The Bihar pollution pool is a large wintertime increase in pollutants over the eastern parts of the Indo Gangetic basin. We use improved carbon monoxide (CO) retrievals from the recent Measurements of Pollution in the Troposphere (MOPITT) version 4 data along with the aerosol data from the latest version 3 of the Cloud-Aerosol Li-5 dar and Infrared Pathfinder Satellite Observations (CALIPSO) lidar instrument and the tropospheric ozone residual products from the Total Ozone Mapping Spectrometer

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... TOMS)/Solar Backscattered Ultraviolet (SBUV) and Ozone Monitoring Instrument (OMI)/Microwave Limb Sounder (MLS) database to characterize this pollution pool. The feature is seen primarily in the lower troposphere from about November to Febru-10 ary with strong concomitant increase in CO, aerosol optical depth and tropospheric ozone columns. The height resolved aerosol data from CALIPSO confirm the trapping of the pollution pool at the lowest altitudes. The observations indicate that MOPITT can capture this low altitude phenomenon even in winter conditions as indicated by the averaging kernels. Fishman et al. (2003) had found high tropospheric ozone residual (TOR) along this basin all through the year. The large sources of emission along the basin has implications also for regional pollution, as deep convection during the monsoon months can efficiently lift the pollution to the upper troposphere which can then be transported westward along the southern edge of the Tibetan anticyclonic circulation. The latter 5 20889 Abstract 25 900 hPa to 100 hpa) from the version 4 MOPITT data. We also use aerosol products retrieved by the CALIPSO lidar instrument which has been providing height resolved 20890 Abstract ACPD Abstract ACPD Abstract 25 winter time aerosols are trapped within the boundary layer. The vertical feature mask shown in Fig. 6b indicates primarily aerosol layers over the land from 20 • N to ∼28 • N Abstract 25 nental and smoke. While dust coming from the northwestern parts declines during the monsoon months, contribution from locally generated dust becomes important during the fall and winter months. This is consistent with the findings of Dey and Di Girolamo (2010), who attribute the low Angstrom exponent and slightly higher non-spherical 20895 Abstract ACPD Abstract 25 altitude ozone in this area in winter. Both TOR (1979TOR ( -2005 and TCO (2004TCO ( -2008 climatologies show enhanced ozone over BPP during the winter (December, January and February) months (not shown). While both show regional enhancements of about Abstract Furthermore, the TOR and TCO residual ozone column data show similar spatial distribution of enhanced ozone over this area. The latter indicates that the TOR and Abstract 25 product: algorithm enhancements, validation and long-term stability, J. Geophys. Res., 115, D07036, Abstract Jethva, H., Satheesh, S. K., and Srinivasan, J.: Seasonal variability of aerosols over the Indo-