Abstract
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.
Highlights
Carbon dioxide (CO2) is the most important anthropogenically emitted greenhouse gas (GHG) and has increased substantially from 278 to 390 parts per million in the atmosphere since the beginning of the industrial era
The preliminary results of these measurements for a 1-month period have been reported in Lal et al (2015). These detailed measurements are utilized for studying the temporal variations of both gases, their emission characteristics on diurnal and seasonal scales using their mutual correlations, estimating the contribution of anthropogenic and biospheric emission components in the diurnal cycle of CO2 using the ratio of carbon monoxide (CO) to CO2 and roughly estimating the annual CO emissions from the study region
There is no specific direction associated with the high levels of these gases. This probably indicates that the transport sector is an important contributor to local emissions since the measurement site is in the midst of an urban city
Summary
Carbon dioxide (CO2) is the most important anthropogenically emitted greenhouse gas (GHG) and has increased substantially from 278 to 390 parts per million (ppm) in the atmosphere since the beginning of the industrial era (circa 1750). Chandra et al.: CO2 over an urban region about 70 % of global CO2 emissions from anthropogenic sources and are projected to increase further over the coming decades (Duren and Miller, 2012) Measurements over these regions are very helpful for understanding emissions growth as well as verifying the mitigation policies. The preliminary results of these measurements for a 1-month period have been reported in Lal et al (2015) These detailed measurements are utilized for studying the temporal variations (diurnal and seasonal) of both gases, their emission characteristics on diurnal and seasonal scales using their mutual correlations, estimating the contribution of anthropogenic and biospheric emission components in the diurnal cycle of CO2 using the ratio of CO to CO2 and roughly estimating the annual CO emissions from the study region. The ventilation coefficient (VC) is obtained by multiplying wind speed and PBL height, which gradually increases from January and attains the maximum value in June, followed by a decrease until November
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