Millions in China rang in the New Year shrouded in a thick blanket of toxic smog, causing road closures and flight cancellations as 24 cities issued alerts that will last through much of the week. On the first day of 2017 in Beijing, concentrations of tiny particles that penetrate deep into the lungs climbed as high as 24 times levels recommended by the World Health Organization. More than 100 flights were cancelled and all intercity buses were halted at the capital’s airport.