AEROSOL PARAMETERS DURING WINTER AND SUMMER SEASONS AND METEOROLOGICAL IMPLICATIONS

The MICROTOPS II aerosol optical depth (AOD) and Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua AOD and angstrom exponent (AE) were collected during December 2016 to May 2017. Higher AOD values were recorded during winter (December February) and summer (March May) Months. These values were observed by MICROTOPS II (0.35 1.279) and MODIS (0.222 1.904) during winter season. During summer AOD values were recorded by MICROTOPS II (0.272 – 1.744) and MODIS (0.227 – 1.33). Whereas MODIS AE (0.218 – 1.799) values were found high during winter season indicates about the dominance of fine particulates. During summer months MODIS AE (.001 – 1.648) values are indicating about the mixing of the aerosol particles. The relative humidity values during December and January months were found to be high, while its values were found decreasing during late February due to transition phase. Relative humidity values were recorded low during summer (36 – 86%) months in compare to the winter (50 100%) months.


INTRODUCTION
The continuously increasing air pollutant concentrations are not only an environmental issue within the cities, while it has also regional and global environmental effects [1,2]. The rapidly increasing population, vehicles, industrial developments and the resulting anthropogenic activities are the major concerns for the air, water and land pollution in most of the developing Asian countries [1][2][3][4][5][6]. So, it becomes very essential to the management of near surface air quality due to its possible implications for the human health, visibility, precipitation and agricultural crop yield [7][8][9]. Indo Gangetic Plain (IGP) in India, experiences a very high aerosol loading with a strong intra-seasonal to inter-annual variability [10]. IGP region traversed by the Ganga river and its tributaries is one of the largest, densely populated, industrialized and developing region of the world where aerosol not only affect the Indian monsoon but also the global climate system [11]. The high mortality estimates from satellite observations in comparison to the ground-based databases were found by [8]. All the anthropogenic activity, including various festivals, affects the environment especially in India.

AOD and AE for the atmospheric aerosols during winter season
During winter months AOD values were in the ranges (0.35 -1.279) collected by MICROTOPS II. The maximum limit is not very large because the AOD data by MICROTOPS II was available from late January 2017 to February 2017. The AOD values were found very high during December month, can be clearly seen from the MODIS collected AOD (0.222 -1.904) data. The temporal variation of MODIS aqua AOD and AE, MICROTOPS II AOD and relative humidity are shown in Figure 1. The decreasing trend of AOD During winter season the aerosols were originated from biomass/biofuel burning and due to vehicular pollution over the site [12].

AOD and AE for the atmospheric aerosols during summer season
The AOD values varies in the different ranges during summer months by MODIS (0.227 -1.33) and MICROTOP II (0.272 -1.744). Good agreement between MODIS and MICROTOPS II AOD was observed. The agreement can clearly be seen by Figure 3. For the most of the cases the AE<1 values during summer months is the indication of the dominance of the coarser particles. Long range transportation of the mineral dust during summer months are the major concern for the dominance of coarser particles. The coarse-mode aerosol concentrations were enhanced because of IGP is strongly affected by frequent and intense dust storms through longrange transport from Arabian Sea, southwest Asia, and the Thar Desert [13]. The Five days back trajectories using HYSPLIT model over Varanasi for 12 May 2017 with huge aerosol loading is shown in Figure 4.

Meteorological implication over the site
Meteorological parameters play a crucial role during observation of aerosol particles and its parameters. During winter months relative humidity was observed very high. While relative humidity values were found to be decreasing in the late February due to transition phase. It's ranging between (50 -100%) during winter months. The exponential increase in AOD in highly humid condition (relative humidity>85%) is mostly attributed to the hygroscopic growth of fine particulates over the region [14]. During summer months relative humidity ranges between (36 -86%) indicating about the dominance of the coarser particles. Lower values of relative humidity were observed during summer months in compare to the winter months.
from IMD centre BHU, Varanasi. Authors are also thankful to MODIS team and NASA (https://giovanni.gsfc.nasa.gov/giovanni/) for the satellite data acquisition. The back-trajectories are generated from website https://www.ready.noaa.gov/HYSPLIT.php.