I will be in transit at Nanjing Airport for about 9 hrs for my onward trip to Lanzhou.

Yes, you can go to explore the city during the layover. Take metro S1 from the airport to South Railway Station (about 35 minutes). Then, change to metro line 3 to Confucious Temple Station (about 20 minutes). Afterwards, you can visit Confucious Temple, Qinhuai River, Presidential Palace, etc.

For the return trip, you can take a taxi for about CNY 110 (about 45 minutes). Remeber to return to the airport at least two hours prior to departure! :)