of the scalar modeling may be as large as −17% for radiance incoming from the horizon for SZA=90.1°. Scalar radiative transfer models underestimate the integral intensity in the principal plane up to 3–4%±0.5% at SZA=90.1° for wavelengths from 320 to 340nm. This should be taken into account in problems of radiative budget estimation and remote sensing of the atmosphere exploiting the twilight period. [Copyright &y& Elsevier]

Size-fractionated ambient aerosol samples were collected at a pasture site and a primary rainforest site in the Brazilian Amazon Basin during two field campaigns (April–May and September–October 1999), as part of the European contribution to the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA-EUSTACH). The samples were analyzed for up to 19 trace elements by particle-induced X-ray emission analysis (PIXE), for equivalent black carbon (BCe) by a light reflectance technique and for mass concentration by gravimetric analysis. Additionally, we made continuous measurements of absorption and light scattering by aerosol particles. The vertical chemical composition gradients at the forest site have been discussed in a companion article (Journal of Geophysical Research-Atmospheres 108 (D18), 4591 (doi:4510.1029/2003JD003465)). In this article, we present the results of a source identification and quantitative apportionment study of the wet and dry season aerosols, including an apportionment of the measured scattering and absorption properties of the total aerosol in terms of the identified aerosol sources. Source apportionments (obtained from absolute principal component analysis) revealed that the wet and dry season aerosols contained the same three main components, but in different (absolute and relative) amounts: the wet season aerosol consisted mainly of a natural biogenic component, whereas pyrogenic aerosols dominated the dry season aerosol mass. The third component identified was soil dust, which was often internally mixed with the biomass-burning aerosol. All three components contributed significantly to light extinction during both seasons. At the pasture site, up to 47% of the light absorption was attributed to biogenic particles during the wet season, and up to 35% at the tower site during the wet-to-dry transition period. The results from the present study suggest that, in addition to pyrogenic particles, biogenic and soil dust aerosols must be taken into account when modeling the physical and optical properties of aerosols in forested regions such the Amazon Basin. [Copyright &y& Elsevier]

COVID-19 is evolving into one of the worst pandemics in recent history, claiming a death toll of over 1.5 million as of December 2020. In an attempt to limit the expansion of the pandemic in its initial phase, nearly all countries imposed restriction measures, which resulted in an unprecedented reduction of air pollution. This study aims to assess the impact of the lockdown effects due to COVID-19 on in situ measured aerosol properties, namely spectral-scattering (bsca) and absorption (babs) coefficients, black carbon (BC) concentrations, single-scattering albedo (SSA), scattering and absorption Ångström exponents (SAE, AAE) in Athens, Greece. Moreover, a comparison is performed with the regional background site of Finokalia, Crete, for a better assessment of the urban impact on observed differences. The study examines pre-lockdown (1–22 March 2020), lockdown (23 March–3 May 2020) and post-lockdown (4–31 May 2020) periods, while the aerosol properties are also compared with a 3–4 year preceding period (2016/2017–2019). Comparison of meteorological parameters in Athens, between the lockdown period and respective days in previous years, showed only marginal variation, which is not deemed sufficient in order to justify the notable changes in aerosol concentrations and optical properties. The largest reduction during the lockdown period was observed for babs compared to the pre-lockdown (−39%) and to the same period in previous years (−36%). This was intensified during the morning traffic hours (−60%), reflecting the large decrease in vehicular emissions. Furthermore, AAE increased during the lockdown period due to reduced emissions from fossil-fuel combustion, while a smaller (−21%) decrease was observed for bsca along with slight increases (6%) in SAE and SSA values, indicating that scattering aerosol properties were less affected by the decrease in vehicular emissions, as they are more dependent on regional sources and atmospheric processing. Nighttime BC emissions related to residential wood-burning were slightly increased during the lockdown period, with respect to previous-year means. On the contrary, aerosol and pollution changes during the lockdown period at Finokalia were low and highly sensitive to natural sources and processes. [ABSTRACT FROM AUTHOR]

Smoke from fires significantly influences climate, weather, and human health. Fire smoke is traditionally detected using an aerosol index calculated from spectral contrast changes. However, such methods usually miss thin smoke plumes. It also remains challenging to accurately separate smoke plumes from dust, clouds, and bright surfaces. To improve smoke plume detections, this paper presents a new scattering-based smoke detection algorithm (SSDA) depending mainly on visible and infrared imaging radiometer suite (VIIRS) blue and green bands. The SSDA is established based on the theory of Mie scattering that occurs when the diameter of an atmospheric particulate is similar to the wavelength of the scattered light. Thus, smoke commonly causes Mie scattering in VIIRS blue and green bands because of the close correspondence between smoke particulate diameters and the blue/green band wavelengths. For developing the SSDA, training samples were selected from global fire-prone regions in North America, South America, Africa, Indonesia, Siberia, and Australia. The SSDA performance was evaluated against the VIIRS aerosol detection product and smoke detections from the ultraviolet aerosol index using manually labeled fire smoke plumes as a benchmark. Results show that the SSDA smoke detections are superior to existing products due chiefly to the improved ability of the algorithm to detect thin smoke and separate fire smoke from other surface types. Moreover, the SSDA smoke distribution pattern exhibits a high spatial correlation with the global fire density map, suggesting that SSDA is capable of detecting smoke plumes of fires in near real-time across the globe. [ABSTRACT FROM AUTHOR]

The angle-dependent scattering effect of aerosols in the atmosphere not only influences climate through radiative forcing effects but also impacts trace gas remote sensing by modifying the path of radiation through the atmosphere. The aerosol phase function, which characterizes the angular signature of scattering, has been continuously monitored from ground-based and space-borne observations. However, the range of scattering angles these instruments can sample is very limited. Here, we report multi-year measurements from a mountain-top remote sensing instrument: the California Laboratory for Atmospheric Remote Sensing Fourier Transform Spectrometer (CLARS-FTS), which overlooks the Los Angeles megacity. The observational geometries of CLARS-FTS provide a wide range of scattering angles, from about 20° (forward) to about 140° (backward), which is larger than the range provided by any existing aerosol remote sensing instrument. We then quantify the aerosol angular scattering effect using the O 2 ratio, which is the ratio of retrieved O 2 Slant Column Density (SCD) to geometric O 2 SCD. The O 2 ratio quantifies the light path modification due to aerosol scattering, with a value of 1 representing an aerosol-free scenario. The lower the O 2 ratio value than 1, the stronger the aerosol loading. CLARS-FTS measurements are highly sensitive to the angular scattering effect of aerosols in the Los Angeles (LA) urban atmosphere, due to the long light path going through the boundary layer and the wide range of observational angles. The differences in aerosol scattering between different surface reflection points targeted by CLARS-FTS can be explained by differences in their angular scattering geometries. The correlation between measurements at different targets can be used to quantify the strength of the angular dependence of the aerosol phase function. Applying the correlation technique to CLARS-FTS measurements, we find that, from 2011 to 2018, there is no significant trend in the aerosol phase function in the LA megacity. Overall, this study provides a practical observing strategy for quantifying the angular dependence of aerosol scattering in urban atmospheres that could potentially contribute towards improved greenhouse gas remote sensing in megacities. • A mountain-top observatory for monitoring aerosols in megacities is introduced. • The observatory makes measurements at a wide range of scattering angles. • Aerosol scattering is quantified based on retrieved oxygen slant column. • The aerosol scattering pattern can be explained by variations in scattering angle. • No significant change in aerosol phase function in LA from 2011 to 2018. [ABSTRACT FROM AUTHOR]