Elsevier

Atmospheric Environment

Volume 84, February 2014, Pages 262-274
Atmospheric Environment

Physical and optical properties of aerosols in a free tropospheric environment: Results from long-term observations over western trans-Himalayas

https://doi.org/10.1016/j.atmosenv.2013.11.029Get rights and content

Highlights

  • Long term aerosol observations from a high altitude location (4.5 km asl) in trans-Himalayas.

  • Black carbon and AOD showed significant seasonal variation with spring maxima.

  • BC contribute significantly to the size range of 200–400 nm of the submicron aerosols size spectra.

  • CWT analysis revealed the role of long-range transport in modulating the aerosol loading.

Abstract

Simultaneous and collocated estimates of near-surface mass concentration of black carbon (BC), number size distribution (NSD) and total number concentration (NT) of composite aerosols, columnar spectral aerosol optical depth (AOD) and local meteorological parameters were made from Aug 2009 to July 2012 at the high altitude (4520 m amsl), remote location Hanle in western trans-Himalayas. Even though all the aerosol parameters remain quite low, large annual variations are seen in the monthly mean values of BC (25 ng m−3–181 ng m−3), total number concentrations of composite aerosols (628 cm−3–1500 cm−3) and AOD (0.05–0.16). Size segregated correlation analysis reveals that the BC mass contributes significantly to the size range of 200–400 nm of the submicron aerosol size spectra. The diurnal variation of BC mostly has been dampened; yet seems to be prominent during spring, showing the presence of weak boundary layer dynamics. In contrast, the diurnal fluctuations in total number concentration have been mostly controlled by the new particle formation events, leading to bursts of large concentrations of ultrafine particles, which subsequently undergo coagulation. Spectral dependence of AOD also shows large monthly variations, with the Angstrom exponent varying from ∼0.52 to 1.3, with a mean value of ∼0.95 ± 0.21. The vertical distribution of extinction coefficients, obtained from CALIPSO data, indicates the presence of elevated aerosol layers, attributed mainly to the influence of long range transport of aerosols from the west Asian and Indian desert region.

Introduction

In recent years, global awareness of the inadequacy of aerosol data in reducing the current uncertainties in aerosol climate forcing estimation (which could be anywhere between 3 and 54%, Yu et al., 2006), calls for more observational data on aerosol parameters from data-sparse regions (Moorthy et al., 2011) through the employment of ground based networks, thematic campaigns, airborne and satellite based measurements and a synergistic modelling effort. In this perspective, information on high altitude aerosols assumes importance (Andrews et al., 2011), and in particular the Himalayas, the world tallest mountain ranges, have an unequivocal relevance. At the higher altitudes above the mixing regions and in the free troposphere, the aerosol characteristics have a more synoptic perspective, would be indicative of the background level and are useful to understand the probable global trends and large scale impacts (Moorthy et al., 2011, Sellegri et al., 2010). Several studies have indicated that increasing pollutant emissions from fast developing countries have led to the progressive increase of aerosol concentrations even over the high altitude Himalayas (e.g., Gautam et al., 2009). The increasing concern of scientific and social importance is about the possible impacts of absorbing aerosols (black carbon and dust) on glaciers and the projected consequences of the changes in the snow albedo forcing on the Asian monsoon circulation, regional climate and hydrological cycles (Marcq et al., 2010). It has also been demonstrated that dust (mixed with soot) aerosols would cause enhanced heating in the middle/upper troposphere, over northern India, the foothills of the Himalayas and Tibetan Plateau, leading to the strengthening of the meridional tropospheric temperature gradient and resulting in the advancement of the monsoon rainfall (Lau and Kim, 2006). Consistent with model simulations, microwave satellite observations of free troposphere since 1979 have also shown an upward trend in the tropospheric temperatures (Santer et al., 2000). Supporting both model simulations and satellite observations, the first observational evidence of elevated atmospheric warming by aerosol absorption and its northward gradients in height and amplitude was unravelled during the multi-platform field experiment ‘Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB, Moorthy et al., 2008). Subsequent investigations under a Regional Aerosol Warming EXperiment (RAWEX) have provided the first direct observational evidence of the large change in environmental lapse rate caused by the atmosphere heating due to absorption of radiation in the elevated layers of BC at ∼4.5 km and ∼8 km (Babu et al., 2011) over central Indian landmass.

In the light of the above, it is extremely important that the aerosol characteristics from high-altitude locations are examined over long periods of time. With this objective, an aerosol observatory has been set-up at Hanle (32.78°N, 78.96°E, 4520 m amsl) in the western trans-Himalayas atop Mt. Saraswati in the Hanle Valley (Moorthy et al., 2011). This is the only aerosol observatory in India at such an altitude that provides continuous measurements of various aerosol parameters.

Section snippets

Site description, prevailing meteorology and seasonal advection pathways

Essential details of the observational site and prevailing meteorology are available in recent papers (Moorthy et al., 2011, Babu et al., 2011). The site is located atop Mt. Saraswati, ∼300 m above the base camp and the scattered settlements in the Hanle Valley (Fig. 1) and is surrounded by several similar or taller peaks; several of them remain snowcapped in summer and snow covered in winter. The valley and the surrounding area is sparsely populated (∼10 per sq km). Rainfall is scanty and the

Measurements and database

The database used for this study was generated using 36 months of continuous measurements of mass concentration of ambient BC aerosol, number size distribution and total number concentration of composite aerosols and columnar spectral AOD since August 2009. Table 1 lists the parameters measured, instruments used, operating conditions and references to relevant publications. Detailed description of instruments, principle of operation and measurement errors are provided in the Supplementary

Near-surface aerosol characteristics

The temporal variations in BC mass concentrations (MB), at diurnal and monthly time scales (climatological monthly mean values considering the data over the 4 year period) are shown in Fig. 4; while the annual variations of monthly mean BC are shown in Fig. 5.

At the shorter, on diurnal time scales (Fig. 4), climatological monthly mean BC concentrations reveal an afternoon low, followed by a nocturnal and early morning high during March through May (spring). The diurnal variation of BC during

Discussions

The information on the long-term characteristics of various aerosol parameters over the high altitude Himalayas is of great scientific relevance in the context of regional and global climatic implications on several concerns. Primarily owing to their presence at high altitude, aerosol absorption can lead to higher warming due to thinnest nature of the atmosphere (Babu et al., 2011). Deposition of BC on snow and glaciers which are abundant on Himalayas lead to snow Albedo forcing besides

Conclusions

Long-term observations of physical and optical properties of aerosols were carried out over a free-tropospheric location Hanle in western trans-Himalayas, as part of the ISRO-RAWEX field experiment, using the simultaneous and collocated measurements of near-surface mass concentration of aerosols BC, number size distribution (NSD) and total number concentration (NT) of composite aerosols, columnar spectral aerosol optical depth (AOD), and local meteorological parameters during August 2009 to

Acknowledgement

This work is carried out under the ARFI Project of ISRO-GBP. We acknowledge NOAA ARL for the provision of the HYSPLIT transport and dispersion model. The CALIPSO data were obtained from the Atmospheric Sciences Data Center at NASA Langley research Center.

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