First results from light scattering enhancement factor over central Indian Himalayas during GVAX campaign

Effects of hygroscopicity on aerosol optical properties and direct radiative forcing in Beijing: Based on two-year observations

The influence of relative humidity on aerosol properties and the direct radiative forcing of PM₁₀ and PM₁ were investigated in Beijing from January 2018 to December 2019. The annual mean scattering hygroscopic growth factor at RH = 80 % [f(80 %)] of PM₁₀ and PM₁ were 1.60 ± 0.24 and 1.58 ± 0.22, respectively. The variation of aerosol hygroscopic growth factors of PM₁₀ and PM₁ aerosols was similar, which is mainly due to the fact that aerosol scattering in Beijing is dominated by fine particles. The seasonal mean f(80 %) of PM₁₀ from spring to winter were 1.66 ± 0.23, 1.71 ± 0.25, 1.51 ± 0.20, 1.49 ± 0.16, respectively, which were higher in spring and summer, and lower in autumn and winter. The diurnal variation of f(80 %) was relatively higher from 12:00 to 18:00, which could be related to the formation of secondary aerosols by photochemical reactions. f(80 %) shows a strong positive relationship with both the scattering Angström exponent (SAE) and the single scattering albedo (ω₀) under dry conditions; therefore, the scattering hygroscopic growth factor could be estimated using these two parameters. The upscatter fraction (β) and single scattering albedo, which are the key aerosol optical properties for the calculation of direct radiative forcing, are also RH-dependent. As RH increases, the upscatter fraction (backscatter fraction) decreases and ω₀ increases. The aerosol radiative forcing at RH 80 % was 1.48 times as that in the dry state. The sensitivity experiment showed that the variation in the scattering coefficient with relative humidity had the greatest influence on radiation forcing, followed by β and ω₀. The seasonal variation of ΔF(80 %)/ΔF(dry) coincides with that of the aerosol hygroscopic growth factor. Our study suggests that understanding the influence of relative humidity on aerosol properties and direct radiative forcing is important for accurately estimating the radiative forcing of aerosols.

The Solar Radiation Climate of Greece

The solar radiation climate of Greece is investigated by using typical meteorological years (TMYs) at 43 locations in Greece based on a period of 10 years (2007–2016). These TMYs include hourly values of global, Hg, and diffuse, Hd, horizontal irradiances from which the direct, Hb, horizontal irradiance is estimated. Use of the diffuse fraction, kd, and the definition of the direct-beam fraction, kb, is made. Solar maps of annual mean Hg, Hd, kd, and kb are prepared over Greece under clear and all skies, which show interesting but explainable patterns. Additionally, the intra-annual and seasonal variabilities of these parameters are presented and regression equations are provided. It is found that Hb has a negative linear relationship with kd; the same applies to Hg with respect to kd or with respect to the latitude of the site. It is shown that kd (kb) can reflect the scattering (absorption) effects of the atmosphere on solar radiation, and, therefore, this parameter can be used as a scattering (absorption) index. An analysis shows that the influence of solar variability (sunspot cycle) on the Hg levels over Athens in the period 1953–2018 was less dominant than the anthropogenic (air-pollution) footprint that caused the global dimming effect.

Simultaneous measurements of PM1 and PM10 aerosol scattering properties and their relationships in urban Beijing: A two-year observation

The aerosol scattering properties of submicron (PM₁) and sub-10 μm particles (PM₁₀) under dry conditions (RH <30%) were investigated in Beijing from 2018 to 2019. Using the simultaneous measurement of PM₁ and PM₁₀, the scattering properties of super micron (PM_10-1) particles were also calculated. At 550 nm, the average of scattering coefficient (σ_sp) and asymmetry parameter (g) were 208.7 ± 204.9 Mm^-1 and 0.61 ± 0.04 for PM₁₀, 140.6 ± 130.2 Mm^-1 and 0.60 ± 0.04 for PM₁, and 69.8 ± 82.2 Mm^-1 and 0.62 ± 0.04 for PM_10-1, respectively, while the backscattering ratio (b) values were 0.13 ± 0.02 for PM₁₀ and PM₁, and 0.12 ± 0.02 for PM_10-1. The mass scattering efficiencies (MSE) for PM₁₀, PM₁ and PM_10-1 were 2.43 ± 2.37, 3.67 ± 0.96, and 1.73 ± 1.82 m² g^-1, respectively. In 2019, σ_sp decreased by approximately 18.4% for PM₁₀, and 16.7% for PM₁ compared with those in 2018, which was quite similar to the decrease of 17% and 19% for PM₁₀ and PM_2.5 mass concentrations during the same time period. The scattering Ångström exponent (SAE_450/700), which was 1.88 ± 0.29 for PM₁ and 1.50 ± 0.27 for PM₁₀ indicated size distributions dominated by fine mode aerosols. This is also evidenced by the high submicron scattering ratio (Rsp) (71.1% ± 7.9%). The high SAE, Rsp, and high PM₁ σ_sp in the study suggest that control of fine particle pollution is important to reduce overall PM pollution in urban Beijing. In addition, with an increase in σ_sp, b, Rsp, and SAE gradually decreased, while g and MSE increased. The clearly scattering coefficient-dependent MSE suggests that high aerosol loading and high MSE both play an important role in degraded visibility during heavy pollution periods.

Identification of key aerosol types and mixing states in the central Indian Himalayas during the GVAX campaign: the role of particle size in aerosol classification

Studies in aerosol properties, types and sources in the Himalayas are important for atmospheric and climatic issues due to high aerosol loading in the neighboring plains. This study uses in situ measurements of aerosol optical and microphysical properties obtained during the Ganges Valley Aerosol eXperiment (GVAX) at Nainital, India over the period June 2011-March 2012, aiming to identify key aerosol types and mixing states for two particle sizes (PM₁ and PM₁₀). Using a classification matrix based on SAE vs. AAE thresholds (scattering vs. absorption Ångström exponents, respectively), seven aerosol types are identified, which are highly dependent on particle size. An aerosol type named "large/BC mix" dominates in both PM₁ (45.4%) and PM₁₀ (46.9%) mass, characterized by aged BC mixed with other aerosols, indicating a wide range of particle sizes and mixing states. Small particles with low spectral dependence of the absorption (AAE < 1) account for 31.6% and BC-dominated aerosols for 14.8% in PM₁, while in PM₁₀, a large fraction (39%) corresponds to "large/low-absorbing" aerosols and only 3.9% is characterized as "BC-dominated". The remaining types consist of mixtures of dust and local emissions from biofuel burning and display very small fractions. The main optical properties e.g. spectral scattering, absorption, single scattering albedo, activation ratio, as well as seasonality and dependence on wind speed and direction of identified types are examined, revealing a large influence of air masses originating from the Indo-Gangetic Plains. This indicates that aerosols over the central Himalayas are mostly composed by mixtures of processed and transported polluted plumes from the plains. This is the first study that identifies key aerosol populations in the central Indian Himalayas based on in situ measurements and the results are highly important for aerosol-type inventories, chemical transport models and reducing the uncertainty in aerosol radiative forcing over the third pole.

Observational study of aerosol hygroscopic growth on scattering coefficient in Beijing: A case study in March of 2018

A humidified nephelometer system was deployed to measure the aerosol scattering coefficients at RH < 30% and RH in the range of 40 to 85% simultaneously in megacity Beijing in March 2018. The aerosol optical properties and aerosol hygroscopicity of two sizes (PM₁₀ and PM₁) during the pollution period, dust period and a new particle formation event (NPF) were analyzed. During the pollution period, the scattering and absorption coefficients increased dramatically with the accumulation of pollutants, while scattering Ångström exponent (SAE), submicron scattering fraction (Rsp), submicron absorption fraction (Rap) decreased, as well as single scattering albedo (SSA) rose slightly, which indicated the increasing contribution of larger particle to scattering and absorption, and enhanced the scattering ability of aerosols. The average PM₁₀ mass scattering efficiency is 3.86 ± 1.19 m² g^-1 with a range of 2.05-5.74 m² g^-1 during the pollution period, and 0.40 ± 0.05 m² g^-1 during the dust period. Rsp at wavelength of 550 nm varied from 55.8% to 89.3% during the measurement period, with the average of 64.8% ± 5.2% and 73.1% ± 6.8% during the pollution period and dust period, respectively, which suggests that the aerosol scattering coefficient is mainly affected by fine particles. The average PM₁₀ and PM₁ aerosol scattering hygroscopic growth factors f(80%) are 1.75 ± 0.05 and 1.75 ± 0.04 during the pollution period, 1.14 ± 0.09 and 1.15 ± 0.06 during the dust period, 1.59 ± 0.05 and 1.60 ± 0.06 during the NPF event period, respectively. Aerosol scattering hygroscopic growth factors showed a strong correlation with the scattering Ångström exponent which suggests the hygroscopicity is much stronger for fine particles (SAE > 1.5) than the coarse particles (SAE < 1.0).

A global view on the effect of water uptake on aerosol particle light scattering

A reference dataset of multi-wavelength particle light scattering and hemispheric backscattering coefficients for different relative humidities (RH) between RH = 30 and 95% and wavelengths between λ = 450 nm and 700 nm is described in this work. Tandem-humidified nephelometer measurements from 26 ground-based sites around the globe, covering multiple aerosol types, have been re-analysed and harmonized into a single dataset. The dataset includes multi-annual measurements from long-term monitoring sites as well as short-term field campaign data. The result is a unique collection of RH-dependent aerosol light scattering properties, presented as a function of size cut. This dataset is important for climate and atmospheric model-measurement inter-comparisons, as a means to improve model performance, and may be useful for satellite and remote sensing evaluation using surface-based, in-situ measurements.

Design Type(s)	spectral data collection and processing objective • data integration objective • time series design
Measurement Type(s)	light scattering
Technology Type(s)	Nephelometry
Factor Type(s)	geographic location • instrument • Environment • temporal_interval
Sample Characteristic(s)	United States of America • climate system • Canada • The Netherlands • Greece • Germany • Portuguese Republic • South Korea • China • United Kingdom • Finland • Switzerland • Maldives Archipelago • Brazil • Republic of Ireland • Niger • India • Kingdom of Spain • Kingdom of Norway

Machine-accessible metadata file describing the reported data (ISA-Tab format)