Ernest Welde says he's been chased, swung at and even pushed by truck drivers, who disapprove of his side gig. From his phone, Welde records trucks that are parked for longer than three minutes with their engines still running, an offense in New York City.

"Every time I go out of my house, I am prepared for an assault," Welde, an environmental attorney by day, told CNBC. "You have to go out expecting there's going to be a confrontation."

Welde's hunt for idling trucks started because of an online citizen reporting program the NYC Department of Environmental Protection launched in 2019. It's called the Citizens Air Complaint Program, and it allows ordinary New Yorkers to receive a monetary reward for their "enforcement efforts."  

Emissions from idling gasoline and diesel motor vehicle engines are known contributors to health problems, including asthma, respiratory issues and cardiovascular harm, according to the agency's website. 

To participate in the program, citizen reporters need to shoot a video showing a commercial vehicle idling for more than three minutes. They then log on to the city's Idling Complaint System to file and track their complaint.

According to the DEP, the fine for a first-time offender is $350, and more for repeat offenders. A 25-percent cut — or $87.50 — is paid to the person who shot the video and filed the complaint.

"I have submitted over 2,000 complaints," said Welde, adding that they're being processed.

Lucrative side hustle

CNBC spoke to other clean-air vigilantes, who are recording idle trucks as a side hustle. They include a pediatrician, a former Wall Street banker and an actress named Rachael Opendaker.

"Just from walking to work, I can get five, or six within a few minutes," Opendaker told CNBC. "I [pretend] I'm on my phone and kind of look around, look like I'm looking for someone."

Patrick Schnell, a pediatrician, estimates he's made $36,000 to date. His technique is to hold his phone in his hand and "try to be inconspicuous," he said. It "kind of works, but not always."

An 81-year-old New Yorker named Paul Slapikas said he's collected $64,000 so far "without even trying." 

"There are idling trucks everywhere," Slapikas said. "Currently, I'm waiting for 42 bounty requests, amounting to $7,300 to be paid."

'Knives pulled on me'

According to the Department of Environmental Protection, the city has collected $2.4 million since the online program started. Collectively, citizen reporters have earned more than $724,000 and counting. The DEP says the number of complaints surged 35% from 9,070 in 2019 to 12,267 in 2021. 

While the program is a revenue generator, it's not always the safest way to make a quick buck.

"I had knives pulled on me three times," Slapikas said, "and never spilled even a drop of blood."

A spokesperson for the DEP says the city is not responsible should a citizen be attacked in a "voluntary act" to report an idling vehicle. 

Former Wall Street banker George Pakenham estimates he's made about $40,000 since the program started, but he said he's not in it for the cash.

"It's a public health issue," Pakenham said.

As a medical professional, Schell, agrees.

"I really do it because I see how bad the air pollution is, especially in big cities where there's a lot of traffic," Schell said.

The DEP's spokesperson said in an email that, in addition to citizen reporters, there are about 70 inspectors who enforce the noise and air codes in the city. 

For the clean-air vigilantes, there's plenty of work ahead.

"Any commercial vehicle that is not engaged in a process and is parked must turn the engine off," said Welde. "I won't stop [reporting trucks] until there's no more idling in New York City."