Tropospheric Greenhouse Gases: Emission Characteristics and Dynamical Effect

Abstract

The urban areas consist of more than half of the world’s population and contribute about 70% of the fossil fuel CO2 emissions (in 2010) as well as significant amount of anthropogenic emissions of CH4 into the atmosphere. India is a fast developing country, where fossil fuel emissions have increased dramatically in last three decade and further predicted to continue to grow at least 6% per year through to 2025. In order to provide independent verification of future mitigation activities and predicting future climate, there is an urgent need of the measurements of greenhouse gases over representative urban regions

Realising the importance of the study of greenhouse gases over urban locations, measurements of CO2 and CH4 along with an anthropogenic emission tracer CO have been initiated at a major urban site Ahmedabad in India, using a state of the art laser based cavity ring down spectrometer. The observed year-long data are analysed using suitable global chemistry transport models (JAMSTECs ACTM for CO2 and CH4, LMDzð€€€ORð€€€INCA for CH4), and correlations among them. The effects of land ecosystem on CO2 variation as well as anthropogenic emissions and atmospheric transport on CO2 and CH4 variations have been identified. The inventory (EDGAR) emissions of CH4 and CO are tested using the observed data for the study location and found that the inventory emissions are underestimated. The CO2 observations and model comparison brings out the need for improvement in the terrestrial flux simulated by the Carnegie-Anes-Stanford Approach (CASA) ecosystem model. Furthermore, comparison of the seasonal cycle of CH4 with both models revel that the afternoon mixing ratios have the potential to represent the footprint of its emission of larger area and hence this data can be used in regional and global CH4 inversion study with some caution.

Analyses of vertical profiles of CO2 and CO in the troposphere using aircraft measurements of CO2 over Delhi and satellite data of CO over five selected regions in India are also conducted. The amplitude of seasonal variability in CO2 shows direct, but a delayed link with the strength of the Indian summer monsoon rainfall in Delhi. The model simulations are more close to the observations in the upper troposphere (3-8 km) as compared to lower troposphere (below 3 km). Average seasonal distributions of CO in the troposphere over all the study regions show very large spatial and vertical variability. Significant contribution at 300 ð€€€ 200 hPa, due to strong convection during the monsoon season as well as long range transport from the biomass burning regions of Central Africa and East Asia during other seasons particularly over south of Ahmedabad, have been observed. The annual variations in the CO concentrations at 900 and 300 hPa are found to be in opposite phase with lower values during monsoon at 900 hPa and higher values at 300 hPa. Simulations from two photochemical-transport models (MOZART and EMAC) are able to capture these variations broadly

Measurements of CO2 and CH4 coupled with the atmospheric transport models could be used to assess the sources and sinks of CO2 and CH4 at regional level by inverse modeling studies. This is, however an objective for the future, since it can only be achieved using data records covering multiple years and at a network of stations. In summary, this thesis contributes in providing precise atmospheric measurements of important GHGs - CO2 and CH4 along with the anthropogenic tracer CO over an urban location in India which is not done so far to the best of our knowledge. This dataset is very helpful for understanding processes and phenomena related to the land-atmosphere exchange of CO2, constraining the CH4 and CO emission inventories as well as understanding the contributions of anthropogenic sources in a mega-city to observed variations.