SATELLITE VALIDATION USING OZONE LIDAR PROFILES

Observations of tropospheric O3 and precursor gases (e.g., CO, NO2, formaldehyde (HCHO)) have been made from space-borne platforms which have led to the better understanding of the tropospheric O3 budget (Sauvage et al., 2007; Martin, 2008; Duncan et al., 2014). Total column (stratosphere + troposphere) O3 has been routinely measured by numerous space-based sensors since the launch of the Total Ozone Mapping Spectrometer (TOMS) in 1978. Tropospheric column O3 has been derived from total column retrievals using strategies such as residual-based approaches which subtract the stratospheric column O3 from total O3 (Fishman et al., 2008 and references therein). Tropospheric O3 profiles have also been directly retrieved from hyperspectral UV (e.g., Liu et al., 2005, 2010) and Thermal Infrared (TIR) (e.g., Bowman et al., 2006) measurements. Currently, sensors measuring tropospheric O3, such as those using UV measurements from the Ozone Monitoring Instrument (OMI) and TIR measurements from the Tropospheric Emission Spectrometer (TES) (Beer, 2006), are from low earth orbit (LEO). While LEO provides global coverage, the observation of tropospheric O3 is limited by coarse spatial resolution, limited temporal frequency (once or twice per day), and inadequate sensitivity to lower tropospheric and planetary boundary layer (PBL) O3 (Fishman et al., 2008; Natraj et al., 2011).

The Tropospheric Emissions: Monitoring of Pollution (TEMPO) satellite, which will be launched between 2019-2021 to geostationary orbit (GEO), is designed to address some of the limitations of current O3 remote-sensing instruments (Chance et al., 2013; Zoogman et al., 2017). TEMPO will provide critical measurements such as vertical profiles of O3, total column O3, NO2, sulfur dioxide, HCHO, glyoxal, and aerosol/cloud parameters over North America. These data products will be provided hourly at a native spatial resolution of ~2.1 x 4.4 km² (at the center of the field of regard) except at the required spatial resolution of 8.4 x 4.4 km² for the O3 profile product (four pixels combined to increase signal to noise ratios and reduce computational resources). TEMPO's domain will encompass the region of North America from Mexico City to the Canadian oil sands and from the Atlantic to the Pacific Ocean. TEMPO will have increased sensitivity to lower tropospheric O3 compared to past/current satellite data by combining measurements from both UV (290-345 nm) and visible (VIS, 540-650 nm) wavelengths (Natraj et al., 2011; Zoogman et al., 2017). The operational TEMPO O3 product will provide vertical profiles and partial O3 columns at ~24-30 layers from the surface to ~60 km above ground level. This product will also include total, stratospheric, tropospheric, and a 0-2 km above ground level O3 columns. TEMPO's high spatial and temporal resolution measurements, including the 0-2 km O3 column, will provide a wealth of information to be used in air quality monitoring and research.

Because of their high spatial (usually 100-500 m in the troposphere) and temporal resolutions (few minutes - half hour), ground-based ozone lidar profiles are ideal sources to validate the tropospheric ozone observations from satellites, especially in the lowermost troposphere (LMT). Using the O3 lidar measurements from multiple TOLNet stations, we have evaluated some potential sources of a priori O3 profiles for use in TEMPO: a tropopause-based O3 climatology (TB-Clim) from ozonesonde observations and O3 profiles from 3 separate models (GEOS-5 forward processing product, MERRA2 reanalysis product, and CTM product). We found that all potential sources of a priori profiles evaluated in this study generally reproduced the vertical structure of summer-averaged observations of O3 profiles. However, larger differences between the a priori profiles and lidar observations were observed when evaluating inter-daily and diurnal variability of tropospheric O3. The TB-Clim O3 profile climatology was unable to replicate observed inter-daily and diurnal variability of O3 while model products, in particular GEOS-Chem simulations, displayed more skill in reproducing these features. Due to the ability of models, primarily the CTM used in this study, on average to capture the inter-daily and diurnal variability of tropospheric and LMT O3 columns, using a priori profiles from these model simulations resulted in TEMPO retrievals with the best statistical comparison with lidar observations. Furthermore, important from an air quality perspective, when high LMT O3 values are observed, using GEOS-Chem a priori profiles resulted in TEMPO LMT O3 retrievals with the least bias.

For details, please read (Johnson et al., submitted to AMT).

REFERENCES

Johnson, M. S., Liu, X., Zoogman, P., Sullivan, J., Newchurch, M. J., Kuang, S., Leblanc, T., and McGee, T.: Potential sources of a priori ozone profiles for TEMPO tropospheric ozone retrievals, Atmos. Meas. Tech. Discuss., https://doi.org/10.5194/amt-2017-484, in review, 2018.