The morning air at Tianqiao Square in Beijing was crisp enough to see one's breath, but by 7 am, a crowd had already gathered. They were waiting for Sun Chubo, a bamboo flute and panpipe performer from the China National Opera and Dance Drama Theater.

There was no stage, no ticket booth, no program. Just a musician, her flutes, and a public square transformed into a concert hall.

Sun has become an unlikely civic phenomenon on social media. Since October, she has given more than 20 such impromptu performances in Beijing, from the Zizhuyuan Park to the Temple of Heaven Park.

The expansion of her stage to include al fresco areas started with a viral social media clip that recorded her playing outdoors in October. Dozens of people stopped to listen, which made her nervous. She then heard someone request a tune for her to play and she developed a rapport with the crowd.

Sun began learning the bamboo flute at the age of 9. By the time she graduated from the Shenyang Conservatory of Music in Shenyang, Liaoning province, she had mostly performed in formal theaters and concert halls.

For Sun, the switch to playing in public parks is profoundly different from the controlled environment of a concert hall.

"On stage, the lights are bright. I can't see the audience's faces," she says. "Here, everything is immediate. I see their smiles or their focused expressions during a sad song. It's intimate."