The University of California discovers new chemical mechanisms for future air pollution

On August 29, 2022, Xuan Zhang’s research team at the University of California, Mercedes-De, published a new study in Chem titled “Probing isoprene photochemistry at atmospherically relevant nitric oxide levels.”

Combining environmental smoke box experiments, three-dimensional chemical transport simulations, and field aerial surveys, the research team discovered a new chemical mechanism that will dominate air pollution changes in the future, and explored in detail how this chemical mechanism acts on the main volatile organic compound isoprene emitted into the atmosphere in nature and the contribution of the atmospheric oxidation process of this substance to ozone and free radical production. The study provides a new perspective on how human activities can alter the future oxidation and self-purification capacity of the atmosphere. The corresponding author of the paper is Zhang Xuan; The co-first authors are Zhang Xuan and Wang Siyuan.

Volatile organic compounds emitted into the atmosphere by nature account for 90% of total organic emissions, of which isoprene is the main component of natural organic emissions. Isoprene also has a very high atmospheric reactivity, so studying the atmospheric oxidation mechanism of isoprene is of great and far-reaching significance for understanding the degree of oxidation and pollution of the entire earth’s atmosphere. For decades, research into the atmospheric chemical reactions of isoprene has focused on two extreme environmental conditions: one is a highly polluted urban environment, which is typically characterized by high concentrations of nitrogen oxides (such as hundreds of ppb); The other is the opposite of a zero-pollution atmospheric background environment in which the concentration of nitrogen oxides is close to zero. However, with the strict control of nitrogen oxide emissions in various countries in recent years, nitrogen oxides in most parts of the world have dropped to the range of dozens to several ppbs, and under such a low concentration of nitrogen oxides, whether the traditional isoprene atmospheric chemical reaction mechanism can still be applied needs to draw a question mark.

The latest environmental smoke box experiment results of the research group found that when the concentration of nitrogen oxides is in the range of several ppts to dozens of ppbs, the atmospheric oxidation mechanism of isoprene no longer follows the traditional reaction path. An atypical feature is that peroxy radicals no longer collide with nitrogen oxides under low concentrations of nitrogen oxides, but instead react in a self-cyclic mutual transformation reaction that completely controls the distribution of isoprene oxidation products. Figure 1 shows that the yields of the two main generation products of isoprene (isobutenal and methylvinyl ketone) are twice as high at low concentrations of nitrogen oxides than at high concentrations of nitrogen oxides.

Figure 1: Yields of the two main generation products of isoprene (isobutenal and methyl vinyl ketone) vary from low to high concentrations of nitrogen oxides.

The team integrated this new mechanism into an observationally controlled zero-dimensional chamber model and compared it to the results of two recent large aerial surveys in the Northern Hemisphere. The two aerial surveys sampled the composition of the air in the southeastern United States and much of North America in different seasons, respectively. The results show that this new mechanism can better explain the distribution of isobutylaldehyde and methyl vinyl ketone in the air than the traditional mechanism. It is worth noting that this new mechanism is far better than the traditional mechanism in different regions and dimensions of North America.

Figure 2: Spatio-temporal distribution of isobutenal and methyl vinyl ketone predicted by aerial surveys and zero-dimensional cabinet models.

Finally, the research team integrated this new mechanism into the three-dimensional chemical transport model, and found that this mechanism not only acts in North America, but also affects the atmospheric reaction of isoprene throughout the world. The most affected areas are concentrated in areas with high vegetation cover and high natural emissions, including the Amazon rainforest. In addition, for areas with relatively high levels of pollution, such as Southeast Asia, this new mechanism will also have a non-negligible effect. Most importantly, this effect is not limited to the distribution of isoprene oxidation products, but also has an important and far-reaching impact on the global distribution of ozone and particulate matter pollution and the self-purification ability of the global atmosphere.

Figure 3: Effect of new atmospheric reaction mechanisms of isoprene on global distribution of isobutyrenal and methyl vinyl ketone.

(Source: Science Network)

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