Quantifying emissions of greenhouse gases from South Asia through a targeted measurement campaign

Abstract

Methane (CH 4 ), nitrous oxide (N20) and sulfur hexafluoride (SF6) are powerful greenhouse gases with global budgets that are well-known but regional distributions that are not adequately constrained for the purposes of mitigation and policy initiatives. Quantifying emissions using inverse approaches at the national scale requires measurements that specifically target the region of interest. Primarily due to the lack of atmospheric measurements from the region, emissions estimates of these greenhouse gases from India have largely been missing. New in situ measurements of atmospheric mole fractions from a Himalayan station in Darjeeling, India (27.03'N, 88.26'E, 2200 meters above sea level) have been collected from December 2011 for CH4 and March 2012 for N20 and SF6 to February 2013 using high-precision instrumentation that is linked to the Advanced Global Atmospheric Gases Experiment (AGAGE). These measurements comprise the first high-frequency dataset of these gases collected in India and are used for measurement-based assessment of emissions. Several features are identified. In SF6 , the signal associated with Northern Hemispheric background is typically present. CH4 and N20 mole fractions are almost always enhanced over the background, suggesting strong regional sources. Additionally, a diurnal signal resulting from thermally driven winds is seasonally present. A particle dispersion model is used to track 'air histories' of measurements, quantifying the sensitivity of concentrations at Darjeeling to surface emissions. The effect of topography on the derived air histories is investigated to test the robustness of the model in simulating transport in this complex environment. The newly acquired data set is used to investigate the ability of the model to reproduce signals that stem from the mesoscale diurnal winds. The sensitivities of meteorological resolution and particle release height are investigated to better quantify some of the uncertainties associated with this chemical transport model. A Quasi-Newton inverse method is used to estimate emissions at monthly resolution. CH4 , N20 and SF6 emissions from India are found to be 44.3% Tg yr- 1, 825 1045/707 GgN yr- 1 and 221 241/205 kton yr-', respectively. Significant uncertainty reduction is seen on emissions from India during the summer when the monsoon results in high sensitivity over the subcontinent.