Simulation shows the molecular footprint of organic air pollutants


Organic pollutants are a major source of air pollution. New simulations developed by Qi Ying and his team can predict the concentrations of certain precursor compounds that contribute to secondary organic aerosols in the ambient air.

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Texas A&M University researchers have joined global air pollution control efforts and developed computer tools to accurately assess the footprint of certain organic air pollutants. Your simulation, described in the magazine environment science and technology, could help government agencies more closely control the human-made sources of carbon-based pollutants.

“Human activities have resulted in a significant increase in organic air pollutants such as aerosols, which has led to deterioration in air quality and even climate change in many places around the world,” said Qi Ying, associate professor at the Zachry Department of Civil and Environmental Engineering. “But by better estimating organic aerosols through specific marker compounds, we can develop better emission control measures for cleaner air for everyone.”

According to published reports, 20-40% of particulate matter in the lower atmosphere comes from organic aerosols. The air pollution caused by these compounds is a constant threat that affects the climate, health, and visibility. For example, depending on the type of aerosol, some may change the amount of heat released into the atmosphere while others affect the amount of heat released. In addition, organic aerosols are easy to inhale and, when present in sufficient concentrations in the body, can exacerbate many health problems from asthma to chronic obstructive pulmonary disease. These compounds can also affect vision by causing haze.

Organic aerosols begin their journey into the atmosphere as volatile compounds released into the air from a variety of natural and man-made sources, such as fossil fuel combustion and vehicle emissions. These precursor aerosols then react with oxidizing agents and condense on existing particles in the atmosphere to form secondary organic aerosols. From the point of view of air quality management, it is therefore necessary to know which precursors contribute to the formation of secondary organic aerosols so that their specific sources can be restricted.

The amount of certain precursor aerosols is calculated from the ratio of the frequency of a marker molecule called 2,3-dihydroxy-4-oxopentanoic acid (DHOPA) to the amount of secondary organic aerosol in air samples from field tests. Historically, this ratio has been determined using laboratory test chambers in which very controlled atmospheric conditions are maintained. However, the ratio may not be suitable for use in various atmospheric conditions in the field.

“We really don’t know whether this relationship is fixed or whether it changes in the open environment,” says Ying Saud. “The humidity of the ambient air, the temperature and other climatic factors are constantly changing, which in turn could influence the estimates of the concentration of the secondary organic aerosols.”

To overcome this disadvantage, researchers used a supercomputer at Texas A & M’s High Performance Research Computing facility to simulate atmospheric chemistry based on environmental conditions across East Asia, including China, Japan, and Korea. Information about the origin of the emissions, the emission rates and the meteorological data at various locations were given to the simulation as inputs. The simulation focused on how many secondary organic aerosols are formed from various precursors, particularly those that contained the molecular signature DHOPA.

While running the simulations, the research team found that the ratio varied with ambient temperature and the level of organic aerosols in the air. They also observed that the ratio obtained from previous chamber studies, if not corrected, would lead to significant errors in the estimation of secondary organic aerosols. However, the correction of the ratio remained the same for regions with comparable air pollution.

“We have come a long way in reducing inorganic air pollutants, but as the contribution of these compounds decreases, there will be a much higher proportion of air pollution from organic precursors,” said Ying. “We started with DHOPA as a marker for some precursor aerosols, but we want to identify molecular markers for other precursors, which is still to be done.”

Other contributors to this research include PhD student Jie Zhang from the Faculty of Civil and Environmental Engineering; Xiao He, Shuhui Zhu, and Jian Zhen Yu from Hong Kong University of Science and Technology; Yaqin Gao, Shengao Jing and Hongli Wang from the Shanghai Academy of Environmental Sciences, China.

This research is supported by the Hong Kong Research Grants Council and the Shanghai City Science and Technology Commission.

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