Study on the variation in the prevalence of smog precursors species in the ambient air of Lahore was carried out to assess the temporal extent of gaseous by-product in air and impact of meteorological factors on their recurring trend. Major pollutants selected for current study were oxides of nitrogen, sulphur, ozone, and particulate matter as they are identified as instigator for smog with substantial evidence. Sources of the release of these air pollutants are dust generating activities, burning of biogenic material and combustion of low-quality fossil fuel which are well apportioned on roads with heavy traffic load, Industries, and agricultural sites near and around Lahore.In present study, the Data set of three years for scrutiny was obtained from the air quality monitoring station that was installed in meteorological department on Jail Road . Jail Road, Lahore is a 5 km long 3 lane road that scored the features of commercial zone e.g., hospitals, educational institutes, police station, departmental stores, restaurants, recreational parks, car showrooms , Fuel pumps stations and housing societies on roadside. This road is located almost on the centre of Lahore city. The intense agglomeration of high grade public and private services cause the high influx of vehicles (>0.1 million) in this area , often create traffic congestion in daytime . Therefore,in current study, temporal monthly mean extent of PM-10 ( 21.98 µg/m3 - 307.76 µg/m3) , PM2.5 (10.49 µg/m3 - 176 µg/m3), NO (04 µg/m3 - 250 µg/m3 ), NO2 (127 µg/m3 - 9.85 µg/m3) was found to be higher than the permissible limits of PEQS . Whereas SO2 ( 99.17 µg/m3 - 9.4 µg/m3) and ozone ( 107 µg/m3 -20.7 µg/m3) was found below standards in the ambient air of Jail road .
First Reason is attributed to type of fuel used in vehicles. Large proportion (> 70 % ) of vehicles use petrol followed by Compressed natural gas (CNG) on second, Liquified petroleum gas (LPG) on third and diesel on fourth . while scientific evidences proves that CNG is more cleaner in term of CO2, CO and SO2 , LPG is cleaner in term of NO2, NO and petrol is cleaner in term of hydrocarbon (Hameed ,2013) . Along with the quality and type of fuel, increase in number of point sources further aggravate the emission factor as the rise in city traffic has reached to 6.2 million vehicles at 10% annual increase rate since 2005. Out of which, 4.2 million motorcycles , 2.4 million non-commercial cars, 0.24 million rikshaw, 0.004 million motorcycle rikshaw are on road while the rest are delivery vans ,buses and trucks (Butt,2018; Shah et al ., 2020).Therefore, 70% air pollution in city is attributed to Traffic pollution .Out of which 40% is from poorly maintained auto rikshaws, pickups and diesel buses. Hence further decline in recurring content of air pollutants could be attributed to the initiative taken by Government in 2016 after worst episode of smog in city (Hameed et al., 2013). Which include the import and supply of low sulphur fuel to local markets . Fuel sector imported first environmental friendly low sulphur diesel of 500 ppm in 2017 , LNG in 2015 and higher 92/95 grade gasoline in November 2016. Sulphur dioxide is formed by the oxidation of sulphur compound in petroleum /coal as fuel which further lead to acid rain (Afon & Ervin , 2008). This minor change in fuel quality has led to visible decrease in sulphur emission. Further governmental actions include the strict implementation of smog policy reforms at provincial level since 2016 which involved shutting down dust generating activities , banning of stubble burning , municipal solid waste burning , closure of brick kiln in winter season for three months fine/challan on pollution generating vehicles and industries every year all over the province. Other structural measures include the widening and construction of signal free roads in Lahore Which contributed in the minimization of fuel combustion emission (PDMA,2016).
Second reason for fluctuation in temporal prevalence of air pollutant is the intermediate gaseous reactions in ambient air because a significant positive correlation of PM10 and PM2.5 with NO (r = 0. 80) > NO2 (r =0.78) > SO2 (r = 0.25) and negative correlation with ozone (r =- 0.45) was analysed from dataset. Particles smaller than 2.5 µm are considered fine particles which originate from secondarily formed aerosols, (gas-to- particle conversion) combustion particles, recondensed organic and metal vapours. According to Park et al. (2007) sulphur dioxide oxidise in the atmosphere to form sulphuric acid (H2SO4) . which can be neutralized by NH3 to form ammonium sulphate. Sulphate aerosols mostly makes the 6%-28% of particulate matter of 2.5 micron Nitrogen dioxide is oxidised to nitric acid (HNO3) which in turn can react with ammonia to form ammonium nitrate (NH4NO3).These secondary particles are the dominant part of fine particles produced from the intermediate reaction of gases. Particulate matter is composed of 28.7% organic matter , 21.4% sulphate, 15% nitrate, 12.8% ammonium, 8.8% elemental carbon and remaining percentage of trace metals . Another study by Viidanoja et al. (2002) revealed that oxidation of VOC gives rise to carbonaceous fractions and black carbon in air . Ozone highly negative association with NO (r = -0.5 ), PM 2.5 ( r = - 0.457) > NO2 (r = -0.26 ) > SO2 (r = -0.247) because oxidation of carbon mono-oxide, nitrogen dioxide , hydroxide and VOCs triggered by photochemical reactions result in elevated ground level tropospheric ozone in summer (Uzoigwe et al., 2013). Whereas lifetime of Sulphur dioxide in atmosphere is 1-3 days due to the ubiquitous photochemical sink because hydroxyl radical oxidize the sulphur dioxide to sulphate aerosols in gas phase in day time (Rotstayn and Lohamann,2002).
Third reason for variation of concentration of PM10, PM2.5, SO2, NO2, O3 at intermonth temporal scale is response of smog precursors to sunshine hours, temperature, Air pressure, Wind speed, relative humidity and cloud percentage. Air flow have positive association with Ozone > SO2 > PM10 >PM2.5.The reason is attributed to the factor that trigger the speed of air flow e.g., In summer, wind convection due to high temperature cause vertically upward movement of pollutant and disperse them in air . The dilution effect of wind speed on pollutant rises at first and then tends to be gentle. Because the particles can be carried out over long distance by wind and then settle on the ground, vegetation or water where they further begin/trigger the disruptions of ecological processes and damages to aesthetic damages (Kgabi and Mokgwetsi , 2009) high wind speed (>12 m/s) cause more dispersion and dilution. Air pressure have positive association with PM2.5 > PM10 > NO2 > NO > SO2 whereas highly negative correlation with O3 because Under the low-pressure circulation situation, there are more rainy days and the wind direction changes more frequently, which helps the diffusion and dilution of particulate matter; while the high-pressure circulation situation brings more sunny days and the weather system is relatively stable, forcing the particulate matter to be stagnate in the near-surface layer. In this way, an inversion can prevent the rise and dispersal of pollutant from lower layer cause a localized air pollution problem Hence, the response of Pollutants to air pressure appears to be positive on a large time scale. Relative humidity and Cloud Percentage have positive association with O3 > SO2 whereas highly negative correlation with PM2.5> PM10 > NO2 >NO because high humidity environment has the effect of agglomerating PM 2.5. High sulphur produce high concentration of sulfuric acid when combine with fog droplets. formation of these acidic particles aggravated by dampness and high concentration of particulate matter in air (Rani et al., 2011). because SO2 react with cloud or fog produce more sulphate aerosol. According to Griffing (1977) NOx concentrations are more elevated during the wet season. Because the frequently recurring thunder storm flashes produce oxide of nitrogen. Temperature and sunshine hours have positive association with O3 and highly negative correlation with PM10 >NO2 > PM2.5 > NO > SO2 .According to Tecer et al. (2008) higher heat deficit is associated with less turbulence thus cause high concentration of Particulate matter while low heat deficit associated with high turbulence that cause low particulate matter in summer . Hence high temperature (>24C) cause more dilution than dispersion which is observed more in summer than winter where under strong temperature inversion condition, wet ground due to precipitation trap more pollutant near to their sources. Whereas Plocoste et al. (2018) reported that temperature, high UV index and sunshine hours strongly influence the amplitude of ozone cycles. Because these factors trigger the high occurrence rate of photochemical reaction in which brownish oxidative smog formed (Derwent et al., 2016).