Meteorological Influences on Seasonal Variations of Air Pollutants (SO2, NO2, O3, CO, PM2.5 and PM10) in the Dhaka Megacity

The study was conducted to evaluate the meteorological influences on seasonal variations of air pollutants load in Dhaka city. In this study we collect air quality data from Darus-salam Continuous Air Monitoring Station (CAMS) and were analyzed to find out their seasonal trends and relation with meteorological parameter. The highest concentration of the major air pollutants showed high values in the dry season (October-April) (SO2=23.45 ppb, NO2=55.33 ppb, O3=11.17 ppb, CO=3.66ppb, PM2.5=125.66 μg/m, PM10=219 μg/m) than those of the wet season (May-September) (SO2=10.26 ppb, NO2=16.36 ppb, O3=2.40 ppb, CO=1.23 ppb, PM2.5=39.65 μg/m, PM10=76.5 μg/m). These results indicate that higher pollutants load in winter are associated with large scale polluted air transported from the brick kilns situated in the northern surrounds of the observing station which are also related with lower boundary atmospheric heights during winter. However, a reverse relation between rainfall and atmospheric pollution load throughout the wet season was observed. This finding revealed that the lowest concentration levels of pollutants during wet season are associated with their atmospheric wash out by precipitation. A strong correlation (R=0.742) was observed between CO and O3 during the study, which indicating huge production of oxidant with increasing CO concentration. Interestingly, O3 showed positive correlation with NO2 (R=0.391). This result may indicate that NO2 is the important precursors of O3 in this study. Similarly, CO and NO2 showed good correlation (R=0.68), indicating that both of NO2 and CO are produced from similar pathways of photochemical oxidation of VOC. However, PM2.5 and PM10 concentrations showed decreasing trends with the onset of monsoon, indicating washout of atmospheric dust load through rainfall during wet season.

due to severe air pollution which is triggered by population growth, industrialization and urbanization (Ahmed and Hossain, 2008;Uddin et al., 2014). Biomass fuels burning during cooking with poor ventilation in rural areas causes indoor air pollution. However, emissions from industries and automobiles are the prime sources of outdoor air pollution (Alam, 2009), which deteriorates ecological conditions (Tripathi and Gautam, 2007).
Air pollution is most dangerous among all type of pollutions because man and even plants need fresh air for their normal metabolic pathways (Azim et al., 2013). According to the World Health Organization (WHO), air pollution is a contamination of the indoor or outdoor environment by any chemical, physical or biological agent enters into the atmosphere and finally modifies the natural characteristics of the atmosphere called air pollution and it sources includes home furnace, vehicular emission, industrial chimney and forest fire (WHO, 2013).
At present, rapid increase in the demand of brick manufacturing and the bunching of brick furnaces are environmental distresses throughout the world. Combustion of coal besides other biomass fuels in brick kilns results in the emissions of particulate matter (PM), sulphur dioxide (SO2), oxide of nitrogen (NOx) and carbon monoxide (CO) (Maithe et al., 2002). Air pollution due to enhanced anthropogenic activities has become an important environmental concern globally, especially in urban areas, in view of its adverse health effects (Dockery et al., 1989;Dockery et al., 1993;and Dockery et al., 1994). Presently, awareness has been created to a great extent among the public and the Government as to the impact of chemical pollutants on the quality of human life and in general the ecosystems. Urban air pollution is acknowledged to be exceedingly deteriorating to community health in Dhaka, and other major cities in Bangladesh. It is estimated that if the exposure to urban air pollution were reduced by 20% to 80% it would result in saving 1200 to 3500 lives annually (WB, 2006).
In addition, it produces negative economic externalities for investment in the country. The level of pollution in these cities if remain upswing due to unplanned urbanization, industrialization and motorization there will be more loads of harmful pollutants in the urban areas and consequently incidences of air pollution related diseases like asthma, bronchial disease, pulmonary diseases and lung cancer will increase manifold which in a way will have profound public health implications in the foreseeable future (CASE, 2016).
Moreover, great dissimilarities can be observed in seasonal variants of concentrations of the main atmospheric pollutants in various urban region of the world. The complex pattern of the air pollutant concentration variations in different seasons is inhomogeneous (Mikhailyuta et al., 2007). Meteorologically, Bangladesh is a subtropical country, which is experienced an extensive periodic dissimilarity in rainfall, soberly warm temperatures, and huge relative humidity having few local climatic metamorphoses (Hossain et al., 2019).
A great periodic distinction is observed among Dhaka air quality (Islam et al., 2015). Dhaka is one of the mega cities in the world, which has perceived a quick progression of municipal inhabitants recently. At present, number of automobiles has amplified expressively in Dhaka city (Ahmad et al., 2018). Similarly, diverse combination of old and date expired vehicles has been amplified in the city area together with narrowing of road space, which finally contributing traffic congestion (Rubel et al., 2019). Consequently, many busy areas of the city have transformed into area of atmospheric pollution from traffic exhausts. Furthermore, atmospheric level of particulate matter (PM) in Dhaka is increasing day by day having contributions from brick field operation, traffic emission, industrial and residential discharges. During winter season brick kilns goes in operation. The pollution due to vehicles and brick kilns is then expected to be high during winter (Hoque et al., 2015).
To obtain reliable information for the urban air quality management, one needs comprehensive information about the seasonal and the diurnal variations of pollutants concentration in Dhaka city. Air pollutants UniversePG I www.universepg.com can travel thousands of miles (Yadav et al., 2013), these air pollutants may destroy the atmospheric stability which in turn can create an environmental menace (Rahman et al., 2010).
Thus, it is needed to make an experimental study based on the impacts of brick field clusters during dry season and motor vehicles which have serious impact on the seasonal variation in the atmospheric concentration level of SO2, NO2, CO, O3, PM2.5 and PM10 in the Dhaka city. Present study was carried out to meet the following objectives: i) To find out the level of SO2, NO2, CO, O3, PM2. 5 and PM10 in ambient air of Dhaka city. ii) To find out the seasonal variation in the atmospheric concentration level of SO2, NO2, CO, O3, PM2.5 and PM10 in the study area. iii) To assess the relationship between PM with meteorological parameters (rainfall and temperature).

MATERIALS & METHODS:
2.1 Study area -The study was conducted at Darussalam (October 2016 to September 2017) in Mirpur of Dhaka city, which is situated at the latitude 23.78ºN and longitude 90.36ºE. Darussalam is a hot spot site for air quality study since several major roadway intersection and large numbers of vehicles plying through this area. This continuous monitoring station is situated about 100 meters away from the main road. The roof height was about 7 m above from the ground and the sampler was located 1.8 m beyond from the roof (CASE, 2016).

Data collection procedure -Measurements of trace gases were done in CAMS in
Darus-salam of Dhaka during both dry (October, 16-February, 17) and wet (March, 17). This location is also characterized as traffic because huge traffics entered in Dhaka city through this way from the northern part of the country and is also influenced by the emission from brick kilns located at northern side of Dhaka during winter season.

Methods of analysis -
There are four gas analyzers at Darussalam CAMS including sulfur dioxide (SO2), carbon monoxide (CO), nitrogen oxides (NOx) and Ozone (O3) analyzer. They are continuously measuring the concentration SO2, CO, NOx and O3 present in the ambient air. These analyzers work in different method. A number of commercial instruments were used for continuous measurement of trace gases. O3 was observed with a UV photometric analyzer (Teledyne Monitor Labs, Inc., model 9810B). CO was measured using nondispersive infrared spectrometer (TML, model 9830B). NO, NO2 and NOx were measured using chemiluminescence analyzer (TML, model 9841B) and SO2 was measured using a pulsed UV fluorescence analyzer (TML, model 9850B). All instruments were housed in an air-conditioned room. Times to time calibration were performed. All calibration processes were traceable to National Institute of Standards and Technology (NIST) standard.

Seasonal variation of SO2 -Fig 1a
shows the seasonal variations of SO2 concentrations of Dhaka city. Concentration of SO2 of the study area shows increasing trends from the month of October to February (Fig 1). During the observations, lowest concentration of SO2 (1.2 ppb) was measured in the September, 2017 and highest concentration of SO2 (37.1 ppb) was measured in February, 2017 ( Table 1). After the February, 2017 peak concentration of atmospheric SO2 in the study area shows a decreasing trend up to May, 2017.
Similar seasonal distributions of SO2 concentration also observed by Kirillova (2003), where maximum concentration was showed in February at St. Petersburg. High peak of SO2 in dry season and base concentration in wet season may happen in Dhaka due to their emission sources are associated with brick UniversePG I www.universepg.com fields operation in the dry season (Sikder et al., 2010). Average concentration of SO2 showed high value (21.7 ppb) in dry season (October-February) and low value (13.4 ppb) in wet season (March-September) ( Table 1) This may happen, because of low temperature in dry season together with high emissions of sulphur from brick kilns, where coal used as the major fuel for burning. Moreover, SO2 cannot spread out through the atmosphere and exist in lower atmosphere during winter season due to difference in atmospheric pressure.  (Fig 1b). Azad and Kitada (1998) also reported a significant concentration of NO2 over Bangladesh during dry season (November-March). NOx found high peak in winter and low peak in summer season (Sikder et al., 2010). As shown in Table 1, average concentration of NO2 in winter (October-April) showed three times high values than those of the summer (May-September). High atmospheric concentration of NO2 in winter may be associated with excessive level of coal burning in the brickfields adjacent to Dhaka city during the winter months. Moreover, positive correlation (r 2 =0.68) was found between NO2 and CO. This finding indicates that both of NO2 and CO produces from similar pathways of photochemical oxidation of VOC.

Seasonal variation of O3 -Fig 1c
shows the winter and summer variations of O3 concentrations in Dhaka city. O3 concentration showed increasing trends in the beginning of winter and reached to the peak value (26.7 ppb) during February (Fig 1c), the coldest months of Bangladesh. Interestingly, the precursor of O3 such and CO also showed peak concentrations during the February (Fig 2c)

Seasonal variation of CO -Fig 2c shows the dry and wet season variations of CO concentrations in the
Dhaka city. CO concentration shows a sharp increment from October, 16-September, 17 (Fig 2c). However, CO shows sudden decrement from March, 17 to September, 17 and showed the lowest value (0.85 ppm) during August, 17. During the observation, average CO concentrations in the winter months were almost more than double than those of the summer months. Sikder et al. (2010) have reported that the seasonal cycle of CO had high peak in winter and base in summer season.  Seasonal variations with maximum concentrations of CO in the winter period and minimum concentrations in the summer period are also observed in Kuwait (Abdul-Wahab and Bouhamra, 2016) and in Indian and Japanese cities (Morikawa, 1998;and Sahu and Lal, 2006).

Seasonal variation of PM2.5 and PM10 -Fig 2b
shows the dry and wet months' variations of PM2.5 and PM10 concentrations in the Dhaka city. Concentrations of PM2.5 and PM10 shows increasing trends from October 2016 to February 2017 and after that it shows decreasing trends up to September, 2017 (Fig 2b). During the observations, highest peaks (PM2.5=183.87μg/m 3 , PM10=303 μg/m 3 ) were in January and February, respectively. However, background concentrations (PM2.5=29.6μg/m 3 and PM10=56.6μg/m 3 ) were detected during June, 2017 (Fig 2b). Islam et al. (2015) have reported that concentration of particulates (PM2.5 and PM10) had exceeded the ideal level during the dry season while remaining as bellow from the standards during the rainy season. Highest concentrations of particulates in Dhaka city were observed in January (Islam et al., 2015). Highest concentrations of particulates in winter may be associated with enhanced atmospheric emissions from fossil fuels combustion, biomass burning and unfavorable meteorological conditions for pollution dispersion.

Relationship between PM2.5 and PM10
with rainfall -Precipitation can effectively reduce atmospheric PM2.5 and PM10 load through wet deposition (Fig 3a, Fig 3b, and Fig 3c). Fig 3 shows the relationship between atmospheric particulates concentrations with rainfall. Interestingly, during dry season (October-March) when rainfall amount is low then particulates loads are high (Fig 3a, Fig 3b, and  Fig 3c). On the contrary, during wet season (April-September) rainfall showed higher values at the same time PM2.5 and PM10 load showed lower values. These findings may specify that atmospheric wet deposition of PM10 accelerated with rainfall (Giri et al., 2008). Similar observation also found by Islam et al. (2015), where they reported that trends in air quality over the past decade had large seasonal variations in PM2.5 and PM10 concentrations during winter due to wind direction which suggested that brick-kilns were major contributors to PM2.5 and PM10 concentrations in Dhaka air during dry season and their concentrations reduced through wet deposition, since the number of rainy days have increased in the onset of monsoon. 3.8 Relationship between PM2.5 and PM10 with ambient Temperature -Fig 4a, Fig 4b, and Fig 4c represent the relation between PM2.5 and PM10 with ambient temperature. As shown in Fig. 4a, b and c, both of the PM2.5 and PM10 shows high values when atmospheric temperature remains as low, indicating that during winter (average low temperature) a high pressure exist in the atmosphere as a result particulates (PM2.5 and PM10) cannot disperse over the long area and concentrated in a local vicinity, which finally contribute to high atmospheric particulate loads.
However, when ambient temperature is high then PM2.5 and PM10 pollution loads remains as lower level.
These results indicate that during high temperature (summer months) a low pressure exist in the atmosphere and an atmospheric instability is existed, which helps for pollutants dispersion over the large area (Nam et al., 2010).