Temporal characteristics of aerosol optical properties over the glacier region of northern Pakistan

Highlights

• The results revealed the presence of absorbing aerosol over the glacier region.

• The analysis showed an increasing trend of AOD, AI & AAOD over studied region.

• CALIPSO showed an evidence of dust, smoke, polluted dust & clean continental aerosol.

• OMI retrieved aerosol types were in good agreement with CALIPSO retrievals.

Abstract

Glacier melting due to light-absorbing aerosol has become a growing issue in recent decades. The emphasis of this study is to examine aerosol loadings over the high mountain glacier region of northern Pakistan between 2004 and 2016, with sources including local emissions and long-range transported pollution. Optical properties of aerosols were seasonally analyzed over the glacier region (35–36.5°N; 74.5–77.5°E) along with three selected sites (Gilgit, Skardu, and Diamar) based on the Ozone Monitoring Instrument (OMI). The aerosol sub-type profile was analyzed with Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used to understand the origin of air masses arriving in the study region. The highest values of aerosol optical depth (AOD) and single scattering albedo (SSA) occurred during spring, whereas aerosol index (AI) and absorption AOD (AAOD) exhibited maximum values in winter and summer, respectively. The minimum values of AOD, AI, AAOD, and SSA occurred in winter, autumn, winter, and autumn, respectively. The results revealed that in spring and summer the prominent aerosols were dust, whereas, in autumn and winter, anthropogenic aerosols were prominent. Trend analysis showed that AI, AOD, and AAOD increased at the rate of 0.005, 0.006, and 0.0001 yr⁻¹, respectively, while SSA decreased at the rate of 0.0002 yr⁻¹. This is suggestive of the enhancement in aerosol types over the region with time that accelerates melting of ice. CALIPSO data indicate that the regional aerosol was mostly comprised of sub-types categorized as dust, polluted dust, smoke, and clean continental. The types of aerosols defined by OMI were in good agreement with CALIPSO retrievals. Analysis of the National Oceanic and Atmospheric Administration Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model revealed that air parcels arriving at the glacier region stemmed from different source sites.

Introduction

The earth's environment is continuously deteriorating due to increasing human population and corresponding activities leading to pollutant emissions. For example, industrial activities, land use, and combustion of fossil fuels emit greenhouse gases and aerosols, resulting in atmospheric composition changes (Mann et al., 1999; Crowley, 2000; Houghton et al., 2001). Aerosols can impact air quality, public health, and climate via both direct and indirect effects. The direct effect on climate occurs when particles scatter and absorb solar radiation, whereas the indirect effect is brought about by how more aerosol particles lead to more numerous but smaller droplets, yielding more reflective clouds at fixed cloud liquid water path (Twomey, 1977).

Scattering of solar radiation by aerosols and clouds causes negative forcing, which tends to cool the surface of the earth, whereas the absorption of terrestrial radiations by aerosols and clouds cause a positive forcing, and thus warming (Hinds, 1999). These effects are quantified in the form of Aerosol Radiative Forcing (ARF). Information needed to robustly calculate ARF includes the concentration of aerosols and its other characteristics, such as composition, size, and optical properties (Russell et al., 2010). Changes in aerosol optical properties arise owing to different emissions sources, chemical transformations, and long range transport of pollution (Ram et al., 2010).

The scattering and absorption of light by aerosols depends on composition and size. Aerosols like inorganic salts (e.g., ammonium sulfate and ammonium nitrate) and sea salt scatter solar radiation back to space resulting in cooling of the planet. However, black carbon (BC) and iron oxides absorb radiation, which leads to heating (Wang et al., 2011). Deposition of BC aerosols on snow/ice surfaces has accelerated snow melt and glacier retreat over the Himalaya and Tibetan Plateau (Zhang et al., 2015). A key source of BC is biomass burning in the region (Rasul et al., 2011). Mineral dust changes the albedo of glacier surfaces, which in turn affects the energy balance of the atmosphere as well as the melting rate of glaciers and seasonal snow (Fujita, 2007). There are different sources of dust deposited on glaciers, including transported desert dust and locally produced mineral dust (Shahgedanova et al., 2013). Analysis of the sensitivity of Vadret da Morteratsch Glaciers showed that a 10% decrease in reflectance from a glacier surface yielded an equivalent impact of 1.7 K warming (Oerlemans et al., 2009); therefore, changes in deposition of absorbing aerosol types can have a major impact on radiative forcing.

Pakistan is known to have the world's highest mountains. Three world famous mountain ranges, specifically the Himalaya, Karakorum, and Hindukush, join in the extreme northern part of Pakistan, which is occupied by glaciers. There are >5000 glaciers in the Pakistani geographical area feeding the Indus River. The frozen water resources are decreasing continuously due to global warming effects, reducing ice mass (Rasul et al., 20011). The increasing trend in temperature has been observed during the last decade in the northern part of Pakistan, which enhances the snow/ice melt (Rasul et al., 2011), affecting agriculture, drinking water supplies, and hydroelectric power, amongst other sociologically-relevant necessities.

In Pakistan, many studies have been carried out on aerosol optical properties over plain areas (Alam et al., 2011, 2012, 2014a,b; Bibi et al., 2015; Tariq and Ali, 2015; Bibi et al., 2017a,b; Iftikhar et al., 2018; Zeb et al., 2018). But no study to our knowledge has been conducted on aerosol optical properties over the glacier areas in northern Pakistan. The aim of this study is to fill in that gap by reporting a comprehensive characterization of aerosol optical properties on both local and regional scales. The scientific gap in our present knowledge about the forcing implications over the glacier region is crucial for climate modelling tasks, and adaptation to the potential effects of changing ice melt rates.