Special Issue on Hazardous and Toxic Pollutants in the Air

Criteria pollutants have been at the centre of air pollution problem for many decades. However, over the years the problem of air pollution has transformed into a more severe form due to increasing emission of hazardous and toxic pollutants into the atmosphere. To address this issue, the ASCE’s Journal of Hazardous, Toxic and Radioactive Waste (ASCE-JHTRW) has brought out the present special issue titled “Hazardous and Toxic Pollutants in Air.” This issue includes papers on various related subthemes such as sources and emissions, transport and transformation, laboratory experiments and case studies, monitoring and characterization, emission inventories, modeling and simulation, parameter estimations, exposure and health effects, environmental impacts and risk assessment, and policy measures and technology interventions for emission reductions and air quality improvement. In response to the call for this special issue, many manuscripts were received, which were screened and thoroughly reviewed to select and include 10 articles in total. Brief descriptions about the included papers are given in the following.

A study on ambient and episodic levels of metals in PM10 aerosols and their source apportionment at central Delhi in India has assessed the levels of metals in PM10 aerosols in ambient air at a site in central Delhi. A total of seven metals were analyzed in PM10 aerosol samples during a postmonsoon period. The average levels of metals were estimated in the order $\mathrm{K}>\mathrm{Zn}>\mathrm{Mg}>\mathrm{Fe}>\mathrm{Mn}>\mathrm{Cu}>\mathrm{Cd}$. The study also covered the metal accumulation due to fireworks on a major festival. A comparison of ambient levels of metal accumulation with the Diwali period revealed that the Diwali festival is a source of severe air pollution when the levels of metals increased by many folds as compared to normal ambient levels. Source apportionment using principal component analysis (PCA) revealed that crustal, industrial, and vehicular sources mainly contributed the metals in air during the normal period whereas fireworks and vehicular sources were identified which contributed metals during the festival period. Further, these metals on deposition may contaminate water bodies, soil, and vegetation cover, which may leave long-term indirect effects on the environment.

Characterization morphology of particulate matter inside farm houses located near biosolids applied agricultural fields reveals that bioaerosols, i.e., airborne particulate matter (PM) is aerosolized during various agricultural processes such as preapplication preparation, disking, application, and postapplication management. This is why, in recent years, there has been concern about the health of people living near the application sites even though these processes have been going on for decades. The focus of this study has been to characterize particulate matter inside households located near fields where biosolids have been applied. The shape factor distribution and shape distribution results indicate that there was a higher percentage of agglomerates which was followed by spherical and pentagonal shaped particles.

A study on commuter exposure to carbon monoxide (CO) and PM10 during transit in the several commonly popular modes of transport in Mumbai, India and Newcastle, UK presents another paper. In Mumbai, real-time exposure concentrations were measured whilst commuting along a route by bus, train, air-conditioned taxi, and a nonair-conditioned private car. In Newcastle, real-time exposure concentrations were measured whilst travelling by electric vehicle, public bus, and bicycle along a route. Average heart rate whilst commuting was monitored and subsequently simulated in a submaximal exercise test to give minute ventilation (VE) associated with each transport mode. The study in Mumbai has indicated that commuters travelling on buses and private nonair-conditioned cars are exposed to very high levels of air pollution compared with the train commuters. In Newcastle, electric vehicle and bicycle displayed the lowest exposure concentrations relative to buses. Higher exposure concentrations were observed for all travel modes in Mumbai as compared to Newcastle. When inhaled amount and the lung deposition of pollutants are considered, cyclists are exposed to higher amounts pollutants in Newcastle due to the considerable increase in VE induced by the physical exertion of cycling. Thus, inhaled quantity and pollutant dosage give a better indication of the possible health risks and should be considered in future studies and in designing the cycle routes in cities. This study provides guidance to the transport planners in Mumbai to focus on reducing exposure to prevent unwanted health effects. The authorities in Mumbai should focus on reducing the exposure of passengers in conventional buses where doors and window are mostly kept open. This can be achieved by replacing these buses with air-conditioned buses. The authorities in Mumbai can also focus on changing the current traffic mix to the newer emission standards as soon as possible to reduce the ambient pollutant concentrations. The buses, taxis, and auto-rickshaws mainly have an old fleet belonging to pre-Euro 1 emission control standards. These will be easier to replace with a newer fleet than the private cars. There is also a need to fast-track the intake of electric and hybrid vehicles by incentivizing their production and adaptation. Also, there is a need to implement traffic management measures like congestion charging, low emission zones, and bus rapid transit systems.

If, based on the previously mentioned study, the society continues to promote cycling, it is necessary that efforts are made to reduce cyclists’ exposure to air pollutants. This investigation recommends a twin-track solution whereby firstly, cyclists are moved further from pollutant sources (namely traffic), reducing exposure, and secondly, cycle routes are designed that do not induce excessive physical exertion, thus reducing ventilation rates (VE) and hence the total pollution dosage. By incorporating these design philosophies into the development of cycling infrastructure, it may be possible to reduce the health risk of cycling. However, further studies are needed to estimate the new health impacts of cycling taking into account the “health loss” due to the extra air pollution dose by way of higher breathing rates and “health gain” due to physical activity while cycling. This is of particular interest to the developed nations which have seen a significant interest and surge in the number of people cycling to work and for leisure. This research has shown the importance of developing cycling infrastructure in such a way so that it is cyclist friendly and has lower air pollution exposure dose. The health risks resulting from air pollution on public buses could be mitigated by improvements to the vehicle fleet. This can be improved by updating, maintaining, or replacing engines on public buses. The provision of air conditioning units on public buses may reduce the need to have open windows which would subsequently reduce infiltration of external pollutants.

Megacities in India (population more than ten million) have taken actions to control air emissions. However, the second-level cities (population between one and ten million) have not drawn any action plan, and people face serious air pollution in these cities. For such a city, Lucknow, a graphical information system (GIS)-based methodology for emission inventory of on-road vehicles has been developed and presented in this special issue. The pollutants include: sulphur dioxide (${\mathrm{SO}}_2$); oxides of nitrogen (NO_x); carbon monoxide (CO); particulate matter (PM); 1,3 butadiene; formaldehyde; acetaldehyde; total aldehydes; and total polycyclic aromatic hydrocarbons (PAHs). Video recording was done at nine road intersections of varying land-use patterns to assess traffic count and vehicle kilometer travel. Parking lot surveys were carried out for assessing engine type, vehicle age, etc., and to arrive at suitable net emission factor for each vehicle category. The 2-Wheelers (2-Ws) and 4-Wheelers (4-Ws) dominate the total traffic with 83% share and are main sources of NO_x (46%) and CO (77%). The heavy duty vehicles (HDVs: buses and trucks), although account for only 2% of the vehicle fleet, emit disproportionately high emissions (23% of ${\mathrm{SO}}_2$, 36% of ${\mathrm{NO}}_x$, and 28% of PM). Spatial cellwise ($2\times 2\mathrm{km}$) emission inventory of pollutants indicates that the city center has the highest pollutant emissions resulting from large number of vehicles, mostly 2-Ws, 3-Ws, and passenger cars. The inventory information can be used for short- and long-term planning to reduce emissions.

Oxides of nitrogen, particulate matter, carbon monoxide, and hydrocarbons are the major air pollutants of vehicular emissions near major intersections and arterials in megacities. Therefore, a case study related to potential assessment of neural network and decision tree algorithms for forecasting ambient PM2.5 and CO concentrations has been conducted in megacity Delhi, which has been included in this special issue of this journal. Artificial neural networks (ANN) and decision tree algorithms (e.g., REPTree and M5P algorithm techniques) have been used to predict hourly fine particulate matter (PM2.5) and carbon monoxide (CO) pollutant concentrations at a hotspot site. Factors and parameters such as those of meteorology, traffic, and vehicular emissions that influence the pollutant concentrations have been used with different combinations for the model development. Performance evaluation of ANN, REPTree, and M5P algorithms for hourly PM2.5 and CO concentrations prediction has been carried out along with the effects of previously mentioned factors. The M5P algorithm performs better than ANN and REPTree algorithms in precisely capturing the relationship amongst the predictor variables and pollutant concentrations.

The free-fall dust in ambient atmosphere provides important information about the nature and sources of pollutants in urban settings because it incorporates the particles settling under gravity’s influence irrespective of their size. With this context, a study was conducted and free-fall dust samples were collected to understand the impact of immediate and ultimate pollution sources, respectively, in Delhi. Samples collected near traffic intersections exhibited impressions of vehicular emissions, resuspended roadside dust, carbon, and trace metals such as Zn, Cu, Pb, Ni, Cr, and Cd. Interestingly, the total carbon fraction contributed more than 10% of total collected mass at all sites. Zn and Ba concentrations dominated over other trace metals in samples at both heights and for all the sites. The results suggest that the presence of carbon in higher quantities has caused a dilution effect on trace metals concentrations. The study confirmed that crustally derived materials dominated the total mass of free-fall dust at both heights. Besides, statistical analysis (ANOVA test) was also performed to find out the differences in samples of two heights. Further, the ionic composition of free-fall dust was also assessed. A significant correlation of total carbon with anions indicated combustion (fossil fuel and biomass) activities are the dominant sources of carbon and influence the chemistry of free-fall dust. The multivariate analysis of the data indicated three prevalent sources of analyzed chemical species: (1) construction activities; (2) resuspension of surface dust/soil; and (3) the fossil fuel and biomass combustion.

Gaseous toxic air pollutants are normally associated with acute short-term and chronic long-term health effects, and hence measurement of these pollutants is essential on a daily basis to understand the science of their occurrences. To do that, a characterization of gaseous pollutants and water-soluble inorganic ions in PM2.5 during summertime in an urban site of north India was conducted. The dilution effect of Si and C was evident on the measured concentrations of major elements. The concentrations of Ca, Fe, and Mg were above the UCC values, which suggests the presence of an additional source, other than the crustal source, for these elements, although due to mixed activities, the sites do not show very clear picture of influence of source type. The analysis (PCA, ANOVA, and Pearson correlation) indicate that the elemental contributions are arising from three major secondary crustal sources: (1) construction material such as cement, (2) resuspension of surface dust and crustal materials, and (3) the biomass and fossil fuel combustion activities. The trace metals also seem to have been contributed by the small-scale metallurgical processes, oil combustion, and incineration in the city. Concentration distribution with respect to mean daily aerosol load indicates dust resuspension as one of the major causes of crustal elements contributes at traffic intersection sites. After comparing data from two sites where both the types of samples (5 and 30,352 ft) are available, it is shown that Zn and Ba are the predominant species at lower as well as higher height sites, but interestingly Pb is one species which is coming in high in only lower-height sites.

Linking air pollution from source to adverse human health effects is a complicated phenomenon that requires a multidisciplinary approach for better understanding. Decision-makers need relevant, comprehensive estimates of the disease burden attributable to different risk factors. Many statistical models have become very relevant for estimating atmospheric concentrations by analysis of complex datasets to produce inferences and predictions that can lead to better management of air pollution. In this paper, we focus on the Indian scenario as a case study and present the current status of air quality in India with special reference to particulate matter. The study presented on this very important issue suggests that air quality networks need to be developed that can depict and forecast pollution levels with health advisories for public and for pollution emergency measures. Development of statistical models and methods for big data analytics can yield a wide array of actionable insights to facilitate policy decisions. Models may also be used to predict the cost of the air pollution control measures as well as the benefits in terms of the control of acute and chronic diseases caused by air pollution. Thus study concludes that the application of statistical models and algorithms can act as an important tool to bridge the gap between science and policy.

In this series, there is another paper that presents the impact of air pollutants on human health and vegetation at selected industrial and control areas of India. Air quality data monitored by a regulatory agency and health data obtained from local hospitals for the period 2010–2012 were analyzed. The particulate matter (PM) size distributions and lung function of the exposed people were also measured at the two study regions using a nonviable cascade impactor and spirometer, respectively. The health data recorded at hospitals indicated maximum hospital admissions during winter period. The lung function analysis at both the study areas revealed that the people residing in the industrial area were having lower air exchange rate as compared to the people residing in the control area. The effect of air quality on vegetation was also investigated. It was found that the plants photosynthetic rate at industrial area was 50% lower than control area.

We hope this comprehensive special issue on “Hazardous and Toxic Pollutants in the Air” will bridge the gap in this important area of atmospheric pollution research and air quality assessment.