Prashant Hegde

Influences of the springtime northern Indian biomass burning over the central Himalayas

The influences of the springtime northern Indian biomass burning are shown for the first time over the central Himalayas by using three years (2007-2009) of surface and space based observations along with a radiative transfer model. Near-surface ozone, black carbon (BC), spectral aerosol optical depths (AODs) and the meteorological parameters are measured at a high altitude site Nainital (29.37 degrees N, 79.45 degrees E, 1958 m amsl) located in the central Himalayas. The satellite observations include the MODIS derived fire counts and AOD (0.55 mu m), and OMI derived tropospheric column NO(2), ultraviolet aerosol index and single scattering albedo. MODIS fire counts and BC observations are used to identify the fire-impacted periods (372 h during 2007-2009) and hence the induced enhancements in surface BC, AOD (0.5 mu m) and ozone are estimated to be 1802 ng m(-3) (similar to 145%), 0.3 (similar to 150%) and 19 ppbv (similar to 34%) respectively. Large enhancements (53-100%) are also seen in the satellite derived parameters over a 2 degrees x 2 degrees region around Nainital. The present analysis highlights the northern Indian biomass burning induced cooling at the surface (-27 W m(-2)) and top of the atmosphere (-8 W m(-2)) in the lesser polluted high altitude regions of the central Himalayas. This cooling leads to an additional atmospheric warming of 19 W m(-2) and increases the lower atmospheric heating rate by 0.8 K day(-1). These biomass burning induced changes over the central Himalayan atmosphere during spring may also lead to enhanced short-wave absorption above clouds and might have an impact on the monsoonal rainfall.

Black carbon aerosols over Manora Peak in the Indian Himalayan foothills: implications for climate forcing

This letter presents the contribution of black carbon (BC) to the total aerosol optical depth (AOD) and subsequently to the direct radiative forcing (DRF) at Manora Peak in the Indian Himalayan foothills. Measurements of the chemical composition of aerosols, carried out from July 2006 to May 2007, together with concurrently measured BC mass concentrations were used in an aerosol optical model to deduce the radiatively important aerosol optical parameters for composite aerosols. On the other hand, BC mass concentrations alone were used in the optical model to deduce the optical parameters solely for BC aerosols. The derived aerosol optical parameters were used independently in a radiative transfer model to estimate the DRF separately for composite and BC aerosols. The average BC mass concentration was found to be 0.98 ( 0.68) g m 3 during the entire observation period, which contributes<3% to the total aerosol mass and 17% to the total AOD at Manora Peak. The mean surface forcing was found to be 14:0 ( 9.7) and 7:4 ( 2.1) W m 2 , respectively for composite and BC aerosols whereas mean atmospheric forcing was about +14 ( 10) and +10 ( 3) W m 2 for these aerosols. These results suggest that BC aerosols exert relatively large surface heating ( 45% higher) as compared to composite aerosols and contribute 70% to the total atmospheric forcing at Manora Peak. Such a large warming effect of BC may affect the strength of Himalayan glaciers, monsoon circulation and precipitation over the Indian region.

The influence of a south Asian dust storm on aerosol radiative forcing at a high-altitude station in central Himalayas

The impact of long-range transported dust aerosols, originating from the Thar Desert region, to a high-altitude station in the central Himalayas was studied with the help of micro-pulse lidar (MPL) observations. A drastic change in lidar back-scatter profile was observed on a dust day as compared with that on a pre-dust day. The back-scatter coefficient on a dust day revealed that the dust layer peaked at an altitude ∼1300 m above ground level (AGL) and extended up to ∼3000 m AGL, with maximum value ∼3 × 10–5 m–1 sr–1. Aerosol Index (AI) and air mass back-trajectory analysis substantiate the transport of dust aerosols from the far-off Thar Desert region to the experimental site. A significant effect of dust aerosols was also observed over the station on the spectral aerosol optical depths (AODs), measured using a Microtops-II Sunphotometer. It showed significantly different spectral behaviour of AOD on a dust day as compared with that on a pre-dust day. The Ångström exponent (α) showed a marked decrease from 0.42 to 0.04 from the pre-dust day to the dust day. The aerosol radiative forcing estimated using the Santa Barbara DISORT (discrete ordinate radiative transfer) atmospheric radiative transfer (SBDART) model, in conjunction with the optical properties of aerosol and cloud (OPAC) model, showed values of about –30, –45 and +15 W m–2, respectively, at top-of-atmosphere (TOA), surface and in the atmosphere on the dust day. The positive atmosphere forcing caused an estimated heating of the lower atmosphere by ∼0.4 K day–1.