Seasonal assessment and classification of aerosols transported to Lahore using AERONET and MODIS deep blue retrievals

• Main Authors: Zafar, Qudsia, Zafar, Sidra, Holben, Brent
• Other Authors: The University of Newcastle. Faculty of Science & Information Technology, School of Mathematical and Physical Sciences
• Format: Article in Journal/Newspaper
• Language: English
• Published: John Wiley & Sons 2018
• Subjects: aerosol optical depth; Ångstrom wavelength exponent; long-range transport; AERONET; HYSPLIT; MODIS deep blue; desert dust/urban-industrial aerosols; Aerosol Robotic Network
• Online Access: http://hdl.handle.net/1959.13/1449095

Daily measurements of aerosol optical depth (τ) and Ångstrom wavelength exponent (α) acquired from Aerosol Robotic Network (AERONET) and Moderate Resolution Imaging Spectrometer (MODIS) are analysed over Lahore – an urban city of Pakistan (period: 2010–2014) to investigate contribution of different types of aerosols originating from both local and regional source locations. The obtained annual averages (mean ± standard deviation) for AERONET retrievals of τ (500 nm) and α (440–870 nm) are 0.68 ± 0.37 (0.08–2.91) and 0.99 ± 0.33 (0–1.8), respectively. Of all the sources, 61% are found within Pakistan, 11% in India, 19% in Afghanistan, 6% in Iran and 2% in Saudi Arabia with seasonal contributions of 35, 25, 23 and 17% in pre-monsoon, monsoon, winter and post-monsoon, respectively. The bimodal distributions of α show dominance of coarse-mode particles during pre-monsoon, fine-mode particles during post-monsoon and presence of both coarse-mode and fine-mode particles during winter and monsoon with winter showing more fine-mode particles. Two broad classes of aerosols namely desert dust (DD) and biomass burning/urban industrial (BU) are identified with criteria, e.g. τ ≥ 0.3 and α ≤ 0.75 indicating presence of DD while τ ≥ 0.2 and α ≥ 1.15 indicating BU. The frequency of occurrence (FOO) of DD and BU aerosols is further identified by applying classification criteria over Aqua-MODIS deep blue retrievals. The FOO identifies anthropogenic activity on-going throughout the year, disrupted with DD aerosols only during pre-monsoon and monsoon. The maximum dust activity is seen over Indo-Gangetic plains (IGP) (localized maxima: 35–45%) and the Arabian peninsula (>55%) during pre-monsoon, while maximum BU aerosols are found over IGP, central and south-eastern plains of India and the state of Gujarat (localized maxima: >70%) in winter and post-monsoon.