In February 1939 Kuo Yao-Nan served in the 17th PS, which was equipped with Polikarpov I-15bis

At 13:50 on 20 February 1939, the Air Defence Command at Sian notified the 1st Air District Command that 30 Japanese aircraft (Mitsubishi Ki-21s and Fiat BR.20s) were heading towards Lanzhou in the Kansu province in Northwest China.
Nine I-15bis from 17th PS were scrambled first to intercept the Japanese aircraft and was later joined by two more. Later Russian volunteers were scrambled at 14:05 with seven I-15bis and one I-16. At 14:10 four I-15bis from the 15th PS, one I-16 and one Curtiss Hawk took off from the Siku field.
The first formation of nine Japanese aircraft attacked the Lanzhou airfield from north-west at 14:50. The second formation of twelve aircraft attacked from west at 15:15 together with a third formation of nine aircraft.
Captain Shen Tse-Liu, commander of the 17th PS, shot down the lead aircraft of a squadron of nine Japanese aircraft. His vice-commander Ma Kwok-Lim shot down the second aircraft of the third flight. Kuo Yao-Nan downed one Japanese aircraft. They were soon joined by the I-15bis flown by Russians and the 15th PS led by Lee Teh-Piao and Chang Tang-Tien. Shen later shot down another aircraft from the third Japanese formation.
Nine of the Russian pilots taking part in this combat was Petr Abramov (I-16), kapitan Nikolay Gavrilov (I-16), leitenant Konstantin Gromov (Polikarpov I-15bis or I-16), leitenant Konstantin Kanteyev (Polikarpov I-15bis or I-16), Mikhail Naydenko (Polikarpov I-15bis or I-16), Vladimir Vasilievich Zolotarev (Polikarpov I-15bis or I-16), leitenant Sergey Donchenko (I-16), starshiy leitenant Timofey Ovsyannikov (I-15bis) and leitenant Andrey Petrunin (I-15bis), who all eight are credited with nine shared enemy bombers over Lanzhou during the day. This indicate that all the Russian volunteers taking part received credits for all the claimed aircraft and that no individual claims were recorded.
Abramov was to become an ace during the Great Patriotic War when he claimed five victories (and a total of 25 shared) before being killed in air combat on 11 September 1941. Naydenko also became an ace during the Great Patriotic War when he claimed seven victories. Zolotarev claimed two victories during the Soviet-Finnish Winter War of 1939-1940.
On this day, the bombs fell on the city and not the airbase and totally nine of the bombers were claimed destroyed by the returning Chinese pilots. One of the Soviet volunteers was wounded during the combat. The Air Defence Command at Sian observed only 21 Japanese aircraft retreating to the east after the battle.
It seems that the Japanese bombers were from the 12th, 60th and 98th Kokutais.

Kuo Yao-Nan ended the war with 1 biplane victory.

Claims:

Kill no.	Date	Time	Number	Type	Result	Plane type	Serial no.	Locality	Unit
	1939
1	20/02/39	14:50-	1	Enemy bomber (a)	Destroyed	I-15bis		Lanzhou area	17th PS

Biplane victories: 1 destroyed.
TOTAL: 1 destroyed.
(a) Probably a Mitsubishi Ki-21 or Fiat BR.20. It seems that the Japanese bombers were from the 12th, 60th and 98th Kokutais.

Sources:
Air Operations in the China Area (July 1937-August 1945)- Military History Section Headquarters, Army Forces Far East, Historica Aviation Publications
All aces of Stalin 1936-1953 – Mikhail Bykov, 2014
Soviet Fighters in the sky of China, Part II - Anatolii Demin, 2000 Aviatsiia Kosmonavtika 10 (translated by George M. Mellinger)
Soviet Fighters in the sky of China, Part IV - Anatolii Demin, 2000 Aviatsiia Kosmonavtika 11 (translated by George M. Mellinger)
Additional Information kindly provided by Tom Chan.