Abstract
Dust storm, a natural hazard, has a direct impact on daily life for a short period. Dust storms are periodic events over India, especially in northern regions. This study has been carried out to investigate the dust impacts on the aerosol characteristics over Dehradun (DDN) during pre-monsoon (March–June), 2012 using ground measurements, satellite observations and model simulations. The measurements illustrate the distinct monthly impact on the aerosol properties with maximum dust loading during May (aerosol optical depth at 500 nm (AOD500) = 0.72 ± 0.18) over DDN, which is confirmed with the Terra-MODIS (AOD550 = 0.70 ± 0.19) measurements. The major dust loading was recorded in aerosol measurements during May at the station, which permitting to examine the influence of dust transports on the aerosol characteristics. Spectral variation of AOD and Angstrom exponent (α) values displayed day to day variation of aerosol during dust episodes. Analysis of aerosol types and seven-day back-trajectories reveal the transportation of desert dust during May over DDN. The Optical Properties of Aerosols and Clouds (OPAC) model was used to compute the aerosol optical properties (e.g., Single scattering albedo (SSA) and asymmetry parameter (g)) and size distribution. The high values of SSA and g are indicating the dust loading in the atmosphere during May. Aerosol volume concentration at the coarse mode (geometric mean radii (RV) = 2.89 ± 0.027 µm) is found to be increased in the May, whereas decrement has been observed in the finer mode (RV = 0.16 ± 0.006 µm). The aerosol direct radiative forcing (ARF) was computed using Santa Barbara Discrete Ordinate Atmospheric Radiative Transfer (SBDART) model in the shortwave (SW) region (0.25–4.00 µm). The mean top of the atmosphere (TOA) and surface forcing come out to be –14.49 W m–2 and –53.29 W m–2 respectively in May. The mean net atmospheric radiative forcing (38.79 W m–2 maximum during May) corresponds to heating rate of ~1.06° K d–1 in the atmosphere.
Highlights
The seasonal variability of the aerosol optical properties and the regional radiative forcing are mainly controlled by aerosol loading, in which dusts are playing an important role in the atmosphere (Tegen and Lacis, 1996)
We present the variation in aerosol characteristics during dust events and their impact on local radiative forcing over Dehradun, Uttarakhand, India
The mainly local diurnal variation controlled the AOD spectrum, but the dust events in May have a strong influence on the AOD spectrum
Summary
The seasonal variability of the aerosol optical properties and the regional radiative forcing are mainly controlled by aerosol loading, in which dusts are playing an important role in the atmosphere (Tegen and Lacis, 1996). The volume size distribution, single scattering albedo (SSA) and asymmetry parameter (g) are used to study the aerosol characteristics over Dehradun, which are simulated using OPAC model. The estimated outputs from the OPAC model along with observations are integrated into the SBDART model to estimate the aerosol radiative forcing (ARF) at the top of the atmosphere (TOA), surface and within the atmosphere during a dust period. Except of SSA, the ARF depends mainly on spectral AOD, volume size distribution, water vapor content, ozone amount, asymmetry parameter, surface albedo and vertical distribution of aerosols (Kedia and Ramachandran, 2011). The observations of spectral AOD and α along with OPAC derived SSA and g are integrated into the SBDART model (Ricchiazzi et al, 1998) to simulate the ARF at the surface and TOA at shortwave region. MODIS derived surface albedo of 0.21 ± 0.009 was integrated in the SBDART as an input for this region
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