In fact, I have left the field of pure scientific research for many years and have no idea what the frontier is. So I only say what I usually hear from hearsay. Please correct me if there is anything wrong. One development direction of air pollution research is macro. With the help of satellite data, the total amount and migration path of atmospheric pollutants are deduced through mathematical modeling, and the data of multiple regions are further integrated to establish the evolution and migration model of pollutants in larger regions and even the whole world.

From the perspective of pure research, how to build a more accurate, reasonable and optimized model is enough for many people to struggle for a lifetime. In the field of water pollution, most of the frontier projects I have come into contact with are like this: new methods to deal with pollutants with too high or too low concentration, methods to deal with a few strange, difficult and strange pollutants (such as nitrogen and benzene rings), and so on. In the field of engineering, I came into contact with the membrane method in water treatment, which can hardly be said to be the theoretical frontier in scientific research. Now it is mainly technical optimization, how to reduce costs, improve design accuracy and so on.