scholarly journals Investigation of Aerosol Climatology and Long-Range Transport of Aerosols over Pokhara, Nepal

Atmosphere ◽  
2020 ◽  
Vol 11 (8) ◽  
pp. 874 ◽  
Author(s):  
Jeevan Regmi ◽  
Khem N Poudyal ◽  
Amod Pokhrel ◽  
Madhu Gyawali ◽  
Lekhendra Tripathee ◽  
...  
Keyword(s):  
Optical Depth ◽  
Monsoon Season ◽  
Single Scattering ◽  
Optical Data ◽  
Logarithmic Scale ◽  
Back Trajectories ◽  
Aerosol Loading ◽  
Post Monsoon ◽  
Polynomial Fit ◽  
Fine Mode

This study presents the spectral monthly and seasonal variation of aerosol optical depth (τAOD), single scattering albedo (SSA), and aerosol absorption optical depth (AAOD) between 2010 and 2018 obtained from the Aerosol Robotic Network (AERONET) over Pokhara, Nepal. The analysis of these column-integrated aerosol optical data suggests significant monthly and seasonal variability of aerosol physical and optical properties. The pre-monsoon season (March to May) has the highest observed τAOD(0.75 ± 0.15), followed by winter (December to February, 0.47 ± 0.12), post-monsoon (October and November, 0.39 ± 0.08), and monsoon seasons (June to September, 0.27 ± 0.13), indicating seasonal aerosol loading over Pokhara. The variability of Ångström parameters, α, and β, were computed from the linear fit line in the logarithmic scale of spectral τAOD, and used to analyze the aerosol physical characteristics such as particle size and aerosol loading. The curvature of spectral τAOD, α’, computed from the second-order polynomial fit, reveals the domination by fine mode aerosol particles in the post-monsoon and winter seasons, with coarse mode dominating in monsoon, and both modes contributing in the pre-monsoon. Analysis of air mass back trajectories and observation of fire spots along with aerosol optical data and aerosol size spectra suggest the presence of mixed types of transboundary aerosols, such as biomass, urban-industrial, and dust aerosols in the atmospheric column over Pokhara.

scholarly journals Analysis of Aerosol Optical Depth and Angstrom Exponents over an AERONET site at Pokhara, Nepal

BIBECHANA ◽  
2021 ◽  
Vol 18 (1) ◽  
pp. 118-127
Author(s):  
Jeevan Regmi ◽  
Khem N Poudyal ◽  
Amod Pokhrel ◽  
Madhu Gyawali ◽  
Anthony Barinelli ◽  
...  
Keyword(s):  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Monsoon Season ◽  
Anthropogenic Aerosols ◽  
Annual Average ◽  
Average Value ◽  
Monthly Average ◽  
Aerosol Loading ◽  
Post Monsoon ◽  
Post Monsoon Season

The monthly variability of Aerosol Optical Depth at 0.50 μm (AOD0.50) and Ångström exponents (AE) based on spectral AODs over an Aerosol Robotic Network (AERONET) site Pokhara, are analyzed by using aerosol data of the year 2017. The AOD0.50 are characterized by low average values (0.21± 0.12) in monsoon, and highest values in pre-monsoon (0.67± 0.14) followed by winter (0.46± 0.28) and post-monsoon (0.33±.02) with an overall mean of 0.43 ± 0.02. The average AE obtained by using AODs at 0.44 μm and 0.87 μm is 1.20± 0.04 in pre-monsoon, 1.37± 0.05 in monsoon, 1.41±.01 in post-monsoon, and 1.37± 0.07 in winter with an annual average value of 1.35 ± 0.08. These overall variations of AE indicate that the majority of aerosol loading during the study period was mixture of fine and coarse mode aerosols and the influence of anthropogenic aerosols. The monthly average AOD suggests low aerosol loading in the months of the monsoon season (June to September) than other months of pre-monsoon season (March to May) and post-monsoon season (October and November). BIBECHANA 18 (2021) 118-127

scholarly journals Retrieval of the Fine-Mode Aerosol Optical Depth over East China Using a Grouped Residual Error Sorting (GRES) Method from Multi-Angle and Polarized Satellite Data

2018 ◽  
Vol 10 (11) ◽  
pp. 1838 ◽  
Author(s):  
Yang Zhang ◽  
Zhengqiang Li ◽  
Zhihong Liu ◽  
Juan Zhang ◽  
Lili Qie ◽  
...  
Keyword(s):  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Residual Error ◽  
East China ◽  
Anthropogenic Aerosols ◽  
Aerosol Model ◽  
Retrieval Accuracy ◽  
Aerosol Loading ◽  
Fine Mode ◽  
Fine Mode Aerosol

The fine-mode aerosol optical depth (AODf) is an important parameter for the environment and climate change study, which mainly represents the anthropogenic aerosols component. The Polarization and Anisotropy of Reflectances for Atmospheric Science coupled with Observations from a Lidar (PARASOL) instrument can detect polarized signal from multi-angle observation and the polarized signal mainly comes from the radiation contribution of the fine-mode aerosols, which provides an opportunity to obtain AODf directly. However, the currently operational algorithm of Laboratoire d’Optique Atmosphérique (LOA) has a poor AODf retrieval accuracy over East China on high aerosol loading days. This study focused on solving this issue and proposed a grouped residual error sorting (GRES) method to determine the optimal aerosol model in AODf retrieval using the traditional look-up table (LUT) approach and then the AODf retrieval accuracy over East China was improved. The comparisons between the GRES retrieved and the Aerosol Robotic Network (AERONET) ground-based AODf at Beijing, Xianghe, Taihu and Hong_Kong_PolyU sites produced high correlation coefficients (r) of 0.900, 0.933, 0.957 and 0.968, respectively. The comparisons of the GRES retrieved AODf and PARASOL AODf product with those of the AERONET observations produced a mean absolute error (MAE) of 0.054 versus 0.104 on high aerosol loading days (AERONET mean AODf at 865 nm = 0.283). An application using the GRES method for total AOD (AODt) retrieval also showed a good expandability for multi-angle aerosol retrieval of this method.

scholarly journals Sunphotometry of the 2006–2007 aerosol optical/radiative properties at the Himalayan Nepal Climate Observatory – Pyramid (5079 m a.s.l.)

2010 ◽  
Vol 10 (1) ◽  
pp. 1193-1220 ◽  
Author(s):  
G. P. Gobbi ◽  
F. Angelini ◽  
P. Bonasoni ◽  
G. P. Verza ◽  
A. Marinoni ◽  
...  
Keyword(s):  
High Altitude ◽  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Radiative Forcing ◽  
Monsoon Season ◽  
Good Reason ◽  
Single Scattering ◽  
Radiative Properties ◽  
Mountain Range ◽  
Monsoon Period

Abstract. In spite of being located at the heart of the highest mountain range in the world, the Himalayan Nepal Climate Observatory (5079 m a.s.l.) at the Ev-K2-CNR Pyramid is shown to be affected by the advection of pollution aerosols from the populated regions of southern Nepal and the Indo-Gangetic plains. Such an impact is observed along most of the period April 2006–March 2007 addressed here, with a minimum in the monsoon season. Backtrajectory-analysis indicates long-range transport episodes occurring in this period to originate mainly in the West Asian deserts. At this high altitude site, the measured aerosol optical depth is observed to be: 1) about one order of magnitude lower than the one measured at Gandhi College (60 m a.s.l.), in the Indo-Gangetic basin, and 2) maximum during the monsoon period, due to the presence of elevated (cirrus-like) particle layers. Assessment of the aerosol radiative forcing results to be hampered by the persistent presence of these high altitude particle layers, which impede a continuous measurement of both the aerosol optical depth and its radiative properties from sky radiance inversions. Even though the retrieved absorption coefficients of pollution aerosols was rather large (single scattering albedo of the order of 0.6–0.9 were observed in the month of April 2006), the corresponding low optical depths (~0.03 at 500 nm) are expected to limit the relevant radiative forcings. Still, the high specific forcing of this aerosol and its capability of altering snow surface albedo provide good reason for continuous monitoring.

scholarly journals Recent increase in aerosol loading over the Australian arid zone

2010 ◽  
Vol 10 (4) ◽  
pp. 1689-1699 ◽  
Author(s):  
R. M. Mitchell ◽  
S. K. Campbell ◽  
Y. Qin
Keyword(s):  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Arid Zone ◽  
Severe Drought ◽  
Near Surface ◽  
Aerosol Loading ◽  
Ephemeral Lakes ◽  
Fine Mode ◽  
Fine Mode Aerosol ◽  
Ephemeral Vegetation

Abstract. Collocated sun photometer and nephelometer measurements at Tinga Tingana in the Australian Outback over the decade 1997–2007 show a significant increase in aerosol loading following the onset of severe drought conditions in 2002. This increase is confined to the season of dust activity, particularly September to March. In contrast, background aerosol levels during May, June and July remained stable. The enhanced aerosol loadings during the latter 5 years of the study period can be understood as a combination of dune destabilisation through loss of ephemeral vegetation and surface crust, and the changing supply of fluvial sediments to ephemeral lakes and floodplains within the Lake Eyre Basin. Major dust outbreaks are generally highly localised, although significant dust activity was observed at Tinga Tingana on 50% of days when a major event occurred elsewhere in the Lake Eyre Basin, suggesting frequent basin-wide dust mobilisation. Combined analysis of aerosol optical depth and scattering coefficient shows weak correlation between the surface and column aerosol (R2=0.24). The aerosol scale height is broadly distributed with a mode typically between 2–3 km, with clearly defined seasonal variation. Climatological analysis reveals bimodal structure in the annual cycle of aerosol optical depth, with a summer peak related to maximal dust activity, and a spring peak related to lofted fine-mode aerosol. There is evidence for an increase in near-surface aerosol during the period 2003–2007 relative to 1997–2002, consistent with an increase in dust activity. This accords with an independent finding of increasing aerosol loading over the Australian region as a whole, suggesting that rising dust activity over the Lake Eyre Basin may be a significant contributor to changes in the aerosol budget of the continent.

scholarly journals Sunphotometry of the 2006–2007 aerosol optical/radiative properties at the Himalayan Nepal Climate Observatory-Pyramid (5079 m a.s.l.)

2010 ◽  
Vol 10 (22) ◽  
pp. 11209-11221 ◽  
Author(s):  
G. P. Gobbi ◽  
F. Angelini ◽  
P. Bonasoni ◽  
G. P. Verza ◽  
A. Marinoni ◽  
...  
Keyword(s):  
High Altitude ◽  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Radiative Forcing ◽  
Monsoon Season ◽  
Single Scattering ◽  
Radiative Properties ◽  
Mountain Range ◽  
Anthropogenic Aerosol ◽  
Aerosol Radiative

Abstract. In spite of being located at the heart of the highest mountain range in the world, the Himalayan Nepal Climate Observatory (5079 m a.s.l.) at the Ev-K2-CNR Pyramid is shown to be affected by the advection of pollution aerosols from the populated regions of southern Nepal and the Indo-Gangetic plains. Such an impact is observed along most of the period April 2006–March 2007 addressed here, with a minimum in the monsoon season. Backtrajectory-analysis indicates long-range transport episodes occurring in this year to originate mainly in the west Asian deserts. At this high altitude site, the measured aerosol optical depth is observed to be about one order of magnitude lower than the one measured at Ghandi College (60 m a.s.l.), in the Indo-Gangetic basin. As for Ghandi College, and in agreement with the in situ ground observations at the Pyramid, the fine mode aerosol optical depth maximizes during winter and minimizes in the monsoon season. Conversely, total optical depth maximizes during the monsoon due to the occurrence of elevated, coarse particle layers. Possible origins of these particles are wind erosion from the surrounding peaks and hydrated/cloud-processed aerosols. Assessment of the aerosol radiative forcing is then expected to be hampered by the presence of these high altitude particle layers, which impede an effective, continuous measurement of anthropogenic aerosol radiative properties from sky radiance inversions and/or ground measurements alone. Even though the retrieved absorption coefficients of pollution aerosols were rather large (single scattering albedo of the order of 0.6–0.9 were observed in the month of April 2006), the corresponding low optical depths (~0.03 at 500 nm) are expected to limit the relevant radiative forcing. Still, the high specific forcing of this aerosol and its capability of altering snow surface albedo provide good reasons for continuous monitoring.

scholarly journals Recent increase in aerosol loading over the Australian arid zone

2009 ◽  
Vol 9 (5) ◽  
pp. 21619-21645 ◽  
Author(s):  
R. M. Mitchell ◽  
S. K. Campbell ◽  
Y. Qin
Keyword(s):  
Aerosol Optical Depth ◽  
Optical Depth ◽  
Arid Zone ◽  
Scattering Coefficient ◽  
Severe Drought ◽  
Near Surface ◽  
Aerosol Loading ◽  
Ephemeral Lakes ◽  
Fine Mode ◽  
Sun Photometer

Abstract. Collocated sun photometer and nephelometer measurements at Tinga Tingana in the Australian Outback over the decade 1997–2007 show a significant increase in aerosol loading following the onset of severe drought conditions in 2002. The mean mid-visible scattering coefficient obtained from nephelometer measurements over the period 2003–2007 is approximately double that recorded over the preceding 5 yr, with consistent trends in the column aerosol optical depth derived from the sun photometer. This increase is confined to the season of dust activity, particularly September to March. In contrast, background aerosol levels during May, June and July remained stable. The enhanced aerosol loadings during the latter 5 yr of the study period can be understood as a combination of dune destabilisation through loss of ephemeral vegetation and surface crust, and the changing supply of fluvial sediments to ephemeral lakes and floodplains within the Lake Eyre Basin. Major dust outbreaks are generally highly localised, although significant dust activity was observed at Tinga Tingana on 50% of days when a major event occurred elsewhere in the Lake Eyre Basin, suggesting frequent basin-wide dust mobilisation. Combined analysis of aerosol optical depth and scattering coefficient shows weak correlation between the surface and column aerosol (R2=0.24). The aerosol scale height is broadly distributed with a mode typically between 2–3 km, with clearly defined seasonal variation. Climatological analysis reveals bimodal structure in the annual cycle of aerosol optical depth, with a summer peak related to maximal dust activity, and a spring peak related to lofted fine-mode aerosol. There is evidence for an increase in near-surface aerosol during the period 2003–2007 relative to 1997–2002, consistent with an increase in dust activity. This accords with an independent finding of increasing aerosol loading over the Australian region as a whole, suggesting that rising dust activity over the Lake Eyre Basin may be a significant contributor to changes in the aerosol budget of the continent.

scholarly journals A multi-model evaluation of aerosols over South Asia: common problems and possible causes

2015 ◽  
Vol 15 (10) ◽  
pp. 5903-5928 ◽  
Author(s):  
X. Pan ◽  
M. Chin ◽  
R. Gautam ◽  
H. Bian ◽  
D. Kim ◽  
...  
Keyword(s):  
South Asia ◽  
Optical Depth ◽  
Agricultural Waste ◽  
Lower Troposphere ◽  
Wide Field ◽  
Gangetic Plain ◽  
Aerosol Loading ◽  
Post Monsoon ◽  
Common Problems ◽  
Waste Burning

Abstract. Atmospheric pollution over South Asia attracts special attention due to its effects on regional climate, water cycle and human health. These effects are potentially growing owing to rising trends of anthropogenic aerosol emissions. In this study, the spatio-temporal aerosol distributions over South Asia from seven global aerosol models are evaluated against aerosol retrievals from NASA satellite sensors and ground-based measurements for the period of 2000–2007. Overall, substantial underestimations of aerosol loading over South Asia are found systematically in most model simulations. Averaged over the entire South Asia, the annual mean aerosol optical depth (AOD) is underestimated by a range 15 to 44% across models compared to MISR (Multi-angle Imaging SpectroRadiometer), which is the lowest bound among various satellite AOD retrievals (from MISR, SeaWiFS (Sea-Viewing Wide Field-of-View Sensor), MODIS (Moderate Resolution Imaging Spectroradiometer) Aqua and Terra). In particular during the post-monsoon and wintertime periods (i.e., October–January), when agricultural waste burning and anthropogenic emissions dominate, models fail to capture AOD and aerosol absorption optical depth (AAOD) over the Indo–Gangetic Plain (IGP) compared to ground-based Aerosol Robotic Network (AERONET) sunphotometer measurements. The underestimations of aerosol loading in models generally occur in the lower troposphere (below 2 km) based on the comparisons of aerosol extinction profiles calculated by the models with those from Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) data. Furthermore, surface concentrations of all aerosol components (sulfate, nitrate, organic aerosol (OA) and black carbon (BC)) from the models are found much lower than in situ measurements in winter. Several possible causes for these common problems of underestimating aerosols in models during the post-monsoon and wintertime periods are identified: the aerosol hygroscopic growth and formation of secondary inorganic aerosol are suppressed in the models because relative humidity (RH) is biased far too low in the boundary layer and thus foggy conditions are poorly represented in current models, the nitrate aerosol is either missing or inadequately accounted for, and emissions from agricultural waste burning and biofuel usage are too low in the emission inventories. These common problems and possible causes found in multiple models point out directions for future model improvements in this important region.

Radiative forcing of atmospheric brown clouds over the Indo-Gangetic Plain

2021 ◽  
Author(s):  
Manish Jangid ◽  
Amit Mishra
Keyword(s):  
Optical Depth ◽  
Radiative Forcing ◽  
Water Cycle ◽  
Monsoon Season ◽  
Regional Climate ◽  
Winter Season ◽  
Gangetic Plain ◽  
Post Monsoon ◽  
Using Data ◽  
Indo Gangetic Plain

<p>Atmospheric brown clouds (ABCs) are a dense and extensive pollution layer and have significant implications on air quality, agriculture, water cycle, and regional climate. The objective of the present study is to observe seasonal and spatial variations in the occurrence of ABCs and its radiative effects. The Indo-Gangetic plain (IGP) is the most populated region of India, which is an extended region in the foothills of the Himalayas. The IGP is one of the ABCs hotspots over the globe. The frequency of ABCs occurrences and radiative forcing were calculated using data from seven ground-based remote sensors situated across the IGP. We have used total ~ 5000 days of Level-2 aerosol measurements from seven AERosol Robotic NETwork (AERONET) stations (Karachi, Lahore, Jaipur, New Delhi, Kanpur, Gandhi college and Dhaka University) for three seasons (Pre-monsoon, Post-monsoon, and Winter) during 2000-2019. An algorithm based on the optical properties of aerosols is used to defined extreme pollution events (ABCs days) for each site. Our results show more frequent occurrences of ABCs over the region in the pre-monsoon out of all three seasons. However, spatial variation is found in all seasons, like maximum frequency of ABCs over western IGP region in post-monsoon and minimum is at eastern IGP region in the winter season. Further, we have used the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model to calculate radiative forcing during ABCs days on all sites of study. Aerosol optical depth (AOD) and absorption optical depth (AAOD) was used to calculate radiative forcing over the IGP region. Radiative forcing of ABCs is negative at both the surface (SRF) and top of the atmosphere (TOA), whereas it is positive in the atmosphere (ATM). In magnitude, it was found minimum in the pre-monsoon season at TOA. However, other seasons have specific features over specific locations, for example, in the winter season, radiative forcing is maximum over Kolkata at TOA, SRF, and ATM, which are -13.81 W/m<sup>2</sup>, -50.90 W/m<sup>2</sup>, and +37.09 W/m<sup>2</sup> respectively. In the pre-monsoon season, radiative forcing is maximum at Delhi (-9.59 W/m<sup>2</sup>) at TOA. In post-monsoon season radiative forcing maximum at Gandhi-college (-11.30 W/m<sup>2</sup>) at TOA. This ground observation is also compared with Modern Era Retrospective analysis and Research and Applications-2 (MEERA 2) modal data. These results indicate the cooling effect of ABCs at the surface and TOA over the IGP region throughout the period.</p>

scholarly journals Global Dust Optical Depth Climatology Derived from CALIOP and MODIS Aerosol Retrievals on Decadal Time Scales: Regional and Interannual Variability

2021 ◽  
Author(s):  
Qianqian Song ◽  
Zhibo Zhang ◽  
Hongbin Yu ◽  
Paul Ginoux ◽  
Jerry Shen
Keyword(s):  
Optical Depth ◽  
Surface Wind ◽  
Active Regions ◽  
Surface Wind Speed ◽  
Single Scattering ◽  
Northwestern Pacific ◽  
Gobi Desert ◽  
Shape Information ◽  
Fine Mode ◽  
Seasonal And Interannual Variations

Abstract. We present a satellite-derived global dust climatological record over the last two decades, including the monthly mean visible dust optical depth (DAOD) and vertical distribution of dust extinction coefficient at a 2º (latitude) × 5º (longitude) spatial resolution derived from CALIOP and MODIS. Dust is distinguished from non-dust aerosols based on particle shape information (e.g., lidar depolarization ratio) for CALIOP, and on dust size and absorption information (e.g., fine-mode fraction, Angstrom exponent, and single-scattering albedo) for MODIS, respectively. On multi-year average basis, the global (60° S–60° N) and annual mean DAOD is 0.029 and 0.063 derived from CALIOP and MODIS retrievals, respectively. In most dust active regions, CALIOP DAOD generally correlates well with the MODIS DAOD, with CALIOP DAOD being significantly smaller. CALIOP DAOD is 18 %, 34 %, 54 % and 31 % smaller than MODIS DAOD over Sahara Deserts, the tropical Atlantic Ocean, the Caribbean Sea, and the Arabian Sea, respectively. Over East Asia and the northwestern Pacific Ocean (NWP), however, the two datasets show weak correlation. Despite these discrepancies, CALIOP and MODIS show similar seasonal and interannual variations in regional DAOD. For dust aerosol over NWP, both CALIOP and MODIS show a declining trend of DAOD at a rate of about 2 % yr−1. This decreasing trend is consistent with the observed declining trend of DAOD in the southern Gobi Desert at a rate of −3 % yr−1 and −5 % yr−1 according to CALIOP and MODIS, respectively. The decreasing trend of DAOD in the southern Gobi Desert is in turn found to be significantly correlated with an increasing trend of vegetation and a decreasing trend of surface wind speed in the area.

scholarly journals Spatio-temporal aerosol optical characteristics over the Arabian Sea during the pre monsoon season

2009 ◽  
Vol 9 (5) ◽  
pp. 22223-22269 ◽  
Author(s):  
D. G. Kaskaoutis ◽  
M. C. R. Kalapureddy ◽  
P. C. S. Devara ◽  
P. G. Kosmopoulos ◽  
P. T. Nastos ◽  
...  
Keyword(s):  
Arabian Sea ◽  
Positive Curvature ◽  
Monsoon Season ◽  
Direct Consequence ◽  
Spectral Variation ◽  
Wavelength Band ◽  
Polynomial Fit ◽  
Fine Mode ◽  
The Mean ◽  
Spatio Temporal

Abstract. Ship-borne Aerosol Optical Depth (AOD) measurements obtained by a sunphotometer have been used to retrieve the Ångström wavelength exponent (α). These measurements were obtained in the Arabian Sea (AS), where the surrounded arid-region influence is expected, during the pre-monsoon season. Spectral variation of α is typically considered in this study. It is found that the polynomial fit becomes more accurate in the wavelength band 340–1020 nm rather than the 340–870 nm. The coarse-mode (positive curvature in the ln τα vs ln λ) aerosols are mainly depicted in the Northern part of the AS closely associated with the nearby arid areas and fine-mode aerosols are mainly observed over the far and coastal AS regions. In the study period the mean AOD at 500 nm is 0.246±0.114 and the α340−1020 is 0.904±0.186. The α340−870 exhibits similar values (0.924±0.179), while significant differences revealed for the constant terms of the polynomial fit (a1 and (a2) proportionally to the wavelength band used for their determination. Observed day-to-day variability in the aerosol load and optical properties are direct consequence of the local winds and air-mass trajectories along with the position of the ship.

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