Bei Bei the giant panda leaves on Tuesday National Zoo in DC for China

Bei Bei is leaving us on Tuesday afternoon. We knew this day would come and it has arrived. The Smithsonian National Zoo's giant panda Bei Bei is taking a one-way non-stop flight for Chengdu, China.

It's one of his last snacks in the United States before he travels some 15 hours to China. "He's very laid back. Look at him," says Timothy McGrath.

All giant panda cubs living at the zoo move to China at age four, due to an agreement with the China Wildlife Conservation Association. Bei Bei celebrated his fourth birthday in August.

He’ll be accompanied by American zoo keepers for a short time in China while he grows accustomed to his new home. The National Zoo says Bei Bei will eventually enter China’s giant panda breeding program in partnership with the China Wildlife Conservation Association.

McGrath, 68, is one of the dozens of people gathered at the giant panda exhibit at Smithsonian's National Zoo before Bei Bei, the cub born here four years ago, leaves for China as part of a research and breeding program.

Bei Bei's crew packed his favorite treats, including bamboo, apples, pears, carrots, cooked sweet potatoes, biscuits, and water for his journey, so he's all set.

Like most 4-year-olds, Bei Bei will have plenty of snacks for his flight. In his case, that will be bamboo, apples, sweet potatoes, leaf eater biscuits and water.

Visitors flocked to Bei Bei’s enclosure for a farewell on Sunday, snapping their last photos of Bei Bei munching on bamboo and a celebratory ice cake.

McGrath has been coming with his wife from upstate New York every few months to visit pandas since they first arrived to the zoo in the 1970s.

"I'm devastated," McGrath says. "I would have been here in a full-body cast to see him off."

So here we are, with bittersweet feelings to say bye, bye to Bei Bei, the cutest 4-year-old panda that has stolen the hearts of many who came to see him.