Satellite retrievals of methane (CH₄) using the Atmospheric Infrared Sounder (AIRS) on the EOS/Aqua platform from 2003–2007 show a strong, plume-like enhancement of CH₄ in the middle to upper troposphere over South Asia during July, August and September, with the maximum occurring in early September. Simulations using the global tracer model version 3 (TM3) also show similar seasonal enhancement of CH₄ in the same region. The model results also suggest that this enhancement is associated with transport processes and local surface emissions, thus the observations of tropospheric CH₄ during the monsoon season may be used to constrain the models for a better estimation of Asian CH₄ sources. Further comparisons between the AIRS retrievals and the model simulations suggest a possible overestimate of emissions from rice paddies in Southeast Asia in the scenario with the global emissions from rice of 60 Tg yr⁻¹. Moreover, the observed tropospheric CH₄ enhancement from AIRS provides evidence for the strong transport of atmospheric pollutants from the lower to the upper troposphere in Asia during the monsoon season. The observed rapid disappearance of the local CH₄ maximum in September may provide valuable information for studying the dissipation of the Tibetan anticyclone and the withdrawal of monsoon.

In the past decades, continuous efforts have been made at a national level to reduce Nitrogen Dioxide (NO₂) emissions in the atmosphere over China. However, public concern and related research mostly deal with tropospheric NO₂ columns rather than ground-level NO₂ concentrations, but actually ground-level NO₂ concentrations are more closely related to anthropogenic emissions, and directly affect human health. This paper presents one method to derive the ground-level NO₂ concentrations using the total column of NO₂ observed from the Ozone Monitoring Instrument (OMI) and the simulations from the Community Multi-scale Air Quality (CMAQ) model in China. One year’s worth of data from 2014 was processed and the results compared with ground-based NO₂ measurements from a network of China’s National Environmental Monitoring Centre (CNEMC). The standard deviation between ground-level NO₂ concentrations over China, the CMAQ simulated measurements and in-situ measurements by CNEMC for January was 21.79 μg/m³, which was improved to a standard deviation of 18.90 μg/m³ between our method and CNEMC data. Correlation coefficients between the CMAQ simulation and in-situ measurements were 0.75 for January and July, and they were improved to 0.80 and 0.78, respectively. Our results revealed that the method presented in this paper can be used to better measure ground-level NO₂ concentrations over China.

Mid-upper tropospheric methane (CH₄), as an operational product at NOAA's (National Oceanic and Atmospheric Administration) Comprehensive Large Array-data Stewardship System (CLASS), has been retrieved from the Infrared Atmospheric Sounding Interferometer (IASI) since 2008. This paper provides a description of the retrieval method and the validation using 596 CH₄ vertical profiles from aircraft measurements by the HIAPER Pole-to-Pole Observations (HIPPO) program over the Pacific Ocean. The number of degrees of freedom for the CH₄ retrieval is mostly less than 1.5, and it decreases under cloudy conditions. The retrievals show greatest sensitivity between 100–600 hPa in the tropics and 200–750 hPa in the mid- to high latitude. Validation is accomplished using aircraft measurements (convolved by applying the monthly mean averaging kernels) collocated with all the retrieved profiles within 200 km and on the same day, and the results show that, on average, a larger error of CH₄ occurs at 300–500 hPa. The bias in the trapezoid of 374–477 hPa is −1.74% with a residual standard deviation of 1.20%, and at layer 596–753 hPa the bias is −0.69% with a residual standard deviation of 1.07%. The retrieval error is relatively larger in the high northern latitude regions and/or under cloudy conditions. The main reasons for this negative bias include the uncertainty in the spectroscopy near the methane Q branch and/or the empirical bias correction, plus residual cloud contamination in the cloud-cleared radiances. It is expected for NOAA to generate the CH₄ product for 20 + years using a similar algorithm from three similar thermal infrared sensors: Atmospheric Infrared Sounder (AIRS), IASI and the Cross-track Infrared Sounder (CrIS). Such a unique product will provide a supplementary to the current ground-based observation network, particularly in the Arctic, for monitoring the CH₄ cycle, its transport and trend associated with climate change.