SEASONAL VARIATIONS IN PROPERTIES OF ATMOSPHERIC AEROSOL IN BENGALURU :A CASE STUDY FOR 2018
Atmospheric aerosols are particles in solid or liquid phase suspended in the atmosphere. The size of these particles ranges from 10-3 to 102 µm depending on their sources and production mechanisms. Although aerosols are present in the atmosphere in trace concentrations, they play a crucial role in the Earth’s weather and climate. The properties of aerosols are highly variable in space and time and their chemical, physical and optical properties are often studied in order to understand their role in the Earth’s climate. The Aerosol Optical Depth (AOD) which represents the columnar aerosol load and mass concentration of the absorbing aerosol Black Carbon (BC) are important properties used to determine the impact of aerosols on the climate. This study addresses seasonal variation in AOD and near-surface BC concentrations in an urban environment, Bengaluru, during the year 2018. It is observed that the AOD and BC mass concentration have large seasonal variations with high values during winter and pre-monsoon seasons and lowest values during monsoon. Two sharp peaks were observed in the diurnal variation of BC mass concentration, one at morning and the other in the evening hours during winter, which is in agreement with existing meteorological studies. These peaks became obscured during monsoon when the aerosol concentrations are brought down by wet scavenging.
Keywords: aerosols, Aerosol Optical Depth (AOD), Black Carbon (BC), atmospheric boundary layer, monsoon,diurnal variation
|AOD||Aerosol Optical Depth|
|ABL||Atmospheric Boundary Layer|
|MBC||Black carbon mass concentration|
Our earth is probably the only planet in the known universe to have a unique atmosphere which can sustain life. The atmosphere acts as a reservoir of gases required for life which also plays a critical role in the distribution of energy within the planet. Our atmosphere is composed of mainly nitrogen (78%) and oxygen (21%) with trace amounts of other gases like water vapor, carbon dioxide etc. and aerosols. Together the gases and aerosols play a crucial role in the Earth’s weather and climate. Atmospheric aerosols are generally particulates suspended in the atmosphere in solid or liquid phase altering the Earth’s radiation budget. Aerosols are naturally produced by mechanical disintegration processes over land, by the action of wind over the ocean or by chemical reactions in atmosphere. They are also emitted from anthropogenic sources like industries, vehicles, open burning, biomass burning etc. Depending on the source and production mechanisms their size can vary from to 10-3 to 102 µm  .Though they are always present in the atmosphere, their concentrations have large spatio-temporal variations. They have a short lifetime of approximately 1 week in the troposphere. Yet they are carried to locations far (thousands of kilometers) away from their sources . Aerosols can interact both directly and indirectly with the Earth’s radiation budget and climate. In the direct interaction, aerosols scatter or absorb the incoming solar radiation sending some part of the solar radiation back to space. Aerosols perturb the radiation balance indirectly, through the modification of cloud properties. Aerosols in the lower atmosphere modify the size of the cloud droplets, changing how the clouds reflect and absorb sunlight thereby affecting the Earth’s energy budget.
Some important characteristics of an aerosol population are size distribution, chemical composition, and shape of the particles. Depending on the size, aerosols are broadly classified into following categories  (Fig.1):
- Aitken mode/Nucleation mode of aerosol particles has diameter between 10-3 μm and 0.1 μm. They result from combustion processes, photochemical reactions, etc. and are generally short-lived.
- Accumulation mode particles has diameter in the range 0.1 and 2.5 μm. They result from coagulation processes of smaller particles. These are generally larger lived and are highly optically active.
- Coarse mode has diameter larger than 2.5 μm. They result from mechanical disintegration processes like wind erosion of dust, sea spray etc.
For particles in the size range from 10-3 to 0.1 µm the average lifetime is typically a few days. Particles larger than 10 µm cannot remain suspended in air for long periods because of gravity.
Sources of aerosols
The sources of aerosol can be either natural or anthropogenic (Table 1). Typical natural sources of aerosol are oceans, deserts, volcanoes, vegetation and wildlife. The main anthropogenic sources of aerosols are emissions from industries and vehicles, burning of fossil fuels, biomass and garbage, emissions from construction sites, emissions from farmlands etc. Although natural aerosols dominate over anthropogenic aerosols globally, the anthropogenic emissions can dominate over natural emissions at regional scales .
|Source||Amount, Tg/yr [106 metric tons/yr]|
|Soil dust||1000 - 3000||1500|
|Sea salt||1000 - 10000||1300|
|Botanical debris||26 - 80||50|
|Volcanic dust||4 - 10000||30|
|Forest fires||3 - 150||20|
|Gas-to-particle conversion||100 - 260||180|
|Photochemical||40 - 200||60|
|Total for natural sources||2200 - 24000||3100|
|Direct emissions||50 - 160||120|
|Gas-to-particle conversion||260 - 460||330|
|Photochemical||5 - 25||10|
|Total for anthropogenic sources||320 - 640||460|
(Reference: W.C. Hinds, Aerosol Technology, 2nd Edition, Wiley Interscience)
(here best estimate = The probability-weighted average)
Common aerosol systems
The common aerosol systems are as follows:
Smoke: Smoke is produced mainly from burning of organic substances like coal, oil, wood or other carbonaceous fuels.
Haze: Particles, which are grown to large sizes by condensation of water vapor. It reduces the visibility as it contains larger aerosols.
Fog: It is a collection of small liquid droplets and when these particles collide with each other, they combine to form large droplets which could even appear as rain.
Smog: A combination of smoke and fog.
Some important definitions
The radiative effect of atmospheric aerosol is of greater interest due to its effect on the climate change. The basic process of aerosol-radiation interaction involves scattering and absorption of solar and terrestrial radiation. Hence in this perspective some terms are used –
i) Extinction coefficient
A proportionality constant which represents the extinction (due to absorption and scattering) of light when it passes through unit length of the medium is called extinction coefficient. Under clear sky conditions the extinction of light (by aerosols and air molecules) in the atmosphere is described by the Lambert-Beer law,
where Io is the incident intensity, I is the transmitted intensity, L is the length of the medium and E is the constant extinction coefficient. The unit of E is m-1.
ii) Aerosol Optical Depth (AOD)
Aerosol optical depth (τ) is a measure of the extinction of the solar beam by aerosol particles. It is a dimensionless number that is related to the amount of aerosol in the vertical column of the atmosphere over the observation location . Extinction coefficient integrated along a vertical column of unit cross section gives total optical depth.
The optical depth due to aerosols only (aerosol optical depth) is obtained by subtracting the contribution due to air molecules from the total optical depth .
iii) Angstrom exponent (α)
The Angstrom exponent is an aerosol optical property which describes the wavelength dependence of AOD. Since the aerosol extinction normally decreases exponentially with wavelength over the visible and near-infrared spectral region it is defined as 
where λ, and β denote the wavelength, AOD, and a wavelength-independent constant known as Angstrom's turbidity coefficient (which equals the AOD at the wavelength of 1 µm).Hence, the AOD can point to the size distribution of particulate matter through α as which shows the relative dominance of fine (submicron) particles over the coarse mode aerosols.
Black Carbon (BC)
Aerosols have a wide range light absorbing properties. The most important light absorbing aerosol is black carbon . BC is an important component of atmospheric aerosols and is produced from the incomplete combustion of hydrocarbon-containing materials, including fossil fuels, biofuels, and biomass. Since the industrial revolution the concentration of BC in the atmosphere has increased significantly due to large use of fossil fuels, organic burnings, vehicle exhaust etc. BC can both directly and indirectly affect global and regional climate. Since it absorbs solar radiation in a broad spectral brand from visible to infrared, it can affect the radiation balance of the earth-atmosphere system.
Aerosols remain in the atmosphere only for a few days (typically 1 week in the troposphere). Removal of aerosols BC from the atmosphere occurs mainly by dry deposition or sedimentation, impaction on surfaces, and wet removal. In sedimentation process particles fall under the influence of gravity. It is more effective in the case of coarse mode particles. Aitken mode particles like BC which gets mixed with heavier species such as dust will also be removed rapidly by sedimentation. Wet removal process is the most efficient sink of aerosols. The removal of aerosols by wet removal processes largely control the global aerosol distribution. In Indian atmospheric conditions, wet removal is mainly effective during the monsoon period (June–September) when more than 90% of annual rainfall occurs . Nucleation mode particles are removed from the atmosphere through coagulation process in which smaller particles combine to form larger particles. Thus the coagulation process generally leads to the increase in the accumulation mode particles.
In this report, a brief analysis is carried out on the seasonal variations in AOD and near-surface BC concentration in an urban environment in southern India- Bengaluru. The seasonality is investigated over a representative year (2018).
AREA OF STUDY AND INSTRUMENTATION
Area of study
Bengaluru is the capital of the south Indian state of Karnataka. It is the third most populous city in the country with a population of ~ 8.443 million [2011 census]. With an elevation of 920 m from sea level, it is located in the south eastern part of the state (Fig .2). The measurements have been carried out in the premises of Indian Institute of Science (IISc), Bengaluru, situated at 13.02o N and 77.57o E.
The aethalometer is used to measure the near real-time BC mass concentration (µg m-3) near the surface. The aethalometer collects aerosol particles continuously by drawing the aerosol-laden air stream through a spot on the filter tape. It measures the transmission of light through one portion of the filter tape containing the sample, versus the transmission through an unloaded portion of the filter tape acting as a reference area. This analysis is done at seven optical wavelengths spanning the range from the near-ultraviolet (370 nm) to the near-infrared (950 nm). It calculates the instantaneous concentration of optically-absorbing aerosols from the rate of change of the attenuation of light transmitted through the particle-laden filter . The BC mass concentration measured at 880 nm is usually for analysis as the absorption at this wavelength is only due to absorption by BC.
The aethalometer Model AE33 was used for measuring the BC mass concentrations in the study area (shown in Fig. 3). Two measurements are obtained simultaneously from two sample spots with different rates of accumulation of the sample. Both spots derive their samples from the same input air stream. The two results are combined mathematically to eliminate nonlinearities and provide the compensated particle light absorption and BC mass concentration. The aethalometer is calibrated to work with a time-resolution of 1 minute and a flow rate of 2 LPM continuously. The air inlet to the aethalometer sets a 2.5 µm cut off to the particle size using a cyclone impactor.
The determination of the BC concentration (Fig.4) is based on the measurement of light absorption on a filter loaded with aerosols :
I0 = reference signal
I = spot signal
S = spot area
F = measured flow
𝜁= leakage factor
𝜎𝑎𝑖𝑟= mass absorption cross-section
C = multiple scattering parameter
k = compensation parameter
t = time
(B) Multi Wavelength solar Radiometer (MWR)
The MWR (Fig.5) is a Sun photometer which is used to measure the AOD. According to the Beer-Lambert law, when a monochromatic light passes through a homogeneous medium, the light extinction in the medium is directly proportional to the concentration of particle in the medium and the path length through it. The MWR measures the AOD at 10 different wavelengths from 380-1025 nm (380, 400, 450, 500, 600, 650, 750, 850, 935, and 1025 nm) . Analysis of solar radiometer data to compute the optical depths involves spectral measurement of ground reaching solar flux, as a function of solar zenith angle. Then a linear least square fit to the Beer-Lambert law is used to connect the ground reaching solar flux to the extra-terrestrial flux. This method is called Langley technique . In Fig.5 the left panel (a) shows the optical unit and the right panel (b) shows the data acquisition/control unit of the MWR.
(C) Microtops II
The Microtops (Fig.6) is a hand held Sun photometer which measures the AOD at five wavelengths (380, 440, 500, 675 and 870 nm) .The Microtops can also be configured to measure the total columnar ozone and water vapor. Unlike the MWR, which is a self-calibrated instrument, the Microtops is calibrated using a standard reference  . The instrument is pointed at the Sun and three consecutive scans are made within a span of 1 minute and the least value of AOD is taken as this would represent measurements from the most accurate Sun pointing. The AOD is calculated based on the extraterrestrial radiation at that wavelength, corrected for the Sun-Earth distance, and the ground level measurement of the radiation.
Except for some missing days in January and November, aethalometer data was collected during all other days for the year 2018.
Compared to BC, the availability of AOD data is less frequent due to limitation set by clear sky conditions. But data is available during all months in 2018.
The data was averaged and the study focused mainly on the properties of aerosols during the four seasons- winter (DJF), pre-monsoon (MAM), monsoon (JJAS) and post monsoon (ON).
RESULTS AND DISCUSSION
The large spatio-temporal variations associated with aerosol properties are well discernable in the near surface BC mass concentrations. For this the diurnal variation of BC was examined. The contour map of the diurnal variation of BC (Fig.7) over the year 2018 has time along the x-axis and seasons along the y-axis. It can be seen that the BC concentration during an average day increases by a factor of ~ 3 times during morning (6 to 10 hrs) and evening (19 to 23 hrs) as compared to the afternoon (12 to 16 hrs) values. This is observed throughout the year but is not well discernible during the monsoon period when the BC values are well below the annual mean values.
These two peaks during morning and evening mark the diurnal variation due to atmospheric boundary layer (ABL) turbulence generated by the forcing from ground. During morning hours the ground heats up due to solar radiation which leads to breaking of stable layer formed during night time, bringing the aerosols trapped in the nocturnal residual layer to the surface giving rise to a sharp peak called fumigation peak . The second peak during evening is due to lowering of ABL on cooling and formation of residual layer. The increase in BC concentration during morning and evening can also be due to high traffic density. The low value of BC during afternoon could be due to increase in boundary layer height on ground forcing and low traffic density. It is to be noted that this variation only suggest the redistribution of BC over a large spatial extent by the boundary layer dynamics and not its loss from the atmosphere. During monsoon the BC concentration is at the lowest with shorter peaks in the diurnal variations.
Following the observation of large variations in the diurnal as well as daily BC values, the variation of daily mean BC mass concentration (MBC) over Bengaluru was examined (Fig. 8). It can be seen from Fig. 8 that there exists large temporal variations in the MBC with the values ranging from 1.77 µg m-3 to 14.09 µg m-3. The red line represents the annual mean MBC which is equal to 5.33 ±2.40 µg m-3 for the year 2018. The MBC values are below the annual mean values during the period June to September which is the monsoon period and some part of post monsoon period. The values are mostly above the annual mean value during other months which shows that MBC is relatively high especially during the cold winter months. In order to understand the association of the temporal variations in MBC ,the diurnal variations in MBC was examined.
The seasonal mean of the BC values (Fig. 9) reveal large seasonal variation with highest values during pre-monsoon and lowest values during monsoon. It is observed that the mean MBC increases from 6.47 ±0.40 µg m-3 during DJF to 6.86 ±0.30 µg m-3 in MAM followed by a sharp decrease in monsoon (JJAS) to 3.19 ±0.81 µg m-3 and a small increase in post monsoon (OS) with a mean concentration of 5.49 ±0.18 µg m-3. Large BC concentrations during cold season could be due to the confinement of aerosols within a shallow atmospheric boundary layer during this period. While the boundary layer confines the particles closer to the surface, the low/calm winds yield very low ventilation apt for buildup of aerosols close to the surface. During monsoon the BC concentration is really low, almost half that of winter due to washout of BC by precipitation. Effective reduction in the BC is observed as a result of the scavenging effect during rainy days compared to normal days. In Indian atmospheric conditions, wet removal is mainly effective during the monsoon period (June–September) when more than 90% of annual rainfall occurs .
Following the study of near surface aerosol properties the columnar properties were also examined. The AOD variations in Bengaluru at 500 nm during the year 2018 were examined. The cloud screened AOD data was used for this analysis. The variations of monthly mean AOD values are shown in Fig. 10. It can be seen that the AOD variations are not as systematic as seen in the BC mass concentrations. The annual mean values for the year is 0.36 ±0.12.
The seasonal variations are shown in Fig. 11. The observation suggests that the AOD is at maximum during pre-monsoon and minimum during monsoon. There is an increase in AOD form DJF (0.347 ±0.144) to MAM (0.402 ±0.134) followed by a decrease in its value to 0.329 ± 0.097 during JJAS, it then increases to 0.359 ± 0.090 during ON. The observed seasonal variation in AOD (with an annual average of 0.36±0.124) is a result of the complexity in the production, transport and removal of aerosols. During monsoon clear sky conditions are scarce and measurements are not too reliable. The wash out of aerosols from the atmosphere is significant due to high precipitation. The higher AOD values observed during summer (pre-monsoon) could be due to the increase in convective activities that enhance wind speed and surface temperature. The boundary layer height is higher and aerosols from both natural and anthropogenic sources (especially the long range transport of dust) are active within the region. The low value of AOD in winter could be caused by the low surface temperature which results in the weak production of aerosol from natural sources, like soil derived mineral dust from the surface. During winter anthropogenic sources dominates in aerosol production while during summer (pre-monsoon) both natural and anthropogenic sources contributes to aerosol concentration in the atmosphere.
Analysing the seasonal variations in BC concentration and AOD in urban city of Bengaluru during the year 2018 suggests that:
- Mean annual BC concentration is 5.33 ±2.40 µg m-3 with maximum values during January to March and minimum during the monsoon seasons (June to August).
- The diurnal variation in near surface BC concentration is seen dominantly during winter compared to other seasons.
- The annual mean AOD is 0.360±0.124 with maximum during pre-monsoon and minimum during monsoon.
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I am highly indebt to all those who gave their valuable suggestions and support which enable me to complete this project.
First and foremost I would like to thank the Indian Academy of Sciences and Dr. C. S. Ravi Kumar, Coordinator, SRPF 2019 for providing me this opportunity as a summer student in Indian Institute of Science,Bengaluru.
I would like to express my sincere gratitude to my guide Prof. S.K. Satheesh, Chairman, Divecha Centre for Climate Change, Professor at Centre for Atmospheric and Oceanic Sciences (CAOS), Indian Institute of Science, Bengaluru for his necessary guidance for the successful completion of my project. I am greatly in debt to my co-guide Mr. Manoj (Project Scientist) for his constant support and guidance.
I would also like to acknowledge my lab members Mr. Anand (Phd scholar), Mr. Adithya, Mr. Eshwin, Mr. Arun, Mr. Abhishek and Mr. Thashwin for their constant support.
I would like to express my sincere thanks to my professors at IISER Pune especially Prof. Neena Joseph Mani and Prof. Suhas Ettamal, Department of Earth Science for their valuable suggestions, support and guidance.
Last but not the least I would like to thank my parents and friends who have always been very supportive, without the help of whom this project would not have been possible.