PM10 Sampling and AOD Trends during 2016 Winter Fog Season in the Islamabad Region

PM10 samples were collected during intensive fog days in Islamabad, Pakistan, to investigate the impact of particulate matter on fog formation. The PM10 concentrations were monitored at the Institute of Space Technology site using a highvolume air sampler and its elemental composition was studied using Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS). Sampling was done for a duration of 24 hours on selected days, including all foggy days in a period from January 2016 to February 2016. The concentration of PM10 varied from 123 μg m to 202 μg m with an overall mean concentration of 177 μg m. On most occasions, PM10 levels were considerably high as compared to permissible limits of both Pak-NEQS and WHO guidelines. It has been observed that the air quality during fog days was much worse, with elevated levels of particulate matter observed during foggy days. The SEM-EDS revealed the presence of different elements including some metals Si, K, Ca, Mg, Zn, Fe, Cr, Pb, Al etc. The morphological studies suggest that most of the particles are crystalline in shape, suggesting their main source as soil. Some samples also showed round spherical shape which refers their anthropogenic source. The sun photometer observations of aerosol optical depth (AOD) and satellite observations from Aqua’s Moderate-resolution Imaging Spectro-radiometer (MODIS) showed significant correlation. Moreover, elevated level of AOD were found during heavy fog days. The validated high satellite AOD were associated with high PM10 concentration during heavy fog days.


INTRODUCTION
The worldwide concern about air pollution is due to its adverse impacts on human health and the environment.Due to anthropogenic activities, along with natural sources, pollution in different regions is rapidly increasing (Afroz et al., 2003;Kappos et al., 2004).Most developing countries still face severe levels of both gaseous and particulate pollution (Dunlap et al., 1993;Brechin and Kempton, 1994).The effects of bad air quality on public health has been reported in various studies ranging from minor physiological variabilities to major diseases such as asthma, lung cancer, premature births, deaths (Delfino et al., 1998;Naeher et al., 1999;Laden et al., 2000;Suresh et al., 2000;Wichmann and Peters, 2000;Calderón-Garcidueñas et al., 2003;Wilhelm and Ritz, 2003;Jonsson et al., 2004;Preutthipan et al., 2004;O'Neill et al., 2004).According to reported studies, 865,000 premature deaths are caused every year by air pollution and about 60% of these deaths found occurred in Asia (WHO, 2002(WHO, , 2007)).
Apart from health impacts, various pollutants absorb and scatter radiation, interfere with vital precipitation and cloud formation processes.Particulate matter is a most harmful air pollutant and has its role in fog formation and is also reported to have impacts on precipitation processes (Seinfeld and Pandis, 1998;Finlayson and Pitts, 2000).
The Asian region, particularly over the last decade has undergone rapid economic development along with urbanization, motorization and extensive energy use.Thus, air pollution and especially particulate matter pollution has emerged as a serious threat to the environment and population health in the region (Gurjar et al., 2008;Hopke et al., 2008).The hazardous impacts of air pollution become worse during the winters due to dense persistent fog especially in the months of December, January and February (DJF).It has been reported that the average concentration of both PM 10 and PM 2.5 increased about two times during intense fog days.Aerosols intensify fog formation in low temperature and high humidity conditions (Mircea et al., 2002;Mohan and Payra, 2009).Many parts of Pakistan experienced severe fog and haze during the months of DJF that lead to significant social and economic problems especially the disruptions of road and air traffic (Saeed et al., 2004).
Earlier studies in the region have reported ground based measurement of particulate matter and satellite observations (Shah et al., 2004a(Shah et al., , b, c, 2006(Shah et al., , 2007(Shah et al., , 2008;;Alam et al., 2014;Bibi et al., 2016Bibi et al., , 2017;;Shahid et al., 2015a, b, Bibi et al., 2015;Shahid et al., 2016).Some of these studies have reported the role of aerosols in fog formation.Mohan and Payra (2009) reported that various urban areas in India are facing increase in fog occurrences due to high aerosol concentration in the atmosphere.Also, the extent of foggy days has increased in China during the recent time because of rapid industrialization and urbanization.The increasing trend of fog frequencies in the last decade results in various kinds of impacts on human life, irrigation networks and economy of the country (Niu et al., 2010).Therefore, particulate matter levels must be monitored continuously to understand its likely impact on fog formation, environment and particularly human health.In this regard, this study is a first of its kind for the city of Islamabad during foggy weather.

Site Description
Islamabad, the Federal Capital of Pakistan is a modern, planned and maintained city located in the Pothohar Plateau in the northeastern part of the country.It is located at 33°26′N 73°02′E at the foot of the Margalla Hills with more than 2 million residents.It covers an area of 906 km 2 with an extension of 2,717 km 2 of Margalla Hills in the north and northeast.Islamabad falls in semi-arid zone with hot humid summers followed by monsoon and cold winter.The modern capital and the ancient Gakhar city of Rawalpindi stand side by side and are known as the Twin Cities.The climate of Islamabad has a typical version of humid subtropical climate, with five seasons: winter (November-February), spring (March and April), summer (May and June), monsoon (July and August) and autumn (September and October) Hameed (2007).
In this study, sampling of PM 10 was carried out at the Institute of Space and Technology (IST) which is located at Zone 5, Islamabad.The sampling site is selected based on its unique features as it is the starting point of Islamabad surrounded by two of the busiest roads of Islamabad and Rawalpindi, namely the GT road and Islamabad expressway.IST is also surrounded by different housing Societies and towns including newly developing areas.Rawat industrial estate and Humak industrial area are also present in the same zone.

PM 10 Sampling
PM 10 sampling was done by using high volume air sampler and filter papers of quartz.Quartz filters were used for PM 10 sampling because of their high collection efficiency as well as their ability to withstand high temperatures.We utilized the Thermo scientific High Volume Sampler, an Air sampling equipment capable of sampling high volumes of air (typically 1,600 m 3 ) at high flow rates (typically 1.13 m 3 min -1 ) over an extended sampling duration (typically 24 hours).The sampling time for each sample was set at 9 AM.The sampler was installed at a height of 10 m on the roof top of the Department of Space Science, IST.Sampling was performed randomly depending on the prediction of foggy days.More samples were collected on fog and hazy days to find the relationship between the fog and particulate concentration.A total of 9 samples of PM 10 were collected during the period of January 2016 to February 2016.Particles in the PM 10 size range were then collected on the filter during the specified 24-h sampling period.Each sample filter was weighed before and after sampling to determine the net weight (mass) gain of the collected PM 10 sample (40 CFR Part 50, Appendix M, US EPA).For better analysis and validation of sources, back trajectories and wind roses were also calculated.Using satellite-based meteorological data from MERRA 2, (available at: https://disc.sci.gsfc.nasa.gov/daac-bin/FTPSubset2.pl), wind and temperature maps of the region has also been developed as shown in Figs.3(a) and 3(b).The wind map shows relatively calm conditions during the 2015-16 winter period while temperature drops during the study period along the Himalaya region are also notable which are favorable conditions for the formation of the high impact South Asian winter fog including the study area of Islamabad.

Elemental Composition and Surface Morphology
The elemental composition and surface morphology of the collected PM 10 samples, the most important step and vital part of the study.Samples were analyzed for their chemical composition and surface morphology by using SEM-EDX (Scanning Electron Microscope-Energy Dispersive X-ray).First, each sample was cut down in small strips of about 0.0254 m.Then these samples strips were coated with carbon to make them nonconductors in the SEM.After carbon coating, strips were placed into the SEM analyzer to study their chemical composition and surface morphology.

PM 10 Variation with AOD from Sun Photometer and Satellite Observations
The AOD data from MODIS was used to compare and generate a good relationship between ground PM 10 data with satellite AOD data.The data of specific days (in accordance to ground sampling) from MODIS website was downloaded.A portable sun photometer was used in this study to validate satellite based measurements and find relationship with data from ground based PM 10 sampling.Ground based AOD data from January to February 2016 were acquired from portable sun photometer at Islamabad, while daily Level-2 AOD from Aqua Satellite's Moderateresolution Imaging Spectroradiometer (MODIS) data (Remer et al., 2005), was accessed and extracted for the Islamabad region from the LAADS site (https://ladsweb.nascom.nasa.gov/) for the period of January until March 2016.The choice of Aqua's MODIS data was due to its extensive validation conducted in Karachi and Lahore using AERONET measurements by Bibi et al. (2015).

PM 10 Concentrations
The main aim of this study was to observe the level and variation of the concentration of PM 10 in ambient air especially the level of particles during foggy and haze days, hence frequently sampling was done when there was prediction for foggier weather.A total of 9 samples were collected during the months of January and February.This study revealed that the values of PM 10 in all collected samples were considerably high as shown in

Backward Air Trajectories and Wind Roses
Overall, Figs.3(a) and 3(b) reveal that during wind speed and temperature dropped considerably low in the Islamabad region in the months of January and February 2016 when heavy fog episodes, in contrast to March 2016 where both wind speed and temperature increase.48-hour backward air trajectories were generated using the National Oceanic and Atmospheric Administration (NOAA) HYSPLIT model (Stein et al., 2015).Some of the trajectories are shown in Fig. 4.These air trajectories indicate a general wind pattern from N, NE, NW and SW and occasionally from S and W. Wind roses were also independently generated for further confirmation of wind directions locally which confirmed that the possible sources for PM 10 during the study period were from industries located nearby in Humak industrial area (N), windblown dust from many newly developed sectors and most of the time the road dust and heavy traffic on the major roads like GT road (S and SW), and Islamabad expressway (NE).These roads typically have heavy traffic  load as they are linked to different cities.Industrial area of Rawat was also found to be major source of particulate pollution as in days with elevated PM 10 levels the wind direction is from NNE. Biomass burning during winters in nearby rural areas and villages (W and SW) was also a major contributor toward particulate pollution.

SEM EDX
To study the chemical composition and surface morphology of the collected 9 filter papers, SEM and EDX analyses were performed.The results showed presence of many elements, namely, Si, O (oxides), Ca and S with higher concentrations while Na, Mg, K, Zn, Pb, Fe and Al were present in less proportions.Elements with higher atomic percentages present in the samples originated from dust.While other elements were anthropogenic, Si was the most sizeable component present.One of the main reason is the quartz filter paper used for sampling.Some of the crustal components Ca, Mg, K, and Fe were also detected.Si, Fe, Al, Ca, and Mg were present in almost all the samples, their origin sources are geological sources and dust.While S, Al, Fe, Zn, Pb come from anthropogenic sources.Zinc in the airborne particles suggesting that its most probable source must be industries and Zn containing particles have released by metallurgical plants and brass/zinc production facilities galvanizing operations in Humak industrial and Rawat industrial estate.The presence of S suggests the most likely source is from fossil fuel burning.Pb was also found in some samples of Islamabad PM 10 which suggested that the major source of it must be from automobile exhaust because the most abundant source of it found to be the use of leaded fuel and that is why most of the developed countries in world banned the leaded fuel to minimize the levels of Pb in air.It can be attributed to heavy vehicular emissions from the nearby Islamabad Highway and GT road, as the wind normally blows from the direction towards sampling site.

Surface Morphology
Surface morphology was conducted by taking images at different wavelength resolutions.As shown in the Figs. 5 and 6, fibers are silicon part of the composition of filter paper, while besides these we can observe different particles with irregular shapes.These irregular shaped particles are the impurities.They can be oxides, carbon or sulphur.The results showed that the particles are round, flaky, spherical and irregular shapes.Depending on the elemental composition and observed shapes the particles were divided into two groups (Wang and Luo, 2009;Pachauri et al., 2013).i) Soil dust particles: these particles possess crystalline or irregular shape shown in Fig. 5.The elemental composition of these particles advocate that these particles originate from soil and the main source of these particles are natural.The elements present in these particles are found to be rich in Si, Ca, Na, K and Zn.Most particles in this study found to be associated with soil and mineralogical sources.ii) Soot particles: Previous studies on the morphological characterization of particles report that most of the urban centers possess particles with round shapes, which is because of the higher quantities of carbon and other metals present.The elevated concentration of these metals and carbon content is due to heavy traffic loads and industries in the cities (Srivastava et al., 2009).In Fig. 6 we observe that the particle morphology indicates species which are rounded and cylindrical in shape and have regular borders.Some samples show high concentrations of carbon and oxygen as the main elements in EDS spectrum.These are presented in previous studies as characteristics of organic biogenic particles that can be identified easily (Srivastava et al., 2009;Wang and Luo, 2009;Pachauri et al., 2013;Singh et al., 2014).Sometimes these particles are found in clusters.Most of the time these clusters were found to be rich in carbon content.These particles were mostly associated with emissions from anthropogenic sources.

AOD Trends via Validation of Sun Photometer and MODIS AOD Retrievals
Sun photometer is a portable device which measures Aerosol Optical Depth (AOD), a column integrated quantity which can be used as a proxy for aerosol concentration levels.This instrument, namely the Microtops II Sun Photometer works in an environment with moderate sunlight, more specifically, limited to optical depth level up to 7.5.It was placed at the Pakistan Meteorological Department (PMD), who provided the Aerosol Optical Depth (AOD) data for January and February 2016.This data was compared with the daily Level-2 AOD from Aqua Satellite's Moderateresolution Imaging Spectroradiometer (MODIS) Collection 6 data using the Deep Blue (DB) algorithm (Levy et al., 2013).Utilizing the deep blue channel, the DB algorithm retrieves AOD at 1 km-resolution and then aggregates them to 10 km resolution.We chose the Aqua satellite AOD retrievals as extensive validation has also been conducted for the Pakistani cities of Karachi and Lahore using the Aerosol Robotic Network (AERONET) sun photometers (Bibi et al., 2015).In this study the authors find that Deep Blue product performs better than the Dark Target algorithm in Lahore city (Bibi et al., 2015), which is near Islamabad at a distance of 270 km.The satellite AOD data was collocated within 0.1° × 0.1° region from PMD site) using the nearest neighbor approach (Nagle and Holz, 2009) for the Islamabad region.The overall comparison of collocated satellite data shows good correlation (R = 0.75) for N = 31 points with ground observations as shown in Fig. 7.In conducting this    validation, we matched the Microtops measurement with the collocated values of MODIS pixel.It is also to be noted here that the Microtop sun photometer is not able to obtain readings in heavy fog days due to lack of moderate sunlight which is required, hence the available data from ground was missing for certain days, and this had to be matched with the appropriate MODIS daily data, which is larger as the satellite sensor does not have this limitation.
After finding that the correlation between satellite and ground observations, we utilized the overall MODIS data from December 1, 2015 to March 13, 2016.In particular, the temporal trends of AOD are depicted in Fig. 8.The large values of AOD reaching peaks on days whenever heavy fog episodes happened, especially from January 29, 2016 until February 2, 2016 period.Here, the peaks of the graph also indicate days of dense fog periods.It is notable that the study regarding possible connections of AOD levels with PM 10 concentrations in this region would require more extensive data, including observations on non-fog days, which is a drawback of this study.However, future work is planned with the aim of surpassing the limitations in the present study.

CONCLUSIONS
The main objective of this study was to provide a description and characterization of particulate matter in the city of Islamabad especially during the winter fog days.This study reveals that during foggy days, the ambient air of Islamabad is contaminated with PM 10 and it also consists of various toxic heavy metals.The concentration of PM 10 varied from 123.59 µg m -3 to 202.65 µg m -3 with an overall mean concentration of 177.33 µg m -3 as shown in Fig. 2. On most occasions, PM 10 levels are considerably high as compared to permissible limits of both Pak-NEQS and WHO guidelines.It has been observed that the air quality during fog days was much worse, and elevated levels of particulate matter were observed during the foggy days.The AOD data for non-foggy days were also observed through MODIS satellite, which shows low AOD levels during non-foggy days.The SEM-EDS revealed the presence of different elements including some metals Si, K, Ca, Mg, Zn, Fe, Cr, Pb, Al etc.The morphological studies also suggested that most of the particles have crystalline shape, suggesting their main source as soil.In general, Islamabad with vast roads and well planned infrastructure is thought to be a relatively clean city as compared to megacities like Karachi, Lahore, and Rawalpindi.But our results showed that the PM 10 levels exceeded the Pak-NEQS in 8 samples out of total 9 samples.The sources of PM 10 are found to be due to both natural as well as anthropogenic sources.Extensive urbanization, industrialization, construction activities and increased vehicular pollution were found to be major contributors of particulate matter.Moreover, we found elevated AOD during heavy fog days.
The investigated levels of particulate matter in ambient air of Islamabad city is likely a threat to public health and environment, if measures are not taken to control harmful emissions.Islamabad is the capital city of Pakistan, which is generally thought to be the green city and less polluted city.However, due to a lot of development works and traffic loads the air quality is getting worse with every passing day.The influx of people from different regions of Pakistan in search of opportunities is also a factor, and as such, the population of the city is also growing with a rapid pace.New towns and areas for living are developed without urban planning.The dust from the construction sites is probably a big factor for elevated levels of particulate matter.The study site is situated in Zone V of Islamabad, which is a newly developed zone of Islamabad.In fact, Islamabad is one of few cities which has a relatively large number of villages and rural areas in its vicinity.These areas do not have gas connections, unlike other sectors of Islamabad.People use wood for cooking and heating purposes and biomass burning in the area is also contributing toward the levels of particulate matter especially in winter season.
Rawat industrial state and Humak industrial area is also present in the same vicinity, which is the largest area of Rawalpindi and Islamabad.This area has many national and international industries.The area is popular with beverage, metal, textile, steel and pharmaceuticals industries.Emissions from these industries is also a driving force for elevated particulate matter levels.
The study area is also adjacent to the Islamabad expressway, which is a signal free highway that connects Islamabad to nearby sectors and different cities.This road has the highest levels of traffic within Islamabad territory and is always busy with vehicular movement throughout the day and night with both light and heavy traffic.The emissions from the vehicles is particularly from heavy duty trucks and loaders.Old vehicles and diesel driven vehicles also contribute to bad air quality.To have proper assessment of the impacts of particulate matter on public health and environmental impacts including influence on fog formation, this work should be extended for continuous monitoring of PM 10 and PM 2.5 , as well as other key air quality parameters.
Fig. 1.The concentration of PM 10 varied from 123 µg m -3 to 202 µg m -3 with an overall mean concentration of 177 µg m -3 as shown in Fig. 2. The permissible limits set by World Health Organization (WHO) and Pakistan National Environmental Quality Standards (Pak-NEQS) is 150 µg m -3 (24 hours) and most of the samples showed higher values than the allowed limits.The highest value of PM 10 recorded during the study period was 202 µg m -3 measured on 27 February 2016 and 10 February 2016.In contrast, the lowest concentrations of PM 10 was 123 µg m -3 , observed on 3 February 2016.

Fig. 2
Fig. 2 depicts daily concentrations along with average wind directions, indicating high PM 10 levels which exceed daily WHO and Pak-NEQS levels.The wind direction and other meteorological parameters were measured through the weather station placed along with the high-volume sampler at the rooftop.

Fig. 2 .
Fig. 2. Observed concentrations of PM 10 during fog days with average wind direction.

Fig. 3
Fig. 3(a).Monthly mean wind magnitude and vectors (m s -1 ) from MERRA 2 satellite data at 10 m height during December 2015 and January, February and March of 2016.

Fig. 3
Fig. 3(b).Monthly mean wind vector (m s -1 ) at 10 m height and temperature from MERRA 2 satellite data at 2 m height during December 2015 and January, February and March of 2016.

Fig. 5 .
Fig. 5. SEM spectrum (top image) and graphical results showing soil dust particles with rough and crystalline shape.(a) part is the actual image (1 kx) while (b) is the zoomed-in version (5 kx) of the image.

Fig. 6 .
Fig. 6.SEM spectrum (top graph) and images showing soot particles with round and cylindrical shape.(a) is the actual image (1 kx) while (b) shows the zoomed-in version (5.01 kx) of the image.

Fig. 7 .
Fig. 7. Scatterplot of MODIS AOD at 550 nm (y-axis, dimensionless) versus Microtops AOD (x-axis, dimensionless) with correlation squared (r 2 ) value.Presented data are Microtops AOD data was collected on non-foggy days in the study period from January to February 2016 and the collocated data points with MODIS observations.