Diurnal and seasonal variation of mixing ratio and δ13C of air CO2 observed at an urban station Bangalore, India

We present here observations on diurnal and seasonal variation of mixing ratio and δ¹³C of air CO₂, from an urban station—Bangalore (BLR), India, monitored between October 2008 and December 2011. On a diurnal scale, higher mixing ratio with depleted δ¹³C of air CO₂ was found for the samples collected during early morning compared to the samples collected during late afternoon. On a seasonal scale, mixing ratio was found to be higher for dry summer months (April–May) and lower for southwest monsoon months (June–July). The maximum enrichment in δ¹³C of air CO₂ (−8.04 ± 0.02‰) was seen in October, then δ¹³C started depleting and maximum depletion (−9.31 ± 0.07‰) was observed during dry summer months. Immediately after that an increasing trend in δ¹³C was monitored coincidental with the advancement of southwest monsoon months and maximum enrichment was seen again in October. Although a similar pattern in seasonal variation was observed for the three consecutive years, the dry summer months of 2011 captured distinctly lower amplitude in both the mixing ratio and δ¹³C of air CO₂ compared to the dry summer months of 2009 and 2010. This was explained with reduced biomass burning and increased productivity associated with prominent La Nina condition. While compared with the observations from the nearest coastal and open ocean stations—Cabo de Rama (CRI) and Seychelles (SEY), BLR being located within an urban region captured higher amplitude of seasonal variation. The average δ¹³C value of the end member source CO₂ was identified based on both diurnal and seasonal scale variation. The δ¹³C value of source CO₂ (−24.9 ± 3‰) determined based on diurnal variation was found to differ drastically from the source value (−14.6 ± 0.7‰) identified based on seasonal scale variation. The source CO₂ identified based on diurnal variation incorporated both early morning and late afternoon sample; whereas, the source CO₂ identified based on seasonal variation included only afternoon samples. Thus, it is evident from the study that sampling timing is one of the important factors while characterizing the composition of end member source CO₂ for a particular station. The difference in δ¹³C value of source CO₂ obtained based on both diurnal and seasonal variation might be due to possible contribution from cement industry along with fossil fuel / biomass burning as predominant sources for the station along with differential meteorological conditions prevailed.