Characterization, Properties and Climatic Implications of Aerosols in Northern Pakistan

Abstract

The microphysical, chemical and optical properties of atmospheric aerosols were studied during the different study periods from 2004-2016 over Peshawar, Glaciated areas (Astore, Gilgit, Sost and Skardu) and Dir (Upper) Pakistan. Moreover, the air quality and climatic implications of aerosols were also studied over these locations. Particulate Matter (PM) plays a vital role in altering air quality, human health and climate change. There are sparse data relevant to PM characteristics in urban environments of the Middle East, including Peshawar city in Pakistan. This work reports on the morphology and composition of PM in two size fractions (PM2.5 and PM10) during November 2016 in Peshawar. The 24 hours mass concentration of PM2.5 varied from 72 μg/m3 to 500 μg/m3 (average 286 μg/m3) and that of PM10 from 300 μg/m3 to 1440 μg/m3 (average 638 μg/m3). The morphology, size and elemental composition of PM were measured using Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray (EDX) Spectroscopy. The major identified sources of PM are vehicular emissions, biomass burning, soil and re-suspended road dust, biological emissions and construction activities in and around the vicinity of the sampling site. Glacier melting due to light-absorbing aerosol has become a growing apprehension in recent decades. The emphasis of this study is to examine absorbing aerosol loading over the high mountain glacier region of northern Pakistan covering a long term of twelve years (2004 – 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 Diamer). 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. The value of AOD decreases in winter and that of AI decreases in iv autumn. Similarly, the value of AAOD decreases in winter and that of SSA in autumn. The results revealed that in spring and summer the prominent absorbing aerosols were dust whereas in autumn and winter mixed anthropogenic aerosols were prominent. Maximum values of radiative forcing within the Atmosphere (ATM) were observed during the summer, followed by spring, autumn and winter. Trend analysis shows 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. With the time, the melting of ice gets accelerated which is suggestive of absorbing aerosol types in the region. CALIPSO data indicate that the regional aerosol was mostly comprised of sub-types categorized as dust, polluted dust, smoke and clean continental. The analysis of the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model revealed that the air parcels were arriving to glacier region from different source sites. Over the high-altitude Himalayan regions in South Asia, absorbing aerosols, particularly black carbon (BC) and dust, have strong effects on the hydrological cycle and climate. This study reports on BC aerosol measurements during May, June, November and December (2016) over Astore, Gilgit, Sost and Skardu (northern Pakistan). Diurnal and monthly variations of BC aerosol were examined in relation to meteorology. BC aerosols exhibited diurnal variations with sharp morning and evening peaks associated with local anthropogenic aerosols in addition to long-range transport. During May, the BC concentrations were in the range of 2.3 ‒ 2.9 μg/m3, 2.1 ‒ 4.7 μg/m3 and 0.9 ‒ 1.6 μg/m3 in Astore, Gilgit and Sost, respectively. During June at Skardu, BC was found in the range from 2.6 – 4.1 μg/m3. During November at Astore and Gilgit, BC varied from 2.5 – 3.7 μg/m3 and from 3.5 – 4.7 μg/m3, respectively. During December at Gilgit, Sost and Skardu, BC concentrations varied from 2.8 – 6.1 μg/m3, 1.5 – 2.7 μg/m3 and 3.2 – 4.7 μg/m3, respectively. BC aerosols exhibited positive correlations with temperature and wind speed, in addition to a negative correlation v with relative humidity. The long-range transport of aerosols to the receptor sites were found to be from central Asia, Eastern Europe, Middle East and India as well. At Astore, Gilgit, Sost and Skardu, the Aerosol Radiative Forcing due to Black Carbon were calculated at TOA, SRF and within ATM during the month of May, June, November and December (2016). BC number density corresponding to BC mass concentration was used in OPAC model for the estimation of aerosols optical properties. The model derived optical properties along with atmospheric parameters were used in SBDART model for the calculation of BC radiative forcing at the TOA, SRF and within ATM. For the entire study period, the surface reflectance varied from 0.09 (17 Nov) at Astore to 0.42 (23 Dec) at Skardu with an average value of 0.21 ± 0.08 over four locations. Daily columnar ozone varied from minimum value of 23 ppb at Sost on 13 December to maximum value of 55.27 ppb at Skardu on 3 June with an average of 36.86 ± 9.6 ppb. Similarly, columnar water ranged from 0.06 cm on 23 May to 0.78 cm on 11 May at Gilgit with an average value of 0.33 ± 0.14. The lowest and highest BC concentration of 0.75 μg/m3 and 6.06 μg/m3 with corresponding modeled derived AOD of 0.19 and 0.25 was found on 2 June and 1 December at Skardu and Gilgit, respectively. Likewise, SSA and AP range from 0.78 (Gilgit) to 0.95 (Skardu) and from 0.68 (Gilgit) to 0.71 (Skardu), respectively. BC radiative forcing at the TOA ranged from minimum value of -0.07 W/m2 at Sost on 16 December to maximum value of 13.92 W/m2 at Skardu on 3 June, while at SRF it ranged from -7.31 W/m2 on 2 June at Skardu to -54.41 W/m2 on 1 December at Gilgit. Similarly, BC radiative forcing within ATM varied from 10.14 W/m2 (2 June) to 48.92 W/m2 (1 December) with corresponding heating rate of 0.28 K/day to 1.37 K/day on the above dates at Skardu and Gilgit, respectively. The study of the optical properties of aerosols over Dir (Upper), a high latitude place of North Pakistan) were carried out from 2004 to 2016 to understand the aerosol loading and its variability. The maximum average AOD value of 0.30 ± 0.05 was found in July while vi maximum AE value of 1.17 ± 0.09 was noted in December. Similarly, the minimum AOD (0.09 ± 0.04) was found in December whereas minimum value of AE (0.51 ± 0.07) in May. The maximum average seasonal AOD (0.28 ± 0.05) and AE (1.030 ± 0.07) was investigated in summer and winter season, respectively. Likewise, the minimum AOD (0.12 ± 0.05) was observed in winter and minimum AE (0.56 ± 0.04) in spring season. Seven-days back trajectories were computed, which shows that the air mass (500 m) reaching the study region during four seasons. Different transported particles were suggested during different seasons. To understand the effects of PM on human health and climate is of great importance. The morphological and chemical characterization of PM has attained significant importance in the recent years, because such data are needed to accurately constrain aerosol radiative properties and health impacts. Particulate Matter like, PM10, PM2.5 and ultra-fine aerosols have different characteristics, sources and potential health effects. Particularly the fine PM fraction can more easily penetrate into the lungs and cause respiratory diseases. BC aerosol can damage the cells of the human body and lead to cancer. The forcing implications over the glacier region are very crucial for climate modelling tasks and adaptation to the potential effects of the change in melting rates of ice. As BC aerosols have an important role on the melting of Himalayan glaciers and on radiative forcing, therefore its study over high-altitude sites at Himalaya has a special importance. The information about BC over these high altitude locations is very helpful for the understanding of local and regional weather and climate change. The current research work will motivate the researchers to make new and efficient instruments for the constant monitoring of atmospheric aerosols. This work will also motivate the researchers to prepare high quality filters which collect PM excellently and identify each element in PM.