Mode Fraction Research Articles

Combined Dust Detection Algorithm for Asian Dust Events Over East Asia Using GK2A/AMI: a Case Study in October 2019

A combined algorithm comprising multiple dust detection methods was developed using infrared (IR) channels onboard the GEOstationary Korea Multi-Purpose SATellite 2A equipped with the Advanced Meteorological Imager (GK2A/AMI). Six cloud tests using brightness temperature difference (BTD) were utilized to reduce errors caused by clouds. For detecting dust storms, three standard BTD tests (i.e., {BT}_{12.3}-{BT}_{10.5}, {BT}_{8.7}-{BT}_{10.5}, and {BT}_{11.2}-{BT}_{10.5}) were combined with the polarized optical depth index (PODI). The combined algorithm normalizes the indices for cloud and dust detection, and adopts weighted combinations of dust tests depending on the observation time (day/night) and surface type (land/sea). The dust detection results were produced as quantitative confidence factors and displayed as false color imagery, applying a dynamic enhancement background reduction algorithm (DEBRA). The combined dust detection algorithm was qualitatively assessed by comparing it with dust RGB imageries and ground-based lidar data. The combined algorithm especially improved the discontinuity in weak dust advection to the sea and considerably reduced false alarms as compared to previous dust monitoring methods. For quantitative validation, we used aerosol optical thickness (AOT) and fine mode fraction (FMF) derived from low Earth orbit (LEO) satellites in daytime. For both severe and weakened dust cases, the probability of detection (POD) ranged from 0.667 to 0.850 and it indicated that the combined algorithm detects more potential dust pixels than other satellites. In particular, the combined algorithm was advantageous in detecting weak dust storms passing over the warm and humid Yellow Sea with low dust height and small AOT.

Climatological variations in aerosol optical depth and aerosol type identification in Liaoning of Northeast China based on MODIS data from 2002 to 2019

The climatological variations of aerosol optical properties are significant in the study of aerosol effect on regional environment. The MODIS level 2 aerosol optical depth (AOD) products (MOD04/MYD04) at 10 km were used to identify the spatial distribution, interdecadal trends, and aerosol types from 2002 to 2019 in Liaoning province over Northeast China. The highest AOD was distributed in central Liaoning, with a peak value of approximately 0.6 and the fine mode fraction (FMF) was higher in eastern Liaoning about 0.8. The overall AOD for Liaoning in summer was significantly higher than that in other seasons, and the higher AOD values were observed in central Liaoning and the area around Bohai Rim (up to approximately 0.7). The frequency of 0.1 ≤ AOD < 0.2 and 0.2 ≤ AOD < 0.3 was highest in 14 cities of Liaoning. For FMF > 0.8, the cities were categorised into those with an increase frequency of 15% to 20%, and those with a decrease frequency of 5% to 10%. During 2014 to 2019, the interannual variation in AOD in Liaoning exhibited a significant downward trend, with a minimum AOD of approximately 0.25 in 2019. The FMF in Liaoning from 2002 to 2019 exhibited a fluctuating trend around 0.5. The AOD in Liaoning showed an increasing trend from 2002 to 2008 with a maximum of approximately 0.02/year and a clear decreasing trend from 2009 to 2019 of approximately −0.02/year. Maritime aerosol mostly occurred in cities in western Liaoning accounted for 20.58% to 36.52%. Dust aerosol had a considerable influence on central Liaoning accounting for 2.95% to 16.18%. In eastern Liaoning, continental background aerosol accounted for 36.69% to 43.30%. These results revealed the influence of AOD and fine particles on aerosol extinction in Northeast China and provided important information for the climate change in Northeast Asia.

Final State Interaction Analysis and Branching Fraction Calculations of $$\varvec{B^+_c\rightarrow D^+{\bar{D}}^0(D^+D^0)}$$ Decays

The LHC experiments have been measured the branching fractions for $$B^+_c$$ decays to two pseudoscalar charm mesons by multiplying the production rates for $$B^+_c$$ relative to $$B^+_u$$ mesons (in units of $$10^{-6}$$ ): $$\begin{aligned} (f_c/f_u){\mathcal {B}}(B^+_c\rightarrow D^+{\bar{D}}^0)_{EXP}=3.04\pm 2.87,\quad (f_c/f_u){\mathcal {B}}(B^+_c\rightarrow D^+D^0)_{EXP}=1.10\pm 2.02. \end{aligned}$$ We have calculated the branching fractions of these decay modes by using the QCD factorization (QCDF) approach and obtained; $${\mathcal {B}}(B^+_c \rightarrow D^+{\bar{D}}^0)_{QCDF}=(0.28\pm 0.08)\times 10^{-4}$$ and $${\mathcal {B}}(B^+_c \rightarrow D^+D^0)_{QCDF}=(0.60\pm 0.17)\times 10^{-6}$$ . Consider to upper limit of production ratio ( $$f_c/f_u=0.012$$ ), the QCDF results are 10 and $$10^2$$ times smaller than experimental results, respectively. The effects of the final state interaction (FSI) have compensated for the smallness of the theoretical results. After entering the effects of FSI, acceptable results have been obtained (in units of $$10^{-6}$$ ): $$\begin{aligned} (f_c/f_u){\mathcal {B}}(B^+_c\rightarrow D^+{\bar{D}}^0)_{FSI}=3.08\pm 0.32,\quad (f_c/f_u){\mathcal {B}}(B^+_c\rightarrow D^+D^0)_{FSI}=0.62\pm 0.07. \end{aligned}$$

Ammonia optical gas sensing based on graphene-covered silicon microring resonators: A design space exploration

This paper discusses the design space exploration for an ammonia optical gas sensor based on a silicon microring resonator that is covered by graphene. A comparative analysis of the transmittance sensitivity of its fundamental quasi-TE and quasi-TM modes is presented. Results show that a large fraction of the fundamental quasi-TM mode’s electrical energy is propagating outside the waveguide, which makes this geometry more sensitive to ammonia induced changes in Fermi energy of graphene. The overall transmittance intensity difference, for the fundamental quasi-TE mode is less than 1 ​dB, independent of ring radius, when the ammonia gas concentration is varied from 0.5 to 1000 ​ppm. For the fundamental quasi-TM mode, results show that a ring radius ~8.4 ​μm can provide critical coupling condition and lead to >20 ​dB modulation of transmittance intensity when the ammonia gas concentration is varied from 0.5 to 1000 ​ppm. Overall, proper selection of mode excitation as well as ring geometry is essential to attain practical sensitivity.

Aerosol Trends during the Dusty Season over Iran

This study assessed the aerosol climatology over Iran, based on the monthly data of aerosol optical depth (AOD) derived from the reanalysis-based Modern Era Retrospective Analysis for Research and Applications (MERRA-2) and the satellite-based Moderate Resolution Imaging Spectroradiometer (MODIS). In addition, sea level pressure, wind speed, temperature, relative humidity, precipitation, and soil moisture from the ERA5 reanalysis dataset were applied to investigate the climate-related effects on temporal AOD changes. Our analysis identified positive and negative AOD trends during 2000–2010 and 2010–2018, respectively, which are likely linked to aeolian dust changes. The dust-driven AOD trends were supported by changes in the Ångström exponent (AE) and fine mode fraction (FMF) of aerosols over Iran. During the early period (2000–2010), results of AOD-meteorology correlation analyses suggest reduced soil moisture, leading to increased dust emissions, whereas our results suggest that during the later period (2010–2018) an increase of soil moisture led to decreased AOD levels. Soil moisture appears to be a key factor in dust mobilization in the region, notably in southwestern Iran, being influenced by adjacent mineral dust sources. These phenomena were affected by large-scale sea level pressure transformations and the associated meteorology in the preceding winter seasons. Using a multiple linear regression model, AOD variability was linked to various meteorological factors in different regions. Our results suggest that climatic variations strongly affect the dust cycle, with a strong dependence on wintertime conditions in the region.

Temporal and spatial variations of aerosol optical properties over the Korean peninsula during KORUS-AQ

We investigated the temporal and spatial variations of aerosol optical properties over the Korean peninsula during the KORUS-AQ (KORea–United States Air Quality) experiment with ground-based aerosol optical properties measured by remote and in-situ techniques. On the ground, AErosol RObotic NETwork (AERONET) and ground-level particulate matter (PM) concentration from air quality monitoring stations were used. From the NASA DC-8 research aircraft, the airborne Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research instrument (4STAR), an aerosol mass spectrometer (AMS), and a single particle soot photometer (SP2) provided aerosol information. Average hourly fine- and coarse-mode aerosol optical depth (AOD) and fine mode fraction (FMF), were divided into four clusters (background, Seoul Metropolitan Area (SMA), southwest, and east) representing different temporal/spatial variations; the results of those clusters were similar to the clustering results using PM from air quality monitoring stations. The downwind region of SMA was dominant by light-scattering fine-mode aerosols likely due to secondary aerosol formation with high fine-mode AOD and single scattering albedo compared to other regions, even Seoul. The fine-mode aerosols were more spatially homogeneous than coarse-mode aerosols, especially in the west-to-east direction, because fine-mode aerosols are usually transported on a regional scale by westerlies rather than emitted from local emission sources. However, the aerosol size distribution was spatially more homogeneous because of a consistent contribution of fine- and coarse-mode AOD to total AOD regardless of direction between the AERONET sites. During high-aerosol loading episodes, the temporal and spatial variations of aerosol optical properties were similar to those derived from 4STAR and ground-level PM concentrations, providing detailed information on aerosol behavior and characteristics. Using missed-approach flight segments (touchdown and take-off without a full-stop), investigation of diurnal variations over the SMA revealed a significant increase in AOD and Angstrom exponent (AE) in the afternoon compared to morning and noon, especially in the downwind region, because of more active secondary formation resulting from advected and/or emitted local pollutants and precursors. The diurnal variation of PM1 in the downwind region was similar to that of AOD; it was mainly increased during the day by secondary organic aerosols and ammonium nitrate due to large amounts of isoprene and meteorological conditions that supported secondary aerosol formation. Ammonium sulfate also partially led to increasing the PM1, but its behavior was unclear, because RH decreased during the day, which would imply the reduced aqueous-phase oxidation (the major pathway for sulfate production). Compared to the downwind region, ammonium nitrate and refractory black carbon decreased in Seoul because of a slightly decreased traffic volume, dissociation of ammonium nitrate, and increased boundary layer height according to meteorological conditions in the afternoon. The results of this study provide detailed information on aerosol behavior given the synergy of the various measurement platforms used. Along with the evidence of active photochemical reactions taking place in the downwind region, such data will be useful in formulating policies that improve air quality in Korea.

Derivation of PM10 mass concentration from advanced satellite retrieval products based on a semi-empirical physical approach

PM10 remote sensing is of great significance in the atmospheric environment studies. Contrary to intuitive perception, deriving PM10 is more difficult than PM2.5 from satellite measurements. This is because although the major satellite parameter Aerosol Optical Depth (AOD) contain contribution of all suspended particles, it is much more sensitive to fine particles than coarse particles. To address this challenge, a physically based remote sensing method for PM10 is developed using two new semi-empirical physical models: the model of columnar volume-to-extinction ratio (VE10) and the model of particle effective density. VE10 is a key parameter bridging the non-linear relationship between aerosol extinction and volume concentration. A semi- empirical VE10 model is developed based on the fine mode fraction (FMF), and the mean relative error of VE10 modeling is 14.5%. Similarly, the particle effective density depends on the ratio of mass to volume. The particle effective density is effectively characterized by refractive index of matter, with the theoretical error of 13.8%. Both semi-empirical physical models are applied to the derivation of PM10, by using aerosol retrieval products of POLarization and Directionality of the Earth's Reflectances (POLDER) produced by the Generalized Retrieval of Atmosphere and Surface Properties (GRASP) algorithm. As a validation, PM10 mass concentration is estimated over China from Jan. to Oct. in 2013. A fairly good correlation and consistency are achieved by inter-comparison with in-situ PM10 measurements.

A time domain approach for shock noise prediction with decomposition analyses of large-scale coherent turbulent structures in jets

Broadband shock-associated noise (BBSAN) is one dominant component from off-design supersonic jets. A time-domain BBSAN model is proposed and evaluated, which is based on the vector Green’s function solution of the decomposed Navier–Stokes equations. The source term is the scalar product of the anisotropic velocities with the mean pressure gradient. The approach uses large-eddy simulation (LES) databases. Proper orthogonal decomposition (POD) is applied to the scalar source field to extract the large-scale coherent structures associated with BBSAN. The key features of the BBSAN spectra are preserved when only a small fraction of POD modes are used to reconstruct the source field. The leading POD modes of the source term show strong amplification in the observer direction. Noise source contour plots calculated with these POD modes reveal that most noise sources are distributed at the shock-turbulence interactions. Predictions are validated with the Ffowcs Williams and Hawkings method and experimental data.

Evolution of Aerosols in the Atmospheric Boundary Layer and Elevated Layers during a Severe, Persistent Haze Episode in a Central China Megacity

Aerosol vertical profiling is crucial to understand the formation mechanism and evolution processes of haze, which have not yet been comprehensively clarified. In this study, we investigated a severe, persistent haze event in Wuhan (30.5° N, 114.4° E), China during 5–18 January 2013 by the use of a polarization lidar, a Cimel sun photometer, meteorological datasets, and the hybrid single-particle Lagrangian integrated trajectory (HYSPLIT) model, focusing on the time–height evolution of aerosols in both the atmospheric boundary layer (ABL) and elevated layers. During the haze period, the integrated particle depolarization ratio was 0.05 ± 0.02, and the fine mode fraction reached 0.91 ± 0.03, indicating haze particles were rather spherical and predominately submicron, that is, of anthropogenic nature. Compared with the clear period, columnar aerosol optical depth at 500 nm tripled to 1.32 ± 0.31, and the strongest enhancement in aerosol concentration occurred from near the ground to an altitude of 1.2 km during the haze period. The daytime evolution of aerosol vertical distribution in the ABL exhibited a distinct pattern under haze weather. Abundant particles accumulated below 0.5 km in the morning hours due to stable meteorological conditions, including a strong surface-based inversion (4.4–8.1 °C), late development (from 1000–1100 LT) of the convective boundary layer, and weak wind (&lt;4 m∙s−1) in the lowermost troposphere. In the afternoon, improved ventilation delivered an overall reduction in boundary layer aerosols but was insufficient to eliminate haze. Particularly, the morning residual layer had an optical depth of 0.29–0.56. It influenced air quality indirectly by weakening convective activities in the morning and directly through the fumigation process around noon, suggesting it may be an important element in aerosol–ABL interactions during consecutive days with haze. Our lidar also captured the presence of the elevated aerosol layers (EALs) embodying regional/long-range transport. Most of the EALs were observed to subside to &lt;1.2 km and exacerbate the pollution level. Backward trajectory analysis and lidar data revealed the EALs originated from the transport of anthropogenic pollutants from the Sichuan Basin, China, and of dust from the deserts in the northwest. They were estimated to contribute ~19% of columnar aerosol-loading, pointing to a non-negligible role of transport during the intense pollution episode. The results could benefit the complete understanding of aerosol–ABL interactions under haze weather and air quality forecasting and control in Wuhan.

The variational method, backreactions, and the absorption probability in Wald type problems

We argue that the variational method in Wald type thought experiments, involves order of magnitude problems when one imposes the fact that delta M is inherently a first order quantity itself. One observes that the contribution of the second order perturbations is actually of the fourth order. Therefore backreactions have to be explicitly calculated. Here, we re-consider the overspinning problem for Kerr–Newman black holes interacting with test fields. We calculate the backreaction effects due to the induced increase in the angular velocity of the event horizon, which brings a partial solution to the overspinning problem. To bring an ultimate solution, we argue that the absorption probability should be taken into account in Wald type problems where black holes interact with test fields. This fundamentally alters the course of the analysis of the thought experiments. Due to the fact that a small fraction of the challenging modes is absorbed by the black holes, overspinning is prevented for both nearly extremal and extremal cases. Some extreme cases are easily fixed by backreaction effects. The arguments do not apply to the generic overspinning by fermionic fields for which the absorption probability is positive definite.

Impact of Nationwide COVID-19 Lockdown on Indian Air Quality in Terms of Aerosols as Observed from the Space

Air pollution in India, especially in cities located in Indo-Gangetic plain (IGP), has exceeded all standards in the recent past. The development of an effective pollution control strategy requires knowledge of the pollutant sources, their spatio-temporal variability, and the efficacy of control strategies. The Indian government has implemented nationwide lockdown (LD) during March and April 2020 to contain the infectious disease COVID-19. LD also provides an opportunity to evaluate the efficacy of restricting anthropogenic activities in controlling air pollution. In the present paper, using data from space-borne sensors, a detailed analysis of the impact of LD on aerosols in the Indian atmosphere is presented. The aerosol optical depth (AOD), fine mode fraction (FMF), absorbing aerosol index (AAI), active fire/thermal anomaly and the HYSPLIT model is used to study the instantaneous state and spatio-temporal transportation of aerosols. On average, an overall decrease in AOD with 60% of the Indian landmass showing an average decrease of 0.16 is observed. A significant decrease in FMF indorses decreased AOD solely due to restricted anthropogenic activities, especially in IGP, which confirms the positive impact of LD on air quality. However, central, northeast and partially western India show some contrasting results which are associated with smoke from active fires. An increase of 30% in fire spots is observed during LD in these regions. The city-level analysis shows improved air quality in most of the cities located in IGP during LD. The cities located in the west, north-west and east India have shown a small decrease in AOD during LD, while in central, north-east, and few in the southern region, the increased AOD is observed in spite of LD due to increase in biomass burning.

Superheated metastable atomized water plume diagnostics peculiarities in confusor-diffuser nozzles; atomizaton process: dimensionless treatment

Micron mode mass fraction ε determination method was tested for dense superheated water atomization plumes based on measurement attenuation coefficients of primary laser radiation. The initial water temperature in experiments was 200 and 240°C. The mass fraction of micron mode ε was determined using Bugger-Lambert equation and Mi theory. The comparison with previous data, which were obtained by scattering indicatrix analysis, was done. In terms of development of dimensionless treatment of experimental data on superheated water spray, the parameter expression for the transition to predominating role of nucleation in superheated water atomization in confusor-diffuser nozzles was revised the base of critical bubble formation.

Impact of Shutdown due to COVID-19 Pandemic on Aerosol Characteristics in Kanpur, India.

Background.Since March 2020, the number of confirmed COVID-19 positive cases have steadily risen in India. Various preventive measures have been taken to contain the spread of COVID-19. With restrictions on human activities, anthropogenic emissions driving air pollution levels have seen a reduction since March 23, 2020, when the government imposed the first nationwide shutdown. The landlocked Indo-Gangetic Plain (IGP) has many densely-populated cities, witnessing high levels of particulate matter due to both nature-driven and anthropogenic elements. Kanpur is an urban metropolis in the IGP with high aerosol loading, and this paper explores the impact of restricted anthropogenic activities on aerosol characteristics in Kanpur.Objectives.This study aims to investigate the change in aerosol optical depth level and its related parameters during the shutdown phases in Kanpur city compared to the same time periods in 2017–2019.Methods.Aerosol optical properties such as aerosol optical depth (AOD) at 500 nm, Angstrom exponent (AE), fine mode fraction (FMF) of AOD at 500 nm and single scattering albedo (SSA) at 440 nm were obtained from the Aerosol Robotic Network (AERONET) station operating in Kanpur from the 1st March to the 30th April for 2017–2020.Results.A significant decrease in aerosol loading was observed during the shutdown period compared to the pre-and partial shutdown periods in 2020 as well as during the same time periods of 2017–2019. Mean AOD, FMF and SSA were 0.37, 0.43 and 0.89, respectively, during the shutdown period in 2020. A 20–35% reduction in mean AOD levels was observed during the shutdown period in 2020 as compared to the same period in 2017–2019.Conclusions.The shutdown led to an improvement in air quality due to decreases in anthropogenic emissions. As fine particles, typically from urban and industrial emissions, dominate episodic air pollution events, this study can be further utilized by the scientific community and regulators to strengthen the emergency response action plan to check high pollution episodes in Kanpur city until cleaner technologies are in place.Competing Interests.The authors declare no completing financial interests.

SIZE DISTRIBUTION AND CONTRIBUTION OF PARTICLES FROM RICE STRAW OPEN BURNING TO THE ATMOSPHERE IN HANOI

Atmospheric ultrafine, accumulation mode and coarse fractions collected at representative rice straw open burning areas in Hanoi were investigated to identify characteristics of size distribution and contribution of particles emitted from rice straw (RS) burning season to the atmosphere. The sampling was conducted in two episodes: RS burning episode and RS non-burning episode at Dong Anh and Quoc Oai, in seven consecutive days for each sampling campaign from 2018 to 2019. In the RS burning episode, PM1-2.5 showed the highest fraction among all collected particles in both sampling sites, while PM2.5-10 was the most abundant in RS non-burning season. The average mass concentration of PM2.5 in RS burning period and RS non-burning period were 79.7  46.5 g m-3 and 65.2  21.9 g m-3, respectively at Dong Anh sampling site. Those values were 90.9  33.2 g m-3 in the QO_RS burning site and 71.9  29.3 g m-3 in the TM_RS non-burning site. The proportion of fine particle (PM2.5) at both sites were considerable higher in RS burning period as compared to non-burning period, while the concentration of ultrafineparticle (PM0.1) and coarse particle (PM&gt;10m) were similar between two episodes. This result provides better understanding on size distribution and contribution of fine particles from open RS burning to the atmosphere in Hanoi, which is an useful information for the environmental managers to control RS open burning in Hanoi as well as in Vietnam.

Optical and chemical properties of long-range transported aerosols using satellite and ground-based observations over seoul, South Korea

Aerosols that are transported long distances to the Korean Peninsula due to seasonal weather conditions can affect the local air quality of South Korea. In general, the long-range transported aerosols are widely distributed and variable with respect to time and space. In this aspect, geostationary satellites can be used as a powerful tool for providing temporal and spatial variation of aerosols by sequentially observing a target area. In this study, select events of high-concentration aerosols transported to Seoul, the capital of South Korea, were identified and analyzed using aerosol optical properties retrieved from geostationary ocean color imager (GOCI) for the cold season (October–March) from 2015 to 2018. In addition to the satellite data, ground-based observations of particulate matter (PM) and backward trajectory models were used. Based on the trajectories of air masses simulated with the backward trajectory models, events of the long-range transported high-concentration aerosols (hereafter referred to as “LRT events”) were selected and classified into three types: yellow dust, PM traversing over the Yellow Sea (PM-YS), and PM passing through North Korea (PM-NK). Optical and chemical properties of the three LRT events were analyzed, and significantly different features were found. Yellow dust showed a relatively high mean depolarization ratio at 1020 nm (0.26), a low Ångström exponent between 440 nm and 870 nm (0.27), fine mode fraction at 500 nm (0.26), single scattering albedo at 440 nm (0.93), and lidar ratio at 675 nm (37 sr) compared with those during the other PM events. Moreover, the proportions of Ca+ concentrations to the water-soluble ions in PM2.5 and concentrations of Ca and Fe among inorganic elements in PM2.5 significantly increased during the yellow dust event. On the contrary, the PM events (PM-YS and PM-NK) had relatively high mean Ångström exponent (1.28 and 1.38), fine mode fraction (0.94 and 0.81), single scattering albedo (0.96 and 0.94), lidar ratio (66 sr and 44 sr), and low depolarization ratios (0.035 and 0.066). The ratios of NO3− and SO42− to water-soluble ions in PM2.5 were significantly increased during the high PM events. By comparing the two PM events, the volume median radius of fine mode particles and fine mode fraction of PM-YS were found to be considerably higher compared with those of PM-NK. Such different characteristics found in this study for the three LRT events indicate that optical and chemical analyses of aerosols using satellite and ground-based observations can give us the possibility of roughly identifying aerosol types (yellow dust or PM) and trajectories (passing through the Yellow Sea or North Korea).

The interaction of ozone and aerosol in a semi-arid region in the Middle East: Ozone formation and radiative forcing implications

Investigations on the variations in ozone concentration, aerosol properties, and ozone-aerosol interactions can help to better understand the atmospheric radiation budget and their effects on climate. In this study, the interaction of ozone and aerosol was analyzed over Zanjan, Iran. The seasonal cycle of total column ozone demonstrated that high ozone concentrations often occur in early spring and winter. However, the seasonal cycle of tropospheric ozone showed that high concentrations of tropospheric ozone often happen in the spring and summer. The high values of Aerosol Optical Depth (AOD), Particulate Matter (PM10) and Absorption Aerosol Optical Depth (AAOD) were observed in the spring and summer, whereas low values in the fall and winter. The results showed that tropospheric ozone and AOD were negatively correlated during the spring and summer seasons. Especially on dusty days, the aerosol has reduced the concentration of ozone in the atmosphere. The correlation coefficients between PM10 and ozone were negative during dusty days/months. Moreover, the relation of ozone with a fine mode fraction of AOD depicted negative correlations in the spring and summer and positive correlation in the fall and winter. A comprehensive study on the effects of the aerosol types on ozone concentration showed that anthropogenic fine mode aerosols such as urban/industrial had a positive impact on ozone production, contrary to the effect of dust, which reduced ozone. This study showed that coarse mode aerosols such as dust with higher AOD, reduced the sunlight to reach the earth's surface. As a result, aerosol radiative forcing (ARF) values were found to be more negative at the surface; thus, the photolysis rate decreased considerably, and the concentration of surface ozone was reduced significantly.

Comparison of Aerosol Properties in the Korean Peninsula Between AERONET Version 2 and 3 Data Set

To investigate the consistency and difference between AERONET version 2 and 3 data set in the Korean peninsula, we compared several aerosol optical properties for 12 Korean AERONET sites having at least 2-year measurement records. Generally, both version products have similar patterns, but we sometimes found a little regional difference. While the aerosol optical depth (AOD) is analogous between version 2 and 3, the version difference of single scattering albedo (SSA) is the largest. Angstrom exponent (AE) and fine mode fraction (FMF) values are sometimes different in maritime conditions. Two AERONET sites in Seoul (Seoul_SNU and Yonsei_Univ) have some differences between version 2 and 3, probably related to the higher sensitivity of version 3 data to the fine mode particle. Since parameters revealing the aerosol radiative property (e.g., SSA) and size information (e.g., AE and FMF) show some version differences regionally, we compared the result of aerosol type classification using these parameters. As a result, we found that the version difference of aerosol size and radiative property can induce the different diagnosis of locally dominant aerosol types. In spite of these findings, consistency between version 2 and 3 is quite large. Thus, regional patterns of aerosol optical properties previously found based on AERONET version 2 look still valid in the Korean peninsula.

Hygroscopicity of urban aerosols and its link to size-resolved chemical composition during spring and summer in Seoul, Korea

Abstract. Chemical effects on the size-resolved hygroscopicity of urban aerosols were examined based on the Korea–US Air Quality Study (KORUS-AQ, 2020) field campaign data. The information on size-resolved hygroscopicity and the chemical composition of aerosols were obtained by a hygroscopic tandem differential mobility analyzer (HTDMA) and a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), respectively. Good correspondence was shown between measured and estimated κ values calculated from the combination of bulk chemical composition data and oxidation parameters of organic aerosols (f44 and O∕C). These results imply that chemical composition is closely associated with aerosol hygroscopicity. However, the correlation between measured and estimated κ values degraded as particle size decreased, implying that size-resolved chemical composition data are required for more detailed hygroscopicity analysis. In addition to size-resolved chemical data, the m∕z tracer method was applied for size-resolved organic factors. Specifically, m∕z 57 and 44 were used as AMS spectral markers for hydrocarbon-like organic aerosol (HOA) and oxygenated organic aerosol (OOA), respectively. These size-resolved chemical composition data were found to be critical in explaining size-dependent hygroscopicity, as well as the diurnal variation in κ for small particles, i.e., low κ in the morning and high κ in the afternoon. Additionally, aerosol mixing state information was associated with the size-resolved chemical composition data. That is to say that the relationship between the number fraction of each hygroscopicity mode and the volume fraction of different chemical composition was investigated. For example, the HOA volume fraction comprised about 60 % of the variation in less hygroscopic (LH) mode number fractions for externally mixed aerosols.

Dust Aerosol Retrieval Over the Oceans With the MODIS/VIIRS Dark Target Algorithm: 2. Nonspherical Dust Model

AbstractThe Dark‐target (DT) aerosol algorithm retrieves spectral Aerosol Optical Depth (AOD) and other aerosol properties from Moderate‐resolution Imaging Spectrometer (MODIS) reflectance observations. Over the ocean, the DT algorithm is known to contain scattering‐angle‐dependent biases in its retrievals of AOD, Angstrom Exponent (AE), and Fine Mode Fraction (FMF) for dust aerosols. Following a two‐step strategy to improve the DT retrieval of dust over ocean, for which the first step is to identify dusty pixels (reported in “Part 1”), in this “Part 2,” we report on construction of a new dust model lookup table (LUT) and the strategy for applying it within the existing DT algorithm. In particular, we evaluate different characterizations of dust optical properties from a variety of frameworks and databases, and compare them with the current DT retrieval assumptions. Substituting the standard operational LUT with a spheroid dust model with identified dusty pixels shows significant improvement when compared with collocated AERONET‐identified dusty pixels. Specifically, the application of the new dust model to dusty pixels reduces their AOD bias from 0.06 to 0.02 while improving the fraction of retrievals within expected error from 64% to 82%. At the same time, the overall bias in AE is reduced from 0.13 to 0.06, and the scattering‐angle‐dependent AE bias is largely eliminated. In testing on two full months of data (April and July), the new retrieval will reduce the monthly mean AOD by up to 0.1 and 0.2 in the north Atlantic and Arabian seas, respectively. The average AE and FMF are also reduced in these dust heavy regions.

Inter-annual and seasonal variations in optical and physical characteristics of columnar aerosols over the Pokhara Valley in the Himalayan foothills

ABSTRACT This study reports comprehensive analysis of seasonal and inter-annual variations of aerosol optical and physical properties over the Pokhara Valley in the foothills of central Himalayas in Nepal utilizing the high-quality multi-year columnar aerosol data observed recently from January 2010 to December 2017. The influence of forest fires and agro-residue fires on aerosol properties is also investigated. Seasonal mean aerosol optical depth (AOD) over Pokhara is ≥0.3 during the year, highest in pre-monsoon and lowest in monsoon season. AOD exhibits a consistent seasonal cycle every year with a peak in pre-monsoon. Fine mode fraction contributing to AOD and Angstrom exponent (α) corroborate seasonal pattern of AODs. AODs show good correlation with fire counts in the Pokhara Valley and surrounding region. The aerosol volume size distributions are bimodal. The peaks in volume size distribution and volume concentrations obtained in 2016 pre-monsoon are the highest during the 8-year observation period which coincided with peaks in the occurrence of highest number of fires in the Pokhara Valley and surrounding region. Effective radius of coarse mode aerosols is an order of magnitude larger than fine mode aerosols. The analysis of optical and physical properties of aerosols over the Pokhara Valley suggest that the aerosols observed over the Himalayan foothills are of urban/industrial, biomass burning and dust origin with proportional contributions varying in different seasons. The study provides observational constraints on aerosol physical and optical properties that can serve as inputs for model simulation of aerosol physiochemical processes and quantification of impacts on air quality, climate and sensitive ecosystems in this data-sparse region.