DIURNAL AND SEASONAL VARIATION OF MEASURED ATMOSPHERIC CO₂ AT DEHRADUN DURING 2009

Abstract. Atmospheric carbon dioxide (CO₂) is the major greenhouse gas. We analysed the diurnal trend and seasonal variations of atmospheric Carbon Dioxide (CO₂) measured at Dehradun during 2009. The measurements are taken from the Vaisala CO₂ probe installed at the top of the IIRS building. A distinct diurnal cycle with seasonal variations is present in these measurements. Diurnal cycle exhibits a half sinusoidal variation with decreasing phase during sunrise to afternoon and increasing phase during afternoon to sunset. CO₂ mostly remains constant with higher values during the night. The largest negative anomaly with respect to the annual mean varies between +/-20 ppmv with positive anomaly during the night time and negative anomaly during the day time. The cause of such variation could be attributed to the local ecosystem activity. During the day time the sinking of CO₂ through the photosynthesis dominates its release through autotrophic and heterotrophic respiration. On the other hand, photosynthesis has been ceased during the night time and respiration only controls the night variation of CO₂.There exists significant change on the diurnal cycle of CO₂ in different seasons during the year. The maximum rapid decrease in CO₂ concentration in early morning hours is due to photosynthetic activity during the monsoon periods (maximum in August -8ppm). During monsoon period rapid night time increase of CO₂ is observed due to the enhancement of respiration. An inverse relationship exist between model simulated net ecosystem productivity and CO₂ deviations on monthly basis. This diurnal and seasonal variations in situ CO₂ are in confirmation with model simulated results.