Not so much air attack, but ground. The TSRR ran very close to the Chinese border from Khabarovsk to Vladivostok, coming within artillery range at Iman (Dal'nerechensk). If Japanese (or Chinese) ground forces occupied this area they could turn the flank of the Soviet forces defending Vladivostok and all territory north of it, creating a siege. The terrain here is favorable for defense: from the attacker's standpoint, in order to capture the town they would first have to get through marshland (unsuitable for tanks) and then cross the Ussuri River.
In September 1941, the correlation of forces between the Soviet and Japanese armies in this area was as follows:
USSR (35th Army)
Infantry: 35th, 66th, 78th Rifle Divisions, 109th Fortified Region (approx. 100 bunkers over 35 km front, depth 1-10 km)
Artillery: 76th, 187th Corps Artillery Regiments; 362nd, 367th Separate Artillery Battalions
Anti-Air: 110th Separate AA Battalion, 3rd Air Defense Area Brigade
Armored/Mechanized: 29th Motorcycle Regiment, Unnamed Armored Train Battalion
Air: 79th Fighter Aviation Division, 18th Reconnaissance Aviation Squadron
Engineer: 402nd Motorized Engineer Battalion, 3rd, 9th, 20th Motorized Pontoon Bridge Battalions, 60th Separate Sapper Battalion
Japan (5th Army)
Infantry/Cavalry: 11th, 24th, 25th Divisions, 3rd Cavalry Brigade, 6th Independent Garrison Unit, 3rd, 4th, 12th Border Garrison Units
Artillery: 8th Artillery Command, 5th, 7th, 12th, 20th Field Heavy Artillery Regiments, 1st Independent Field Artillery Regiment, 5th, 9th Independent Heavy Artillery Battalions, 6th Independent Rapid Fire (AT) Battalion
Anti-Air: 12th Field Air Defense Command, 53rd, 54th Field AA Battalions, 20th, 24th, 27th Field Machine Cannon Companies
Armored/Mechanized: 2nd Tank Group (4th, 10th, 11th Tank Regiments)
Air: No organic air units (JAAF in Manchuria had its own centralized command structure, the Soviet air force was allocated piecemeal to ground army HQs)
Engineer: 1st Engineer Command (HQ, 22nd, 24th Independent Engineer Regiments), 7th, 23rd Independent Engineer Regiments, 21st, 22nd, 29th Bridging Materials Companies, 13th River Crossing Materials Company, 38th, 42nd, 44th, 45th Field Road Construction Units, 17th Field Duty Unit, 8th Field Construction Unit
In the event of war, the Japanese planned to strengthen the 5th Army to 5 divisions, with 3 more in front reserve and 5 more in theater reserve. Because of the war in Europe, Soviet emergency reserves were small: for the entire Far Eastern Front (i.e, forces east of Skovorodino - 5 armies including the 35th) only 2 tank brigades, 3 artillery regiments with 76.2mm guns, 3 guards mortar regiments, and 5 armored trains were available.