Environmental Monitoring and Assessment (2005) 104: 269–280 DOI: 10.1007/s10661-005-1615-7
c Springer 2005
FACTORS INFLUENCING AEROSOL CHARACTERISTICS OVER URBAN ENVIRONMENT
K. MADHAVI LATHA and K. V. S. BADARINATH∗
Forestry and Ecology Division, National Remote Sensing Agency, Department of Space, Government of India, Balanagar, Hyderabad, India (∗ author for correspondence, e-mail: badrinath kvs@nrsa.gov.in)
(Received 21 October 2003; accepted 28 May 2004)
Abstract. Atmospheric aerosols are an important contributing factor to turbidity in urban areas besides having impact on health. Aerosol characteristics show a high degree of variability in space and time as anthropogenic share of total aerosol loading is quite substantial and is essential to monitor the aerosol features over long time scales. In the present study extensive observations of columnar aerosol optical depth (AOD), total columnar ozone (TCO) and precipitable water content (PWC) have been carried over a tropical urban city of Hyderabad, India. Significant variations of AOD have been observed during course of the day with low values of AOD during morning and evening hours and high values during afternoon hours. Spectral variation of AOD exhibits high AOD at smaller wavelengths and vice versa except a slight enhancement in AOD at 500 nm. Anomalies in AOD, particulate matter and black carbon concentrations have been observed during May, 2003. Back trajectory analysis of air mass during these episodes suggested variation in air mass trajectories. Analysis of the results suggests that air trajectories from land region north of study area cause high loading of atmospheric aerosols. The results are discussed in the paper. Keywords: aerosol optical depth, black carbon, columnar ozone, atmospheric turbidity, angstrom coefficients
1. Introduction The role of regional synoptic scale air mass types in influencing the aerosol properties and optical depth are quite recognized (Pillai and Moorthy, 2001). Besides these, in the recent years there is an increase in the awareness of the potential of air trajectories in advecting aerosols from distinct source regions and causing changes in optical depths/composition/physical characteristics at far off locations (Tyson et al., 1996). Atmospheric aerosols, water vapor and ozone plays an important role in the urban environment. Aerosols interact with both incoming solar shortwave (SW) and outgoing terrestrial longwave (LW) as they scatter and absorb the short and longwave radiation and exert more complex effect on climate (Charlson et al., 1999). Aerosols are generated from different sources and are distributed in the atmosphere through transport of air masses over large spatial scale. The size of particles controls the dynamics of aerosol particle population. Meteorological features affect size distribution of aerosols but the detailed mechanisms like radiation, precipitation, atmospheric transport and turbulence etc. and their importance need further
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investigations. Size distribution of ambient aerosol particles depends on origin and removal of particles together with meteorological parameters and other processes acting on the aerosols during its lifetime. Origin of submicrometer particles in the atmosphere is one of the major open questions in ambient aerosol research (Makela et al., 1998). However, when urban air is considered, origin of particles is more complicated. It is well known that the traffic and other anthropogenic combustion sources are most important sources of all air pollution compounds in urban air including fine particles (Derwent et al., 1995). Nucleation mode particles (particle diameter less than 20 nm) have been found to be originating from traffic (Vakeva et al., 1999). The present study has been taken up to addresses high aerosol optical depth (AOD) anomalies, high particulate matter (PM) and black carbon (BC) concentrations over tropical urban city of Hyderabad, India and its relation to synoptic air masses. 2. Experimental Setup The study area Hyderabad extends over 17◦ 10 to 17◦ 50 N and 78◦ 10 to 78◦ 50 E and is the fifth largest city in India (Figure 1). Measurements have been carried out in the premises of National Remote Sensing Agency (NRSA) at Balanagar (17◦ 28 N and 78◦ 26 E) located well within the urban center. The twin cities of Hyderabad and Secunderabad extend up to 16 km. In the present study meteorological parameters viz., air temperature, relative humidity and wind speed have been measured using meteorological station. AOD has been measured at wavelengths viz., 380, 440, 500, 675, 870 and 1020 nm using MICROTOPS-II Sunphotometer/Ozonometer having an accuracy of ±2%. The instrument is equipped with five optical collimators, actually aligned with a 2.5◦ full field of view and internal baffles eliminating internal reflections. The detector consists of a silicon photodiode mounted behind a set of continuous variable interference filters. The viewing angle of the instrument is 1.8◦ . The extinction coefficient δA (λ) was retrieved from the measuring data by accounting Rayleigh scattering δR (λ) and the contribution of gas absorbers. δA (λ) = δ(λ) − δR (λ) − δO3 (λ) − δH2 O (λ) Rayleigh scattering has been calculated by the formula δR (λ) = (P/P0 ) × 0.008735 × λ−4.08 (Leckner, 1978). In this formula P is the actual air pressure in hPa and P0 = 1013.25 hPa. Precipitable Water vapor (PWV) and Total Columnar Ozone (TCO) (the equivalent thickness of pure ozone layer at standard pressure and temperature) have been measured using MICROTOPS-II Sunphotometer/Ozonometer from measurements of three wavelengths in the UV region. The precipitable water column is determined based on measurements at 940 nm (water absorption peak) and 1020 nm (no absorption by water). Continuous and nearreal-time measurements of mass concentration of aerosol BC have been carried
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Figure 1. Location map showing study area.
out from January to May, 2003 using an Aethalometer; model AE-21 of Magee Scientific, U.S.A. The instrument aspirates ambient air from an altitude of ∼3 m above the ground using its inlet tube and its pump. BC mass concentration has been estimated by measuring the change in transmittance of a quartz filter tape, on to which the particles impinge. The instrument has been operated at a time base of 5 min, round the clock with a flow rate of 3 LPM. The instrument has been factory calibrated and errors in the measurements are ∼±2%. Total particulate
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matter has been measured using Quartz Crystal Microbalance (QCM) Impactor from California Instruments Inc, U.S.A. The instrument sucks in the ambient air and segregates the aerosols in accordance with the aerodynamic diameter into one of its 10 size bins in 10 stages ranging from 0.05 to 25 µm with an accuracy of ∼±20%. 3. Methodology AOD is a possible measure of atmospheric turbidity. The most frequently used turbidity coefficient is obtained by Angstrom (Angstrom, 1961). τpλ = βλ−α , where β is the Angstrom turbidity coefficient, α is the Angstrom exponent (wavelength exponent) and wavelength λ is in µm. Graph of ln τpλ vs. ln λ is plotted the slope of which gives coefficient α and its intercept ln β at λ µm. β is related to the amount of aerosols present in the atmosphere while α is related to the size distribution of aerosols. α yields information on the predominant size of suspended particles. Like many other climatic variables, β and α can vary throughout an individual day because of changes in air temperature that causes evaporation or condensation of moisture in the atmosphere (Moorthy et al., 2003a,b; Satheesh et al., 2002). 4. Results and Discussion Temporal variations of AOD at different wavelengths, TCO and Precipitable Water Content (PWC) on a typical day (4 May 2003) have been shown in Figures 2 and 3 respectively. Synchronous measurements of PWC also provides a means to cross check for cloud contamination in the measured AOD data sets. Significant variations
Figure 2. Temporal variation of AOD on a typical clear sky day on 4 May 2003.
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Figure 3. Temporal variation of air temperature, relative humidity and wind speed on 4 May 2003.
of AOD have been observed during course of the day with low AOD during morning and evening hours and high AOD values during afternoon hours. Convective turbulent processes cause mixing of existing particles and lifting of fresh lighter aerosol particles that are generated due to anthropogenic activities around the measuring site. Changes in AOD during mid-day hours has been attributed to the advection of pollution from surroundings and convective activity leading to changes in aerosol particle number distributions and gas-to-particle conversions (photochemical processes) while those during forenoon hours could be attributed to the impacts of radiative cooling and turbulent processes on aerosol characteristics during previous night (Heisler and Friedlander, 1977). Breakup of inversions and associated ventilation of aerosol particles and their further modification during afternoon hours may cause significant difference in AOD during forenoon and afternoon hours. Figure 2 suggests high AOD during afternoon hours (around 11:00 h) which coincides with monotonic increase of air temperature which leads to gas-to-particle conversion. Higher wind speed during mid day results in horizontal advection of pollution leading to high aerosol column content. These results are in the general agreement with those reported by earlier investigators (Shaw, 1979; Pinker et al., 1994; Devara et al., 1996; Dani et al., 2003). Significant changes in AOD can be expected when the relative humidity exceeds the value of 80% (Nilsson, 1979). In the present study, variations in AOD (Figures 2 and 4) seems to be influenced more by the variations in wind speed and air temperature. Spectral variation of AOD exhibits high AOD at smaller wavelengths (Figure 2) which is expected from the Mie theory except a slight enhancement in AOD at 500 nm. Spectral variation of AOD with high optical depths at short wavelengths and increases towards longer wavelengths resembles that of a continental environment (Moorthy et al., 2003a,b), rather than the flat spectra generally expected over marine environment (Hoppel et al., 1990; Moorthy and Satheesh, 2000) or when a strong marine air prevails (Moorthy et al., 2001). The steep increase in AOD towards shorter wavelengths is indicative of abundance
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Figure 4. Temporal variation of Precipitable Water Vapor and Total Column Content.
of fine (sub-micron) aerosols over the study region. TCO showed gradual decrease during morning hours and increase towards evening hours with a maximum during afternoon hours which can be explained on the basis of photochemistry (Devara et al., 2002). Comparison between Figures 2 and 3 suggests growth of aerosol particles associated with higher PWC values. Day-to-day variations in spectral dependence of AOD for the entire period during May 2003 has been shown in Figure 5. It is interesting to note from Figure 5 that the AOD variations during May 2003 showed three distinct groups. Group-1 corresponds to 1–8 May 2003 (Julian days of 121– 128) indicating low AOD values ranging from 0.46 to 0.50; Group-2 corresponds to 9–16 May 2003 (Julian days of 129–136) indicating relatively high AOD values ranging from 0.64 to 0.83; Group-3 corresponds to 17–26 May 2003 (Julian days of 137–146) indicating low AOD values ranging from 0.2 to 0.5. In order to explain these results, Back Trajectory Analysis has been performed using the NOAA Hybrid
Figure 5. Variation of day average AOD during May 2003.
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Single-Particle Lagrangian Integrated Trajectory (HYSPLIT 4) model (Draxler and Hess 1998). This analysis provides lat–long distributions of kinetic wind field including horizontal and vertical wind velocities. The (5 days back) trajectories have been computed at 500, 1000 and 1500 m AGL during May 2003. Figures 6a–c show trajectories on some selected days (3 May 2003; 16 May 2003 and 23 May 2003) of observations. These trajectories basically reveal transport of air mass and
Figures 6(a–c). Back trajectory analysis of air mass characteristics on 3 May 2003; 16 May 2003 and 23 May 2003.
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hence the source and sink characteristics of aerosols in the study area and adjoining regions (Estelles et al., 2004). In contrast, the observations influenced mostly by air mass from East and West of the study site on 3 and 23 May 2003 causes low AOD values. The day average variations of AOD with meteorological parameters
Figure 6(b).
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Figure 6(c).
such as wind speed, air temperature and relative humidity during May, 2003 are shown in Figures 7 and 8. Figure 9 shows variations of day average wavelength exponent (α) and angstrom turbidity (β) during May, 2003. Both these parameters have been found to be high during 9–16 May 2003 suggesting abundance of
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Figure 7. Variation of day average air temperature and relative humidity during May 2003.
sub-micron size aerosol particles. Total particulate matter (PM) and BC also showed high concentrations during 9–16 May 2003 (Figure 10). Higher values of β (more attenuation) and lower values of α (contribution from bigger aerosol particles) observed on the days of first and third groups suggest the influence of air mass from East to West side of the study site (Dani et al., 2003; Moorthy et al., 2003a,b). The results of the study suggest that there is an impact of advecting aerosols from distinct source regions that causes changes in the optical depths over urban regions. 5. Conclusions The present study addresses the variations of atmospheric turbidity and its influencing factors over urban region. Results of the study suggest that, air trajectories from land regions located north of the study site causes high AOD, PM and BC
Figure 8. Variation of day average wind speed during May 2003.
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Figure 9. Variation of turbidity coefficient and wavelength exponent during May 2003.
Figure 10. Variation of PM and BC during May 2003.
over the study area. The air trajectories from East and West of study site contribute to lower AOD with low values of α and high values of β compared to trajectories from north which are rich in fine aerosols. References
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