AIRS retrieved CO2 and its association with climatic parameters over India during 2004-2011

Regional estimation of methane emissions over the peninsular India using atmospheric inverse modelling

Accurate renditions of country-scale methane (CH₄) emissions are critical in understanding the regional CH₄ budget and essential for adapting national climate mitigation policies to curtail the atmospheric build-up of this greenhouse gas with high warming potential. India housing 30% of the Asian population is currently appraised as a region of CH₄ source based on the inventories. To date, there have not been many reported efforts to estimate the regional CH₄ emissions using direct measurements of boundary layer CH₄ concentrations at multiple locations over India. Here, 2 years (2017-2018) of in situ CH₄ observations from three distantly placed stations over the peninsular India is combined with state-of-the-art inversion using a Lagrangian particle dispersion model for the estimation of CH₄ emission. This study updates CH₄ emission over the peninsular India (land area south of 21.5°N) as ~ 10.63 Terra gram (Tg) CH₄ year^-1, which is 0.13 Tg CH₄ year^-1 higher than the existing inventory-based emission. On seasonal scale, the changes from the existing CH₄ emission inventories are 0.12, 0.05, 0.055 and 0.28 Tg CH₄ year^-1 during winter, pre-monsoon, monsoon and post-monsoon seasons respectively. Spatial distributions of seasonal variability of posterior emissions suggest an enhancement over the eastern region of peninsular India compared to the western part. The study with observations from three stations over the peninsular India provides an update on the inventory-based estimation of CH₄ emissions and urges the importance of more observations over the Indian region for the accurate estimation of fluxes.

Benefits of satellite XCO2 and newly proposed atmospheric CO2 observation network over India in constraining regional CO2 fluxes

Top-down modeling estimates are among the most reliable information available on the CO₂ fluxes of the earth system. The inadequate coverage of CO₂ observing stations over the tropical regions adds a limitation to this estimate, especially when the satellite XCO₂ is strictly screened for cloud contamination, aerosol, dust, etc. In this study, we investigated the potential benefit of a global ground-based observing station network, 17 newly proposed stations over India, and global satellite XCO₂ in reducing the uncertainty of terrestrial biospheric fluxes of Tropical Asia-Eurasia in TransCom cyclo-stationary inversion. The data from selected 80 global ground-based CO₂ observation stations, together with two additional stations from India (i.e., Cape Rama and Sinhagad) and satellite XCO₂, helps to reduce the temperate Eurasian terrestrial flux uncertainty by 23.8%, 26.4%, and 36.2%, respectively. This further improved to 54.7% by adding the newly proposed stations over India into the inversion. By separating the Indian sub-continent from temperate Eurasia (as inspired by the heterogeneity in the terrestrial ecosystems, prevailing meteorological conditions, and the orography of this vast region), the inversion evinces the capacity of existing CO₂ observations to reduce the Indian terrestrial flux uncertainty by 20.5%. The largest benefit (70% reduction of annual mean uncertainty) for estimating Indian terrestrial fluxes could be achieved by combining these global observations with data from the newly proposed stations over India. The existing two stations from India suggest Temperate Eurasia as a mild source of CO₂ (0.33 ± 0.57 Pg C yr^-1), albeit with prominent anthropogenic influences visible in these two stations during the dry seasons. This implies that the proposed new stations should be cautiously placed to avoid such effects. The study also finds that the newly proposed stations over India also have an impact in constraining nearby oceanic CO₂ fluxes.

Dynamics of atmospheric carbon dioxide over different land cover types in India

This study presents an analysis of high-resolution space borne retrievals of the column-averaged dry-air mole fraction of carbon dioxide [Formula: see text] and the role of vegetation in controlling atmospheric CO₂ dynamics over the Indian region. Nadir and glint mode [Formula: see text] retrievals from the Orbiting Carbon Observatory-2 (OCO-2) spectrometer for the period September 2014-July 2017 are studied with satellite-derived normalized difference vegetation index (NDVI) and rainfall over different land cover types. The atmospheric [Formula: see text] variability shows a strong negative correlation with satellite-derived NDVI. Higher rainfall favours the vegetative growth and photosynthetic activity, thus lowers atmospheric [Formula: see text] concentration. The mean monthly [Formula: see text] over terrestrial region of India is observed as 400.18 ± 3.85 ppm with seasonal variations over different land cover types. The correlation of [Formula: see text] with NDVI over mixed forest and deciduous broadleaf forest types was estimated as - 0.86, - 0.76 (p < 0.01), respectively. The study could also highlight strong seasonal [Formula: see text] variability at higher latitudinal zones of India. It is significant to note that space-based observations indicate that atmospheric carbon dioxide levels have surpassed a significant milestone of 400 ppm in recent times. The study provides significant inputs towards improving our understanding of terrestrial biogeochemical carbon cycle over India.

Important evidence of constant low CO2 windows and impacts on the non-closure of the greenhouse effect

The CO₂ distribution in the atmosphere remains unclear for the complexity of the long-range vertical transport process and other influencing factors. In this work, regression analysis was used to verify the accuracy of CO₂ concentrations datasets. Geostatistical analyses were used to investigate the spatiotemporal distributions of CO₂ at 7 levels from near the surface to the mid-troposphere (0~5 km). Spatial correlation and time series analyses were used to further determine the diffusion characteristics of the CO₂ concentration based on the horizontal wind (NCEP R2), which is one of the main driving factors. The results showed that the horizontal, not vertical, diffusion of CO₂ becomes increasingly more prominent with the decrease in atmospheric pressure to the mid-troposphere, whereas many regions, such as the Rocky Mountains and Qinghai-Tibet Plateau, have constant low values throughout the year due to the influence of high topography (up to 10.756 ppmv lower than that near the surface). These areas form low CO₂ concentration 'windows' keeping letting thermal infrared energy out into space. This study is the first to question the existing view of the closure of the 'greenhouse effect'. Future research studies should more precisely determine the closure threshold and the uncertainties about the surface fluxes.

The Global Spatiotemporal Distribution of the Mid-Tropospheric CO2 Concentration and Analysis of the Controlling Factors

The atmospheric infrared sounder (AIRS) provides a robust and accurate data source to investigate the variability of mid-tropospheric CO2 globally. In this paper, we use the AIRS CO2 product and other auxiliary data to survey the spatiotemporal distribution characteristics of mid-tropospheric CO2 and the controlling factors using linear regression, empirical orthogonal functions (EOFs), geostatistical analysis, and correlation analysis. The results show that areas with low mid-tropospheric CO2 concentrations (20°S–5°N) (384.2 ppm) are formed as a result of subsidence in the atmosphere, the presence of the Amazon rainforest, and the lack of high CO2 emission areas. The areas with high mid-tropospheric CO2 concentrations (30°N–70°N) (382.1 ppm) are formed due to high CO2 emissions. The global mid-tropospheric CO2 concentrations increased gradually (the annual average rate of increase in CO2 concentration is 2.11 ppm/a), with the highest concentration occurring in spring (384.0 ppm) and the lowest value in winter (382.5 ppm). The amplitude of the seasonal variation retrieved from AIRS (average: 1.38 ppm) is consistent with that of comprehensive observation network for trace gases (CONTRAIL), but smaller than the surface ground stations, which is related to altitude and coverage. These results contribute to a comprehensive understanding of the spatiotemporal distribution of mid-tropospheric CO2 and related mechanisms.

Variability in AIRS CO2 during active and break phases of Indian summer monsoon

Due to human activities, the atmospheric concentration of Carbon Dioxide (CO2) has been rising extensively since the Industrial Revolution. Indian summer monsoon (ISM) has a dominant westerly component from ocean to land with a strong tendency to ascend and hence may have role in CO2 distribution in lower and middle troposphere over Indian sub-continent. A substantial component of ISM variability arises from the fluctuations on the intra-seasonal scale between active and break phases which correspond to strong and weak monsoon circulation. In view of the above, an attempt is made in this study to examine the AIRS/AQUA satellite retrieved CO2 distribution in response to atmospheric circulation with focus on active and break phase. Correlation analysis indicates the increase in AIRS CO2 linked with strong monsoon circulation. Study also reveals that anomalous circulation pattern during active and break phase show resemblance with high and low values of AIRS CO2. Homogeneous monsoon regions of India show substantial increase in CO2 levels during active phase. Hilly regions of India show strong contrast in CO2 and vertical velocity during active and break phases.