Abstract: The evaluation of the potential of the synergic reduction of greenhouse gases (GHG) and air pollutants emissions was carried out in the transportation sector, with Tangshan City as a case study, taking into account the GHG emission caused by refrigerant leakage in automobile air conditioning in the synergic emission reduction for the first time. The side effects of phasing out old vehicles were pointed out, and the indicator of synergy degree in the synergic theory was adopted to quantify the synergic reduction capability of the corresponding measures to reduce pollutants and GHG emissions. The results showed that under the implementation of all emission reduction measures, GHG emission from the transportation sector would peak in 2030, and the emission of air pollutants would achieve peak value at the end of the 14th Five-Year Plan. The GHG emission effect of current automobile refrigerant leakage in Tangshan City reached 280 800 tons of CO₂ equivalent, accounting for about 4.7% of the total GHG emission. 50% of the leakage came from automobiles, and the leakage would ramp up with the increase in automobile ownership. Therefore, specified measures should be taken to mitigate the leakage of the air conditioning refrigerant in the operation, maintenance, and scrapping processes. The degree of synergic reduction could be effectively distinguished by using synergy evaluation criteria. The absolute emission reduction of phasing out vehicles of national stage Ⅲ and below was large and had good synergy. Although it may cause an increase in refrigerant leakage during the implementation period, it was able to achieve a synergic reduction of GHG and air pollutants overall. Replacing diesel trucks with natural gas ones and improving the "road to water" ratio had a good reduction synergy degree. However, they had a negative synergy in reducing single air pollutant and GHG emissions, which needed to be carried out simultaneously with other emission reduction measures.