Aerosol Physical Properties Research Articles

Seasonal Behavior of Aerosol Vertical Concentration in Dakar and Role Played by the Sea-Breeze

The Westward transport of mineral dust from the North Africa continent to Atlantic Ocean can produce poor air quality, low visibilities, and negatively impacting respiratory and cardiac health due to the optical and physical properties of aerosols. The dynamical impact of the sea-breeze on the dust vertical distribution in West Africa remains unknown. To investigate this issue, we have used in-situ measurements from lidar. We have focused on the attenuated backscatter of aerosols to study the effect of the local circulation on the vertical profile of mineral dust at land-sea transition. The results highlight a strong diurnal cycle of mineral dust associated with the nocturnal low-level jet (NLLJ). The jet is located between 500 m and 1000 m and crucially affected by the dynamic of the sea-breeze circulation.

Optical and physical properties of aerosols during active fire events occurring in the Indo-Gangetic Plains: Implications for aerosol radiative forcing

The climatological characteristics of biomass burning over the western Indo-Gangetic Plain (IGP) are examined during the post-monsoon (October-November) season from 2001-2018 using the Moderate resolution Imaging Spectrometer (MODIS) fire products. The intensity of the biomass burning are estimated from the fire pixels detected by the MODIS satellites in the form of fire counts, surface brightness temperature, and fire radiative power. Such biomass burning are mostly affected over Lahore and Patiala region of the western IGP. These fire products are examined along with optical and physical properties of aerosols and its radiative forcing estimated from AErosol RObotic NETwork (AERONET) data. Due to non-availability of long-term aerosol data, the study was performed only at Lahore (AERONET) and Patiala (MODIS) of the western IGP region. The current study shows increasing trends of average brightness temperature by 16% year−1 over Patiala and 14% year−1 over Lahore, which enhances the aerosol optical depth (AOD) by 3%–7% year−1 over the study region. The sphericity of the fine-mode aerosols are rising from 23% to 61% during non-fire to fire events and the dominance of fine-mode aerosols are observed during the active fire events. Further, the absorptivity of AOD (at 440 nm) has enhanced from 0.07 to 0.14 during non-fire to fire events. Asymmetric parameter (AS) of fine-mode aerosol increases from 0.65 to 0.68 during non-fire to active fire events. The characteristics of elevated aerosol layers due to the active fire events are clearly distinguished from the non-fire events within 0-2 km above the surface as noticed from high aerosol extinction and attenuated back scatter coefficients, obtained from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). Absorbing aerosols associated from biomass-burning contribute to the surface radiative forcing by -173.96 ± 47 W m−2 and atmospheric forcing by 123.57 ± 41 W m−2 over Lahore. The impact of the biomass-burning over Lahore has making the atmosphere more warming by 3.50 ± 1.14 K Day−1. The estimated heating rates (HR) in the atmosphere are rising during both fire and non-fire events by 3.79% and 3.44% per year, respectively.

Study of aerosol optical properties in the Middle East during 2013.

Aerosols affect the earth's atmospheric radiative fluxes via direct, semi-direct, and indirect mechanisms. Aerosols also are one of the main sources of uncertainty in climate models. In the Middle East, in addition to climate effects, various problems such as reduced visibility, human health hazards, and air pollution are caused by aerosols. Studying the optical and physical properties of aerosols on local and global scales helps reduce the uncertainties in climate forcing. In this study, aerosol optical properties, including Aerosol Optical Depth (AOD), Angstrom Exponent (AE), ASYmmetry parameter (ASY), Single Scattering Albedo (SSA), and phase function were analyzed. These properties were investigated over five sites in the Middle East during 2013 using the Aerosol Robotic NETwork (AERONET) data. The results revealed an inverse relationship between AOD and AE in all sites. A high AOD value and a low AE value were detected in spring and summer in all studied sites, suggestive of coarse mode dust particles. ASY initially decreased due to the dominance of absorbing type aerosols in the visible spectrum with the increase in wavelength. Afterwards, ASY increased with the increase in wavelength in the infrared region due to the dominance of the coarse mode particles. In most sites, SSA increased, particularly in spring and summer, with the increase in the wavelength because of the dominance of desert dust. In spring and summer, the phase function was high over all sites. High phase functions associated with small scattering angles were caused by the coarse mode particles.

Wintertime aerosol properties in Beijing

Abstract. Severe wintertime haze events with exceedingly high levels of aerosols have occurred frequently in China in recent years, impacting human health, weather, and the climate. A better knowledge of the formation mechanism and aerosol properties during haze events is helpful for the development of effective mitigation policies. In this study, we present field measurements of aerosol properties at an urban site in Beijing during January and February 2015. A suite of aerosol instruments were deployed to measure a comprehensive set of aerosol chemical and physical properties. The evolution of haze events in winter, dependent on meteorological conditions, consistently involves new particle formation during the clean period and subsequently continuous growth from the nucleation mode particles to submicron particles over the course of multiple days. Particulate organic matter is primarily responsible for producing the nucleation mode particles, while secondary organic and inorganic components jointly contribute to the high aerosol mass observed during haze events. The average effective density and hygroscopic parameter (κ) of ambient particles are approximately 1.37 g cm−3 and 0.25 during the clean period and increase to 1.42 g cm−3 and 0.4 during the polluted period, indicating the formation of secondary inorganic species from the continuous growth of nucleation mode particles. Our results corroborate that the periodic cycles of severe haze formation in Beijing during winter are attributed to the efficient nucleation and secondary aerosol growth under high gaseous precursor concentrations and the stagnant air conditions, highlighting that reductions in emissions of aerosol precursor gases are critical for remedying secondary aerosol formation and thereby mitigating haze pollution.

Investigation of physical and optical properties of aerosol over high altitude stations along the sub-Himalayan region of North-Eastern India

The present study focuses on exploring the spectral dependency of atmospheric extinction, the size distribution of aerosols, black carbon concentration and absorption and compensation parameter along the sub-Himalayan region of northeastern India, a field campaign was conducted from 1st February to 20th February 2018. Measurements were taken at three different stations in Arunachal Pradesh i.e., Tawang (2990 m a.s.l), New Palin (1300 m a.s.l.), and Geku (200 m a.s.l.). Techniques of Ångström exponent's curvature effect has been used to determine the aerosol size distribution over the stations. The mean black carbon concentration was 3.98 ± 1.03 μg m−3, 4.27 ± 1.22 μg m−3 and 2.38 ± 1.05 μg m−3 at Tawang, New Palin, and Geku respectively. The daily mean absorption coefficient at 370 nm (σ370) was (1.46 ± 0.56) × 10−4, (1.84 ± 0.42) × 10−4, and (1.85 ± 1.8) × 10−4 at Tawang, New Palin, and Geku. Whereas daily mean absorption coefficient at 880 nm (σ880) was (0.33 ± 0.08) × 10−4, (0.42 ± 0.05) × 10−4, and (0.2 ± 0.17) × 10−4. The σ370 was 126%, 125%, and 164% higher than the absorption at the longer wavelength (880 nm) which indicates a higher biomass burning BC. The average Absorption Ångström Exponent (AAE) at 370–880 nm were 1.59 ± 0.23, 1.63 ± 0.23 and 2.28 ± 0.57 at Tawang, New Palin, and Geku. In addition, at Tawang, New Palin, and Geku, the average AAE in the shorter spectral range (370–520 nm) were 2.43 ± 0.97, 2.41 ± 1.22 and 3.09 ± 1.54, whereas in longer spectral range (520–880 nm) were 1.05 ± 0.05, 1.13 ± 0.18 and 1.44 ± 0.33, respectively.

Transport of Asian aerosols to the Pacific Ocean

Abstract Recent efforts reported that the aerosols even can be transported from a long distance to the Pacific Ocean (PO) and potentially influence the marine primary productivity. In this study, based on the multi-source satellite observations and meteorological reanalysis data, the source and transport of aerosols from Asia to the PO region are investigated. The result shows that the satellites capture 104 aerosol events during 2007–2016, which occur in Asia, and can further influence the PO region. The Asian source regions of aerosol over the PO region mainly include eastern and central China (ECC), eastern and central Russia (ECR), Inner Mongolia and Gansu of China and Mongolia (IMGM), and Southeast Asia (SEA), corresponding to sulfate, organic carbon, dust, and organic carbon aerosol sources, respectively. Among the aerosol events detected over the PO region, most of them are sourced from ECC region, accounting for 54.8% of the total aerosol events, followed by ECR region (accounting for 22.1%). Statistical analyses show that, during 2007–2016, the aerosol events observed over the PO region mainly occur in the spring (47 times), followed by the autumn (25 times); and the least of them occur in the winter (14 times). Correspondingly, the months with the most and least aerosol events are March and February, respectively. When the aerosols enter the PO region, the average optical depths of aerosols sourcing from ECC, ECR, IMGM and SEA are 0.24, 0.38, 0.29 and 0.19, respectively. Besides, the transport of aerosols from Asia to the PO region is mainly influenced by westerly winds and troughs of low-pressure system. This study can provide some evidence of understanding the aerosol type, physical and optical properties over the PO region.

A Cloud Detection Algorithm Over Land Based on the Polarized Characteristics Difference Between Cloudless and Cloud Targets

AbstractThe accurate identification of cloud over land is one of the key issues of the satellite data processing and the product retrievals. This paper describes a new cloud detection algorithm based on Level 1 data of Polarization and Directionality of Earth's Reflectance. The simulation of multiangular normalized polarized reflectance is done for cloudless targets over land before the cloud identification processing. First, the Normalized Difference Vegetation Index over land and reflectance of 670 nm are used as two initial criterions for the cloud mask. Then, the difference between the simulation and Polarization and Directionality of Earth's Reflectance observation of polarized reflectance is used as the third criterion to distinguish cloudless pixels from cloud ones. And this algorithm is proved to be more convenient and effective. This algorithm is also applied to cloud mask processing of the Multi‐Angular Polarization Imager (onboard the Tiangong‐2) observation. The results show that this algorithm can effectively detect cloud targets over land, and its consistency with Polarization and Directionality of Earth's Reflectance official cloud mask products is about 90%. This algorithm can provide reliable cloud mask products for the retrieval of optical and physical properties of land aerosol using Multi‐Angular Polarization Imager data.

Dust Properties and Radiative Impacts at a Suburban Site during 2004–2017 in the North China Plain

Aerosols and their radiative effects are of primary interest in climate research because of their vital influence on climate change. Dust aerosols are an important aerosol type in the North China Plain (NCP), mainly as a result of long-range transport, showing substantial spatiotemporal variations. By using measurements from the Aerosol Robotic Network (AERONET) between September 2004 and May 2017, and the space-borne Moderate Resolution Imaging Spectroradiometer (MODIS) and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) aerosol products, we investigated the properties of dust aerosols and their radiative effects at Xianghe (XH)—a suburban site in the NCP. Dust events occurred most frequently during spring (a total of 105 days) relative to the other three seasons (a total of 41 days) during the periods concerned. The dust aerosol optical depth (AOD) at 675 nm was at a maximum in spring (0.60 ± 0.44), followed (in decreasing order) by those in autumn (0.58 ± 0.39), summer (0.54 ± 0.15), and winter (0.53 ± 0.23). Cooling effects of dust aerosol radiative forcing (RF) at the bottom and top of the atmosphere tended to be strongest in spring (−96.72 ± 45.69 and −41.87 ± 19.66 Wm−2) compared to that in summer (−57.08 ± 18.54 and −25.54 ± 4.45 Wm−2), autumn (−72.01 ± 27.27 and −32.54 ± 15.18 Wm−2), and winter (−79.57 ± 32.96 and −37.05 ± 17.06 Wm−2). The back-trajectory analysis indicated that dust air mass at 500 m that arrived at XH generally originated from the Gobi and other deserts of northern China and Mongolia (59.8%), and followed by northwest China and Kazakhstan (37.2%); few dust cases came from northeast China (3.0%). A single-peaked structure with the maximum occurring at ~2 km was illustrated by all dust events and those sorted by their sources in three directions. Three typical dust events were specifically discussed to better reveal how long-range transport impacted the dust properties and radiative effects over the NCP. The results presented here are expected to improve our understanding of the physical properties of dust aerosols over the NCP and their major transport path and significant impacts on the regional solar radiation budget.

Relationship between Structure, Functionality, and Viscosity for Aerosol-Mimicking Solutions Containing Ammonium Sulfate, Glyoxal, and a Series of Oxidized C1–C5 Compounds

The physical properties of aqueous secondary organic aerosol (SOA) can be influenced by the chemical composition of the solvent matrix within which SOA-forming reactions take place. The chemical structure and functionality of perturbing solutes on solution viscosity in bulk-phase SOA-modeling reaction mixtures containing glyoxal and ammonium sulfate were measured for a series of oxidized C1–C5 compounds. In general, as the solute concentration increased, solution viscosity increased, with the magnitude of those increases dependent upon the solute species added. The presence of glyoxal increased viscosity in about 50% of mixtures as a result of specific solute/solvent/product interactions. The oxygen/carbon (O/C) ratio was found to have a linear relationship with relative viscosity at a high solute concentration such that every −CH2CH2OH moiety addition increased solution viscosity by 25%. Solutes with identical O/C ratios but different structural characteristics (for example, acetone, 1-propanol, and 2-propanol) showed 7–8% differences in viscosity as a function of the chain length or ability to act as a hydrogen bond donor. The addition of an alcohol group increased viscosity by 10–20% over hydrogen or methyl groups in the same position. The solute structure appears to play a role in mediating intrasolvent interactions, and increases in hydrogen bonding and intersolute interactions lead to increased bulk solution viscosity, as shown in a series of experiments varying the timing and order of solute addition. These solutes are models for oxidized aerosol components, and the structure-dependent results suggest the importance of the aqueous layer functional group presence on aerosol physical properties, such as viscosity.

Variations in submicron aerosol liquid water content and the contribution of chemical components during heavy aerosol pollution episodes in winter in Beijing.

The aerosol liquid water content (ALWC) of submicron particles (PM1) was calculated in this work by three methods based on the aerosol physical and chemical properties measurement campaigns in winter in Beijing, including (a) the PM1 volume difference between the ambient and dry states by applying the particle number size distribution and particle hygroscopicity measurement; (b) the thermodynamic equilibrium model (ISORROPIA II) based on the chemical composition; and (c) the κ-Köhler theory of chemical composition with a volume mixing scheme. The three methods agreed well with reasonable uncertainties. The ALWC showed an exponential trend depending on the relative humidity (RH), and an abundant ALWC was also favored by the high PM1 mass loading. The contribution of different chemical component to the ALWC was evaluated by the κ-Köhler method, which revealed that during the measurement, the inorganics and organics could contribute to ~80% and ~20%, respectively, under ambient RH conditions, with the largest contributor of ammonium nitrate. When the RH was above 85%, the mass concentration of ALWC was comparable to, or even larger than, that of the dry PM1.

Observed below-Cloud and Cloud Interstitial Submicron Aerosol Chemical and Physical Properties at Whiteface Mountain, New York, during August 2017

A pilot study took place at Whiteface Mountain (WFM) in the Adirondacks of upstate New York during the summer of 2017 to evaluate the chemical processing of aerosol within clouds. Below-cloud and cloud interstitial submicron aerosols were characterized in real-time using an Aerodyne High Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) and other instruments deployed via the Atmospheric Sciences Research Center (ASRC) sprinter van mobile laboratory. The primary observation is a dominance of organic aerosol mass, averaging 66%–78% of PM2.5 during urban-influenced periods and 83–93% during biogenic-influenced periods, in stark contrast to observations in the northeastern U.S. in past decades when sulfate dominated the aerosol mass. The observations also show chemical differences between the below-cloud layer, the transition layer (just below-cloud), and the in-cloud layer. Comparison between these layers revealed enhanced inorganic nitrate in interstitial aerosol sampled in the transition and in-cloud layers compared to the below-cloud aerosol. Enhanced particle nitrate mass concentrations were more significant during urban influenced cloud events than biogenic influenced ones. The below-cloud and transition layer aerosols were characterized by similar bimodal number and mass size distributions, and when entrained into the cloud, aerosols with number mean dry diameters of > ∼50 nm were incorporated into cloud droplets, suggesting that cloud supersaturations at WFM can be quite high (>0.6%). The organic component of the interstitial aerosols in the transition and in-cloud layers is slightly less oxidized than the below-cloud aerosol. The hygroscopicity parameter (κ), as determined from size-resolved cloud condensation nuclei (CCN) measurements at water vapor supersaturations ranging from 0.23% to 0.78%, was 0.13–0.19 throughout the majority of the pilot study. Exceptions were two of the five urban-influenced periods (with κ values of 0.27–0.33) and at the lowest supersaturation (0.23%) for two of the three biogenic-influenced periods (with κ values of 0.23–0.25), the latter of which may be attributed to greater accumulation of soluble aerosol mass during aqueous/cloud processing of larger particles. The hygroscopicity of the organic component was estimated to range from 0.08 to 0.22, and was well correlated with the organic oxidation state. The below-cloud and cloud interstitial aerosol observations shown here comprise an important part of the 2017 pilot study at WFM. These observations also provide a baseline for future intensive field campaigns focused on cloud processing at WFM, which will ultimately inform atmospheric models to better simulate cloud processing in the northeastern U.S in comparison to other biogenic-dominated locations.

Properties of Arctic Aerosol Based on Sun Photometer Long-Term Measurements in Ny-Ålesund, Svalbard

On the basis of sun photometer measurements located at the German-French polar research base AWIPEV in Ny-Ålesund ( 78.923 ° N, 11.928 ° E), Svalbard, long-term changes (2001–2017) of aerosol properties in the European Arctic are analyzed with the main focus on physical aerosol properties like Aerosol Optical Depth (AOD) and the Ångström exponent during the Arctic haze season in spring compared with summer and autumn months. In order to gain more information from the photometer data and to reduce the error of fitting the data to the Ångström law, a new approach with an Ångström exponent, which depends linearly on wavelength, is presented in this paper. With the Mie program of libRadtran, a calculator for long- and short-wave radiation through the Earth’s atmosphere, artificial aerosol size distributions were created to extend the physical understanding of this modified Ångström law. Monthly means of the measured AOD of the years 1994–2017 are presented to analyze long-term changes of aerosol properties and its load. Because photometer data in general have no height information, a comparison with a Lidar located at the same site is presented. The so-obtained data are then compared with the previous Mie calculus. More homogeneous aerosol properties were found during spring and more heterogeneous in summer. To study possible aerosol sources and sinks, five-day back-trajectories were calculated with the FLEXPART model at three different arriving heights at 11 UTC in the village Ny-Ålesund. Besides the pollution pathway of the aerosol into the European Arctic based on the calculated back-trajectories, the influence of the boundary layer parameterized by the lowermost 100 hPa atmospheric layer is analyzed and compared to the measured aerosol load by the photometer in Ny-Ålesund additionally. During spring, the open ocean acts as a sink for aerosols, whereas sea ice clearly reduces their sinks. Hence, trajectories over sea ice are correlated to higher aerosol loads. Thus, both sources and sinks must be considered to understand aerosol occurrences in the Arctic.

Size distribution and concentration of aerosol particles in Yinchuan area, China

ABSTRACTThe size distribution and concentration of atmospheric aerosol particles are important parameters that characterize the physical and optical properties of aerosols. Understanding the spectral distribution of atmospheric particles is fundamental for studying particulate pollution. Based on observational data acquired by an aerodynamic particle sizer APS-3321 and a laser dust meter LD-5 from September 2017 to July 2018, the temporal evolution of particle size distribution and concentration of aerosols were studied at the North Minzu University in Yinchuan (38°29’52"N 106°06’28"E), China. By cluster analysis of particle surface area distribution in different weathers, seven clusters were obtained and discussed. The experimental results showed that the particle number and mass concentrations were different in the four seasons of the year. Compared with the unimodal distribution of the particle number concentration and the doublet mass concentration distribution in sunny weather conditions, there were great differences in the particle spectrum distribution in haze, snowfall, and dusty weather conditions. The particle surface area distributions of the seven clusters were clearly different from each other and influenced by meteorological conditions. Additionally, the PM2.5 value has a relationship with the meteorological elements. The research is helpful to reveal the formation mechanism and source of aerosols in Yinchuan, China.

Retrieval of the water ice column and physical properties of water-ice clouds in the martian atmosphere using the OMEGA imaging spectrometer

Using spectral images recorded by the OMEGA instrument on Mars Express (Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité), we are able to derive physical properties of aerosols in water-ice clouds on Mars for a distribution of pixels over an observed cloud formation. These properties, mean effective radius, reff, and optical depth (at 0.67 μm), τi, were used to estimate the water ice-column (WIC), and we found an empirical relationship between the WIC and an ice cloud index (ICI). The overall mean of retrieved reff is ∼2.2 μm, with a standard deviation of 0.8 μm, and cloud formations with reff between 4.4 and 5.4 μm are observed. The optical depth varies between 0.2 and 2.0. The OMEGA spectra are primarily sensitive to water ice mass due to absorption, and we find that the ICI, very easy to compute, is a good proxy for the mass of the water-ice column (WIC) along the optical line of sight. Our retrieval of physical properties is limited in time (to before 2010) by the exhaustion of coolant for one of the OMEGA channels, and in space (to equatorial observations between 140∘W and 90∘E) by the availability of surface albedo measurements. However, we used the ICI to compute WIC values for the entire OMEGA data set, which has near-global coverage for Mars years 26–32, and we present a climatology of the WIC derived from the OMEGA data, which features enhancements on the order of 1.2–1.6 pr. μm over the aphelion cloud belt, and 1.5–2.5 pr. μm over the polar hoods. The data set analyzed is for observations between 140°W and 90°E, and between 35∘S and 35∘N. No restriction is placed on season, but the majority of cloudy observations were during the aphelion period from Ls 35∘ to 135∘. This work was motivated by the ability of the OMEGA instrument to observe the distribution of water-ice cloud physical properties, and by the availability of new a priori data sets, especially multi-spectral, aerosol-free surface albedo retrieved from a subset of the OMEGA data featuring a cloud-free sky. The main limitations of the retrieval algorithm are linked to the uncertainties on surface albedo, the dust opacity, and the quantity of water-ice suspended in the atmosphere, which can lead to spectral fits with lower accuracy or unrealistic results. We present distributions of each retrieved parameter, goodness of fit, ICI, and cloud mass, and our investigation of relationships between each parameter. Our approach was to maximize the amount of data analyzed, apply stringent data quality cuts and take a statistical approach to interpretation.

Case study of the effects of aerosol chemical composition and hygroscopicity on the scattering coefficient in summer, Xianghe, southeast of Beijing, China

The aerosol scattering coefficient is mainly affected by various factors, such as aerosol chemical composition, hygroscopicity and relative humidity. A comprehensive observation of the chemical and physical properties of aerosols was conducted in Xianghe, southeast of Beijing, from June 15 to 26, 2018. Five specific cases were analysed during this observation: (1) rainy (RA); (2) low visibility during the day (LD); (3) high visibility during the day (HD); (4) low visibility at night (LN); and (5) high visibility at night (HN). The mass percentages of a secondary inorganic aerosol (nitrate, sulfate and ammonium) measured with a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) were 64.5% and 68.3% during LD and LN, which were higher than those (63.6% and 46.1%) observed during HD and HN, respectively. The mass percentages of secondary organic aerosol (SOA) and primary organic aerosol (POA) to organic aerosol were 76.6% and 23.4%, respectively, during LD. The hygroscopic parameters κ were 0.59 and 0.60 during LD and LN, which were slightly higher than those (0.57 and 0.58) observed during HD and HN, respectively, indicating that the variation of aerosol chemical composition had a limited impact on hygroscopicity. The average proportion, measured with a single particle aerosol mass spectrometer (SPAMS), of biomass burning, dust, heavy metal, soot-like, sea salt, secondary aerosols and other particles were 12.3%, 2.9%, 16.2%, 36.1%, 8.8%, 21.0% and 2.7%. The average proportions of soot-like and secondary aerosols were 42.4% and 19.2% from 0.2 to 1.0 μm. Although the variation and height of the mixing layer were similar between LD and HD, clouds reduced the solar radiation during LD, which caused a higher relative humidity (RH) (67.5%) than that (45.9%) observed during HD. A larger wind speed below 850 hpa during HN caused the mixing layer height (MLH) to reach 1910 m, which resulted in a lower RH (64.8%) than that (68.0%) observed during LN. The light scattering enhancement factors f(RH) during low visibility events were almost greater than those observed during high visibility events, regardless of they were measured during the day (0–9.7%) or night (0–13.1%). The aerosol hygroscopic growth factor (GF) maintained a similar trend with RH, indicating that RH was the main factor affecting the hygroscopic growth of the aerosol, resulting in the decrease in visibility.

Long-term measurements of aerosol optical and physical properties over the Eastern Mediterranean: Hygroscopic nature and source regions

Long-term aerosol optical and physical properties such as aerosol optical thickness and angstrom exponent were obtained from AERONET ground based sun sky radiometer located at the rural coastal Erdemli (36.6°N, 34.3°E) site in the Eastern Mediterranean from January 2000 to December 2014. Aerosol Optical Thickness (AOT) and Angstrom Exponent (α) exhibited a great short-term variability, varying up to couple of fold from day to day. The lowest AOT values was found in winter due to the removal of particles by wet deposition. Elevated AOT values with low α440-870 was observed in spring owing to sporadic mineral dust transport from North Africa and the Middle East. High AOT at 440 nm and α440-870 were characterized in summer because of high gas-to-particle conversion, sluggish air masses and absence of rain. Enhanced AOT and low α (0.7) and fine mode fraction (47%) were associated with air flow from North Africa and the Middle East with a slight decrease from 440 nm to 1020 nm, showing importance of coarse particles. Air flow from Mediterranean Sea, Eastern Europe, Western Europe and Turkey, on the other hand, showed comparatively higher α (>1.1) and fine mode fraction (>62%). Three classes of aerosol were identified namely non-hygroscopic dominated, moderately hygroscopic and hygroscopic dominated. Non-hygroscopic particles were dust dominated aerosol population originated from North Africa (Southern Tunisia, Western Libya, Libya/Chad border, southern Egypt northeastern Mauritania, north Mali, southern Algeria, Northwestern Niger) and the Middle East (extending from Iraq to eastern Saudi Arabia). Moderately hygroscopic particles were a mixture of mineral dust and anthropic aerosols when the dust reached receptor site after passing through industrialized and populated sites (such as Spain, France and Italy). Hygroscopic particles, however, were ascribed to nearly mesoscale formation of secondary aerosols under the prevailing summer conditions.

Physical properties of secondary photochemical aerosol from OH oxidation of a cyclic siloxane.

Cyclic volatile methyl siloxanes (cVMS) are high-production chemicals present in many personal care products. They are volatile, hydrophobic, and relatively long-lived due to slow oxidation kinetics. Evidence from chamber and ambient studies indicates that oxidation products may be found in the condensed aerosol phase. In this work, we use an oxidation flow reactor to produce ~ 100 μgm-3 of organosilicon aerosol from OH oxidation of decamethyl-cyclopentasiloxane (D5) with aerosol mass fractions (i.e., yields) of 0.2-0.5. The aerosols were assessed for concentration, size distribution, morphology, sensitivity to seed aerosol, hygroscopicity, volatility and chemical composition through a combination of aerosol size distribution measurement, tandem differential mobility analysis, and electron microscopy. Similar aerosols were produced when vapor from solid antiperspirant was used as the reaction precursor. Aerosol yield was sensitive to chamber OH and to seed aerosol, suggesting sensitivity of lower-volatility species and recovered yields to oxidation conditions and chamber operation. The D5 oxidation aerosol products were relatively non-hygroscopic, with an average hygroscopicity kappa of ~ 0.01, and nearly non-volatile up to 190 °C temperature. Parameters for exploratory treatment as a semi-volatile organic aerosol in atmospheric models are provided.

Aerosol variability induced by atmospheric dynamics in a coastal area of Senegal, North-Western Africa

During the first Intensive Observation Period (IOP1) of the SHADOW (SaHAran Dust Over West Africa) campaign carried out during March–June 2015, the Mbour site (Senegal) was reinforced with a large setup of in-situ and remote sensing instruments to measure the aerosol optical, physical and chemical properties as well as the atmospheric dynamical features over the site. Here, we use this dataset to study the impact of Sea Breeze (SB) on the aerosol properties.Two case studies were selected: (i) one representative of SB events observed during typical Harmattan conditions, (ii) and one representative of SB events observed during a Harmattan desert dust event. Both cases show a strong influence of the SB period on the dry Mass Scattering Efficiency (MSE), which increases by a factor of up to 1.4. Given the fact that SB occurred every other day at this coastal site during IOP1 (dry season) and that the relative humidity may reach 96%, the effect of such event on the radiative forcing may not be negligible.In order to evaluate the effects of different flows on aerosol transport, recirculation and stagnation factors were retrieved from wind components. The air mass recirculation factor implies that aerosols previously transported from the land over the sea are then brought back inland. In both cases, high recirculation factors were calculated (between 0.84 and 0.98) and air mass stagnation was observed right before the SB establishment. However, this stagnation is observed over the sea during the first case while it is located near the coast during the second case, which has a significant impact on the aerosol evolution. Indeed, a stagnation of air masses over the sea is associated with an increase in fine aerosol mass concentrations (PM1, PM2.5) due to a higher contribution of marine and anthropogenic sources notably for organic aerosols, and a higher contribution of more-oxidized organics. This contribution is lower when the air mass stagnation occurs over the shoreline during the desert dust event.

Aerosol hygroscopic growth, contributing factors, and impact on haze events in a severely polluted region in northern China

Abstract. This study investigates the impact of the aerosol hygroscopic growth effect on haze events in Xingtai, a heavily polluted city in the central part of the North China Plain (NCP), using a large array of instruments measuring aerosol optical, physical, and chemical properties. Key instruments used and measurements made include the Raman lidar for atmospheric water vapor content and aerosol optical profiles, the PC-3016A GrayWolf six-channel handheld particle and mass meter for atmospheric total particulate matter (PM) that has diameters less than 1 and 2.5 µm (PM1 and PM2.5, respectively), the aerosol chemical speciation monitor (ACSM) for chemical components in PM1, and the hygroscopic tandem differential mobility analyzer (H-TDMA) for aerosol hygroscopicity. The changes in PM1 and PM2.5 agreed well with that of the water vapor content due to the aerosol hygroscopic growth effect. Two cases were selected to further analyze the effects of aerosol hygroscopic growth on haze events. The lidar-estimated hygroscopic enhancement factor for the aerosol backscattering coefficient during a relatively clean period (Case I) was lower than that during a pollution event (Case II) with similar relative humidity (RH) levels of 80 %–91 %. The Kasten model was used to fit the aerosol optical hygroscopic growth factor (GF) whose parameter b differed considerably between the two cases, i.e., 0.1000 (Case I) versus 0.9346 (Case II). The aerosol acidity value calculated from ACSM data for Case I (1.35) was less than that for Case II (1.50) due to different amounts of inorganics such as NH4NO3, NH4HSO4, and (NH4)2SO4. Model results based on H-TDMA data showed that aerosol hygroscopic growth factors in each size category (40, 80, 110, 150, and 200 nm) at different RH levels (80 %–91 %) for Case I were lower than those for Case II. For similar ambient RH levels, the high content of nitrate facilitates the hygroscopic growth of aerosols, which may be a major factor contributing to heavy haze episodes in Xingtai.

A shape model of internally mixed soot particles derived from artificial surface tension

Abstract. To retrieve the physical properties of aerosols from multi-channel ground-based and satellite measurements, we developed a shape model of coated soot particles and created a dataset of their optical properties. Bare soot particles were assumed to have an aggregate shape, and two types of aggregates with different size–shape dependences were modeled using a polyhedral Voronoi structure. To simulate the detailed shape properties of mixtures of soot aggregates and adhered water-soluble substances, we propose a simple model of surface tension derived from the artificial surface potential. The light-scattering properties of the modeled particles with different volume fractions of water-soluble material were calculated using the finite-difference time-domain method and discrete-dipole approximation. The results of the single-scattering albedo and asymmetry factors were compared to those of conventional internally mixed spheres (i.e., effective medium spheres based on the Maxwell-Garnett approximation and simple core-shell spheres). In addition, the lidar backscattering properties (i.e., lidar ratios and linear depolarization ratios) of the modeled soot particles were investigated. For internally mixed soot particles, the lidar backscattering properties were sensitive to the shape of the soot particles and the volume mixing ratio of the assumed water-soluble components. However, the average optical properties of biomass smoke, which have been reported from in situ field and laboratory measurements, were difficult to explain based on the individually modeled particle. Nonetheless, our shape model and its calculated optical properties are expected to be useful as an alternative model for biomass smoke particles in advanced remote sensing via multi-channel radiometer and lidar measurements.