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In a corona discharge the field created in the surrounding gas (air in your case) by the charged object will ionise the gas forming + & - charged particles in it. If the body is negatively charged, it will attract the positively charged ions to itself and discharge them (they will become neutral). The negatively charged particles will be repelled by the body & will stream away from it. The net effect is that the negatively charged body produces negative charges in the surrounding air.