This study presents a comprehensive source-meteorology-receptor (SMR) approach to understand the fine particulate (PM2.5, particles less than 2.5 µm in diameter) problem in three major cities of Ohio viz., Cleveland, Columbus, and Cincinnati. The work emphasizes a three pronged combined approach using the available receptor, meteorology and source data to analyze the PM2.5 concentrations to identify, establish and quantify the PM2.5 behavior, the effect of meteorology and responsible factors, and the relative source contributions to the pollutant in the three urban areas under consideration. The different analyses are directed towards better understanding PM2.5 by characterizing the pollutant, studying the behavior, establishing the effect of meteorology, and delineating specific major sources and their relative contributions to the problem. Every step of the SMR approach helps in better understanding the pollutant by revealing the trend and the seasonal variations, by establishing favorable conditions for higher concentrations, and by identifying the problem-causing sources. Review of the literature indicates that such an approach has not been developed for urban areas where only intermittent concentration data are available and meteorology plays a minor role in fine particulate pollution problem.
The trend study revealed that all three cities exhibited a seasonal PM2.5 concentration pattern with higher concentrations in summer and lower in winter. PM2.5 and its species both showed seasonal and spatial variations. In all the three cities, ammonium sulfate and organic carbon (OC) comprised the largest fraction of PM2.5 followed closely by ammonium nitrate concentrations. The three major components ammonium sulfate, organic carbon, and ammonium nitrate accounted for about 37-46%, 20-22%, and 15-20% of the total PM2.5 in the three cities. Sulfates dominated the summer-time PM2.5, and nitrates contributed to the winter concentrations in all the areas under study. Seasonal differences in the sulfate concentration ranged from 3-7 µg/m3 from one season to another in each of the cities. The average summer sulfate concentrations were approximately 45% more than the winter time concentrations. The nitrate concentrations were generally higher in winter than in summer, probably due to a combination of lower temperatures and meteorology. Crustal components did not show much seasonal variation for Cincinnati and Columbus, but a summer high was observed for Cleveland. The correlation analysis revealed strong component to component associations at all the sites. Total PM2.5 was found to be strongly correlated with sulfate, ammonium, and OC. Ammonium was correlated better with sulfate than nitrate; organic carbon and elemental carbon (EC) were strongly correlated with each other suggesting similar emission sources. The OC/EC ratio was consistently higher in summer and winter showing similar strengths of the pollutant emissions during both the seasons. The annual average nitrate/sulfate mass ratios for the three cities were consistently below 1 suggesting stationary source emissions as the dominant sources in the three cities studied. In addition, an evaluation of episode days when PM2.5 concentrations were over 35 µg/m3 identified summer episodes characterized by high sulfate concentrations and winter episodes with high nitrates. OC concentrations were similar during both of the seasonal episodes in all the three cities suggesting the local emissions of OC.
The meteorological effect and contribution study revealed wind speed to be the most significant meteorological variable for the three cities. Wind speeds lower than 5 miles/hour, temperature greater than 60 degree F, and relative humidity higher than 60% were all significant variables during different times. Temperature and wind speed were highly correlated to PM2.5 during summer, with wind speed being important in all the seasons for all the cities. Significant PM2.5 species such as sulfates, nitrates and OC showed strong positive correlation to temperature and inverse relation to wind speed in all the three cities. Nitrates and OC showed a positive and negative relation to relative humidity, respectively. Relative humidity was important in all the seasons and more significant in Cincinnati than in the other two cities. Different combinations of variables explained the trend of the particulates in different cities. In general, wind speed below 5 miles/hour resulted in higher concentrations in all the seasons. In summer, temperature above 65 degree F resulted in higher concentrations along with lower wind speeds or higher humidity over 60%. Overall, the considered meteorological variables accounted for only approximately 29-41% of PM2.5 variability in Cincinnati, 25-30% of variability in Columbus and 27-37% of variability in Cleveland. The meteorological contributions suggest a stronger contribution from direct emission sources and transported secondary pollutants.
To further understand the remaining variability in the PM2.5 concentrations and trends, a detailed source apportionment analysis was conducted for all the three cities to identify the responsible sources and their relative contributions. Speciated fine particulate data collected as a part of the Speciation Trends Network at the three cities were modeled using the U.S. Environmental Protection Agency (U.S.EPA) positive matrix factorization model (PMF) to understand sources contributing to PM2.5 mass. The model resolved seven sources for Cincinnati and Columbus and twelve sources for Cleveland. The common sources included secondary sulfate, secondary nitrate, OC/EC factor related to mobile sources, crustal, wood/biomass burning, and an industrial zinc factor. The model also resolved source profiles identifying local industrial processes including – industrial iron/copper (for Cincinnati, Cleveland), iron/calcium limestone factor (for Columbus), an industrial manganese factor, a chromium/nickel factor, an industrial lead factor and a copper/nickel factor (for Cleveland). The identified factors for the study areas contributed about 88% to the total mass in Cincinnati, 82% in Cleveland and 95% in Columbus. The factorial contribution plots for the different seasons correlated well with conclusions from the trend and the meteorological analysis of the SMR approach. The secondary sulfate contribution to PM2.5 was higher during summer; crustal contribution was higher during spring and summer and during weekdays; and secondary nitrate contributions were higher during winter and spring and during weekdays. Regional influence of secondary sulfate was more dominant in Cleveland than in the other two cities. Local industrial factors emitting trace metals played a minor role in the contribution to total PM2.5 mass, but their role in increasing the toxicity of pollutant is quite significant.
In a nutshell, this study has developed a three pronged SMR approach to systematically analyze the fine particulate problem and the approach was successfully applied to the three major cities in Ohio. The approach incorporates both basic and advanced analyses to fully capture the PM2.5 problem in its entirety and can be applied for major urban areas in the world for a detailed understanding and quantification of the problem. Such a detailed study and analysis of fine particulate matter and its chemical composition will provide crucial information to air quality regulators and decision makers to determine the efficacy of the imposed air quality standards and lead to the development of more specific control strategies, instead of a regular capping limit.