New York City’s Vehicle Emissions Get Absorbed by Its Greenery on Many Summer Days, Study Finds

A recent study of New York City’s vegetation found that a remarkable amount of the city’s vehicle carbon emissions — plus that of some neighboring areas — are absorbed by grasses and trees. On many summer days not only are all of the carbon dioxide emissions from buses, cars and trucks soaked up by the area’s greenery, but some additional emissions are as well.

The study used high resolution local vegetation maps to record small areas of greenery that had been previously overlooked, some in highly developed areas, a press release from Columbia Climate School said. The researchers found that the photosynthetic process of the greenery is contributing a great deal to the exchange of carbon dioxide emissions for sugars and oxygen.

“There is a lot more greenery than we thought, and that’s what drives our conclusion,” said postdoctoral researcher at the Columbia Climate School’s Lamont-Doherty Earth Observatory Dandan Wei, who was lead author of the study, reported Yale E360. “This tells us that the ecosystem matters in New York City, and if it matters here, it probably matters everywhere else.”

Urban areas produce more than 70 percent of the carbon dioxide emitted by humans, with New York City being the third biggest emitter of the greenhouse gas in the world and the biggest in the U.S., the press release said.

The study, “High resolution modeling of vegetation reveals large summertime biogenic CO2 fluxes in New York City,” was published in the journal Environmental Research Letters.

In earlier studies, scientists have mostly examined contiguous areas of grassland and forest, which make up only about 10 percent of the city.

The new aerial radar images used by the researchers mapped the city’s vegetation in six-inch grids that included areas left out in previous representations, like vacant lots, single trees on streets, backyard gardens and other small locations. Around one third of the 837.8 square mile total study area extended beyond New York City’s five boroughs and was split up into approximately 98-foot grids.

“Most people have assumed that New York City is just a grey box, that it’s biogenically dead,” said Lamont-Doherty atmospheric chemist Roísín Commane, a co-author of the paper, in the press release. “But just because there’s a concrete sidewalk somewhere doesn’t mean there’s not also a tree that’s shading it.”

The researchers found that 22 percent of New York City — or 65.64 square miles — is covered by trees, and about 12 percent — or 36.29 square miles — is blanketed in grasses.

In order to determine how all of this greenery affected the city’s carbon dioxide emissions, the researchers looked at data from June to August of 2018, when the New York metro area produced about 14.7 million tons of carbon.

The study found that the biggest carbon dioxide emitters were vehicles and the energy used by buildings. Commane said the average carbon dioxide levels in New York City are frequently 460 parts per million or more, while the global average is about 417.

According to the study, these levels would be greater if it wasn’t for the greenery. In the developed parts of the city it was responsible for almost 85 percent of the daily carbon absorption.

The study found that on a high number of summer days, 40 percent of the carbon emissions from all sources was absorbed. Carbon levels were found to increase in the morning and decrease some in the afternoon. Wei said in cities with climates warmer than New York, vegetation most likely has an even bigger effect.

The research team has plans to expand their study of the city’s trees to species types in order to uncover their specific benefits. Oaks are common in the region, but have been determined to produce a compound — isoprene — that reacts with vehicle emissions to produce the air pollutant ozone.

“More trees are always going to be better, no matter what they are,” Wei said in the press release. “But we could use an assessment of which ones are the best.”