In 2013, severe haze weather occurred continuously across the country, causing great harm to people's travel and health. Therefore, research on NO2, SO2, CO, etc.

(1) ① Flue gas containing SO2 can be desulfurized by seawater. The purpose of introducing air into the aeration tank is to oxidize H2SO3, HSO3-, etc. into SO42-, so the answer is: H2SO3, HSO3-, etc. HSO3-, etc. are oxidized to SO42-;

② From the previous question, it can be seen that the sulfate concentration will definitely increase. Natural seawater is weakly alkaline. After the air is introduced, the seawater in the aeration tank will generate acidic substances. When HCO3- is consumed, HCO3- will decrease, so the answer is: a;

(3) ① According to the movement direction of anions and cations in the electrolytic cell: it can be judged that cations move towards the cathode ① The a pole in the figure must be connected to the power supply The negative electrode, SO32- loses electrons at the anode and becomes SO42-, so the substance flowing out of port C is H2SO4,

So the answer is: negative; sulfuric acid;

②SO32- loses electrons and is Oxidized to SO42-, the electrode reaction formula is: SO32--2e-+H2O=SO42-+2H+, so the answer is: SO32--2e-+H2O=SO42-+2H+;

③At the cathode H+ is discharged to generate H2, c (H+) decreases, and the ionization balance of water H2O?H++OH- moves forward, so the alkalinity increases,

So the answer is: H2O?H++OH- , H+ is discharged at the cathode to generate H2, c (H+) decreases, the ionization balance of water moves forward, and the alkalinity increases;

(3) Since Ka2=5.61×10-11

So the answer is: CO32-+H2S=HCO3-+HS-.