Temporal variations of atmospheric CO₂ and CO at Ahmedabad in western India

Abstract. About 70 % of the anthropogenic carbon dioxide (CO₂) is emitted from the megacities and urban areas of the world. In order to draw effective emission mitigation policies for combating future climate change as well as independently validating the emission inventories for constraining their large range of uncertainties, especially over major metropolitan areas of developing countries, there is an urgent need for greenhouse gas measurements over representative urban regions. India is a fast developing country, where fossil fuel emissions have increased dramatically in the last three decades and are predicted to continue to grow further by at least 6 % per year through to 2025. The CO₂ measurements over urban regions in India are lacking. To overcome this limitation, simultaneous measurements of CO₂ and carbon monoxide (CO) have been made at Ahmedabad, a major urban site in western India, using a state-of-the-art laser-based cavity ring down spectroscopy technique from November 2013 to May 2015. These measurements enable us to understand the diurnal and seasonal variations in atmospheric CO₂ with respect to its sources (both anthropogenic and biospheric) and biospheric sinks. The observed annual average concentrations of CO₂ and CO are 413.0 ± 13.7 and 0.50 ± 0.37 ppm respectively. Both CO₂ and CO show strong seasonality with lower concentrations (400.3 ± 6.8 and 0.19 ± 0.13 ppm) during the south-west monsoon and higher concentrations (419.6 ± 22.8 and 0.72 ± 0.68 ppm) during the autumn (SON) season. Strong diurnal variations are also observed for both the species. The common factors for the diurnal cycles of CO₂ and CO are vertical mixing and rush hour traffic, while the influence of biospheric fluxes is also seen in the CO₂ diurnal cycle. Using CO and CO₂ covariation, we differentiate the anthropogenic and biospheric components of CO₂ and found significant contributions of biospheric respiration and anthropogenic emissions in the late night (00:00–05:00 h, IST) and evening rush hours (18:00–22:00 h) respectively. We compute total yearly emissions of CO to be 69.2 ± 0.07 Gg for the study region using the observed CO : CO₂ correlation slope and bottom-up CO₂ emission inventory. This calculated emission of CO is 52 % larger than the estimated emission of CO by the emissions database for global atmospheric research (EDGAR) inventory. The observations of CO₂ have been compared with an atmospheric chemistry-transport model (ACTM), which incorporates various components of CO₂ fluxes. ACTM is able to capture the basic variabilities, but both diurnal and seasonal amplitudes are largely underestimated compared to the observations. We attribute this underestimation by the model to uncertainties in terrestrial biosphere fluxes and coarse model resolution. The fossil fuel signal from the model shows fairly good correlation with observed CO₂ variations, which supports the overall dominance of fossil fuel emissions over the biospheric fluxes in this urban region.