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

Abstract

Daily measurements of aerosol optical depth ( ) and Ångstrom wavelength exponent ( ) acquired from Aerosol Robotic Network (AERONET) and Moderate Resolution Imaging Spectrometer (MODIS) are analysed over Lahore -an urban city of Pakistan (period: 2010Pakistan (period: -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 (500 nm) and (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 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. ≥ 0.3 and ≤ 0.75 indicating presence of DD while ≥ 0.2 and ≥ 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. KEY WORDS aerosol optical depth; Ångström wavelength exponent; long-range transport; AERONET; HYSPLIT; MODIS deep blue; desert dust/urban-industrial aerosols