Variations in the aerosol optical properties and types over the tropical urban site of Hyderabad, India

Abstract

[1] Aerosol measurements over the tropical urban site of Hyderabad, India, provide a way of determining the variability of the aerosol characteristics over a duration of 1 year (October 2007 to September 2008. The meteorological pattern over India, mainly driven by the regional monsoons, has a great effect on the amount and size distribution of the aerosols. Higher aerosol optical depth (AOD) values are observed in premonsoon, while the variability of the Å ngström exponent (a) seems to be more pronounced, with higher values in winter and premonsoon and lower values in the monsoon periods. The AOD at 500 nm (AOD 500 ) is very large over Hyderabad, varying from 0.46 ± 0.17 in postmonsoon to 0.65 ± 0.22 in premonsoon periods. A discrimination of the different aerosol types over Hyderabad is also attempted using values of AOD 500 and a 380 -870 . Such discrimination is rather difficult to interpret since a single aerosol type can partly be identified only under specific conditions (e.g., anthropogenic emissions, biomass burning or dust outbreaks), while the presence of mixed aerosols, without dominance of the coarse or accumulation mode, is the usual situation. According to the analysis the three individual components of differing origin, composition and optical characteristics are (1) an urban/industrial aerosol type composed of aerosols produced locally and all year round by combustion activities in the city or long-range transported (mainly in spring) biomass burning, (2) an aerosol type of mineral origin raised by the wind in the deserts (mainly in premonsoon) or constituting coarse-mode aerosols under high relative humidity conditions mainly in the monsoon period, and (3) an aerosol type with a marine influence under background conditions occurring in monsoon and postmonsoon periods. Nevertheless, the mixed or undetermined aerosol type dominates with percentages varying from 44.3% (premonsoon) to 72.9% (postmonsoon). Spectral AOD and a data are analyzed to obtain information about the adequacy of the simple use of the Å ngström exponent for characterizing the aerosols. This is achieved by taking advantage of the spectral variation of lnAOD versus lnl, the so-called curvature. The results show that the spectral curvature can be effectively used as a tool for aerosol types discrimination, since the fine-mode aerosols exhibit negative curvature, while the coarse-mode particles are positive.

Contrasting aerosol characteristics and radiative forcing over Hyderabad, India due to seasonal mesoscale and synoptic‐scale processes

“…The results reveal a rather significant monthly variation with high values during the period March-June and decreasing towards the post-monsoon (October-November) and winter (December-February) seasons. The annual mean AOD 500 over the two-year period is found to be 0.52 ± 0.23, slightly lower than that reported by Kaskaoutis et al (2009) for the urban region of Hyderabad (0.57 ± 0.22) but significantly higher than those at other Indian cities not so industrialized (Suresh and Desa, 2005;Moorthy et al, 2007;Pantithurai et al, 2007;Gogoi et al, 2009). An interesting feature is the different monthly variation in the spectral AODs suggesting difference in the aerosol load and particle size.…”

“…The WS is generally low (<5 m s −1 ) in all seasons, enhanced in the monsoon and late pre-monsoon to values up to 7 m s −1 . During the monsoon the prevailing wind is southwesterly and the aerosols over Hyderabad are influenced by marine air masses from the Arabian Sea (AS) and Indian Ocean (Kaskaoutis et al, 2009). In winter and post-monsoon the winds flow from northeasterly directions mainly, are generally dry, and are rich in continental aerosols with a large anthropogenic component (Raghavendra Kumar et al, 2009).…”

“…Viewed in the light of the above, the environment over Hyderabad constitutes an excellent atmospheric laboratory for examining the optical and microphysical aerosol characteristics, since it is affected by locally produced anthropogenic aerosols and naturally produced particles that travelled large distances before reaching the site. In addition, the seasonally changing air masses and the meteorological parameters strongly affect the aerosol load and properties over the urban site (Kaskaoutis et al, 2009). Therefore, studying aerosol characteristics over Hyderabad on a seasonal basis provides a view of aerosols over central-south India that can be inferred in the framework of the Aerosol Radiative Forcing over India (ARFI) project.…”

“…Lau et al, 2006), hydrological cycle (Ramanathan et al, 2001a) as well as biochemical cycling (Xin et al, 2005). Because of the variety of sources, their short residence time and the dynamic processes that may alter their properties, aerosols can be divided into several types having different physical, chemical and optical properties (Kaskaoutis et al, 2009). To better understand the effect of aerosols on the Earth-atmosphere system, it is essential to study the optical and physical properties, which are controlled by aerosol size distribution and refractive index.…”

“…Kaskaoutis et al, 2009). The anthropogenic aerosols originate either from gas-phase reaction of low volatile vapours in the atmosphere or by direct emissions, and are physically and chemically different from those in remote areas and over marine environments (e.g.…”