UNESA Ketintang Street Campus, Surabaya, is the biggest campus in South Surabaya. Academic activity on the UNESA Ketintang Street Campus to some extent contributed to the increase of CO₂ concentration in the Surabaya City. The increase of CO₂ concentration can be balanced by Green Open Space. This study goal is to determine the adequacy of Green Open Space in Unesa Ketintang Street Campus in reducing CO₂.

The study used quantitative research method. The study location was UNESA Ketintang Street Campus, Surabaya. Data acquisition is by observation technique. Observation was carried out for 6 days in July 2021. Analysis Unit uses Box Model. Adequity of existing Green Open Space was analysed by consideration of CO₂ reduction, the reduction indicator of CO₂ concentration (Net_CO₂-Con) in cumulative concentration of CO2 on ambient air.

The result of study shows that from 7 point of observation, the average CO₂ concentration on ambient air is around 346 – 570 ppmV. The value of reduction of CO₂ concentration (Net_CO₂-Con) is around -19,46 to -87,69 ppmV. The minus sign (-) on the reduction value shows that Green Open Space in UNESA Ketintang Street Campus, Surabaya, was adequated to reduce CO₂ concentration on ambient air.