Would a “City Tree” that is like a Green Wall, provide an environmental impact equal to that of up to 275 normal urban trees.  In any case, the generalisation of the “City Tree” and use them like a Green Wall has begun being used to filter toxic pollutants from the air of some cities in Europe and possibly alleviate Climate Change.
The decision of Donald Trump to withdraw the US from the Paris Agreement was not a surprise. It was part of his electoral campaign anyway.  It is still too early to assess the consequences of the withdrawal from a text that will not take effect until 2020.  By the way, that will be election year when D. Trump would go back to the polls for presumably a possible second mandate.
Meanwhile, China’s commitment would be beside the fact that its leaders have expressed their disappointment and concern at the announced withdrawal of America from the Paris Agreement, part of all those countries who intend to support it.
There are several reasons for this.
The first reason is institutional. For Beijing an agreement signed by several countries in an international forum is worth more to the Chinese leaders than bilateral commitments.  The second reason is environmental and is obvious that pollution is at unprecedented levels in that country, recurrently causing social tensions mostly related to the quality of air or water, power or respect for nature.
Would we be wrong if we assume that D. Trump envisages building a number of these walls?
An CNN article written by Chris Giles, CNN and updated on June 8, 2017 is about a particular solution that could make some difference to the issue of air pollution.  It is per this article already being put to use in Europe. 

This ‘tree’ has the environmental benefits of a forest

The “CityTree” has the same environmental impact of up to 275 normal urban trees. Using moss cultures that have large surface leaf areas, it captures and filters toxic pollutants from the air.

CNN) Air pollution is one of the world’s invisible killers.

It causes seven million premature deaths a year, making it the largest single environmental health risk, according to the World Health Organization.

In urban areas, air quality is particularly problematic. More than 80% of people living in areas where pollution is monitored are exposed to air quality levels that exceed WHO limits. And given that by 2050 two thirds of the global population will be urban, cleaning up our cities’ air is a matter of urgency.

One well-established way to reduce air pollutants is to plant trees, as their leaves catch and absorb harmful particulates.

But planting new trees is not always a viable option.

That’s why the “CityTree”, a mobile installation which removes pollutants from the air, has been popping up in cities around the world, including Oslo, Paris, Brussels and Hong Kong.

A CityTree in Paris, France.

Moss is in the air

Each CityTree is just under 4 meters tall, nearly 3 meters wide and 2.19 meters deep, available in two versions: with or without a bench. A display is included for information or advertising.

Berlin-based Green City Solutions claims its invention has the environmental benefit of up to 275 actual trees.

But the CityTree isn’t, in fact, a tree at all — it’s a moss culture.

“Moss cultures have a much larger leaf surface area than any other plant. That means we can capture more pollutants,” said Zhengliang Wu, co-founder of Green City Solutions.

The CityTree includes Wi-fi enabled sensors that measure the local air quality.

The huge surfaces of moss installed in each tree can remove dust, nitrogen dioxide and ozone gases from the air. The installation is autonomous and requires very little maintenance: solar panels provide electricity, while rainwater is collected into a reservoir and then pumped into the soil.

To monitor the health of the moss, the CityTree has sensors which measure soil humidity, temperature and water quality.

“We also have pollution sensors inside the installation, which help monitor the local air quality and tell us how efficient the tree is.” Wu said.

Its creators say that each City Tree is able to absorb around 250 grams of particulate matter a day and contributes to the capture of greenhouse gases by removing 240 metric tons of CO2 a year.

Read more in the original document.