Observing atmospheric formaldehyde (HCHO) from space: validation and intercomparison of six retrievals from four satellites (OMI, GOME2A, GOME2B, OMPS) with SEAC⁴RS aircraft observations over the Southeast US

Lei Zhu¹, Daniel J Jacob^{1

2}, Patrick S Kim², Jenny A Fisher^{3

4}, Karen Yu¹, Katherine R Travis¹, Loretta J Mickley¹, Robert M Yantosca¹, Melissa P Sulprizio¹, Isabelle De Smedt⁵, Gonzalo Gonzalez Abad⁶, Kelly Chance⁶, Can Li^{7

8}, Richard Ferrare⁹, Alan Fried¹⁰, Johnathan W Hair⁹, Thomas F Hanisco⁸, Dirk Richter¹⁰, Amy Jo Scarino¹¹, James Walega¹⁰, Petter Weibring¹⁰, Glenn M Wolfe^{8

12}

Affiliations

¹ John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA.
² Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA.
³ Centre for Atmospheric Chemistry, School of Chemistry, University of Wollongong, Wollongong, NSW, Australia.
⁴ School of Earth and Environmental Sciences, University of Wollongong, Wollongong, NSW, Australia.
⁵ Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium.
⁶ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA.
⁷ Earth System Science Interdisciplinary Center, University of Maryland, College Park, Maryland, USA.
⁸ NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.
⁹ NASA Langley Research Center, Hampton, VA 23681, USA.
¹⁰ Institute of Arctic and Alpine Research, Univ. of Colorado, Boulder, CO, USA.
¹¹ Science Systems and Applications, Inc., Hampton, VA, USA.
¹² Joint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, Maryland, USA.

PMID: 29619044
PMCID: PMC5880299
DOI: 10.5194/acp-16-13477-2016

Observing atmospheric formaldehyde (HCHO) from space: validation and intercomparison of six retrievals from four satellites (OMI, GOME2A, GOME2B, OMPS) with SEAC⁴RS aircraft observations over the Southeast US

Lei Zhu et al. Atmos Chem Phys. 2016.

. 2016;16(21):13477-13490.

doi: 10.5194/acp-16-13477-2016. Epub 2016 Nov 1.

Authors

Affiliations

¹ John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA.
² Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA.
³ Centre for Atmospheric Chemistry, School of Chemistry, University of Wollongong, Wollongong, NSW, Australia.
⁴ School of Earth and Environmental Sciences, University of Wollongong, Wollongong, NSW, Australia.
⁵ Belgian Institute for Space Aeronomy (BIRA-IASB), Brussels, Belgium.
⁶ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA.
⁷ Earth System Science Interdisciplinary Center, University of Maryland, College Park, Maryland, USA.
⁸ NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.
⁹ NASA Langley Research Center, Hampton, VA 23681, USA.
¹⁰ Institute of Arctic and Alpine Research, Univ. of Colorado, Boulder, CO, USA.
¹¹ Science Systems and Applications, Inc., Hampton, VA, USA.
¹² Joint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, Maryland, USA.

PMID: 29619044
PMCID: PMC5880299
DOI: 10.5194/acp-16-13477-2016

Abstract

Formaldehyde (HCHO) column data from satellites are widely used as a proxy for emissions of volatile organic compounds (VOCs) but validation of the data has been extremely limited. Here we use highly accurate HCHO aircraft observations from the NASA SEAC⁴RS campaign over the Southeast US in August-September 2013 to validate and intercompare six retrievals of HCHO columns from four different satellite instruments (OMI, GOME2A, GOME2B and OMPS) and three different research groups. The GEOS-Chem chemical transport model is used as a common intercomparison platform. All retrievals feature a HCHO maximum over Arkansas and Louisiana, consistent with the aircraft observations and reflecting high emissions of biogenic isoprene. The retrievals are also interconsistent in their spatial variability over the Southeast US (r=0.4-0.8 on a 0.5°×0.5° grid) and in their day-to-day variability (r=0.5-0.8). However, all retrievals are biased low in the mean by 20-51%, which would lead to corresponding bias in estimates of isoprene emissions from the satellite data. The smallest bias is for OMI-BIRA, which has high corrected slant columns relative to the other retrievals and low scattering weights in its air mass factor (AMF) calculation. OMI-BIRA has systematic error in its assumed vertical HCHO shape profiles for the AMF calculation and correcting this would eliminate its bias relative to the SEAC⁴RS data. Our results support the use of satellite HCHO data as a quantitative proxy for isoprene emission after correction of the low mean bias. There is no evident pattern in the bias, suggesting that a uniform correction factor may be applied to the data until better understanding is achieved.

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Figures

**Figure 1**
Formaldehyde (HCHO) concentrations along SEAC⁴RS aircraft flight tracks (5 August–25 September 2013). The top left panel shows the DC-8 flight tracks (in grey) and the CAMS measurements aboard the aircraft in the mixed layer. The mixed layer is the convectively unstable region of the atmosphere in contact with the surface, diagnosed locally from aerosol lidar observations aboard the aircraft [DIAL-HSRL Mixed Layer Heights README, 2014] and typically extending to 1–3 km altitude. The states of Missouri (MO), Arkansas (AR), and Louisiana (LA) are indicated. The right panel shows the mean vertical profiles observed by the CAMS and ISAF instruments, and simulated by GEOS-Chem, for the Southeast US domain (30.5°–39°N, 95°–81.5°W) defined by the black rectangle in the top left panel. Horizontal bars represent observed standard deviations. GEOS-Chem is sampled along the flight tracks at the time of the measurements. The dashed black line shows the mean vertical CAMS profile in marine air over the Gulf of Mexico (22°–28°N, 96.5°–88.5°W), which is used in determining background HCHO column (0.40×10¹⁶ molecules cm⁻², see Section 3). The bottom left panel shows the mean HCHO columns on a 0.5°×0.5° grid derived from the CAMS measurements after normalizing for temperature, for mixed layer depth, and for the contribution from HCHO aloft (see text in Section 3).

**Figure 2**
Comparisons between HCHO measurements from the CAMS and ISAF instruments aboard the SEAC⁴RS aircraft, and simulated by GEOS-Chem, for the Southeast US flight tracks (box in Figure 1). The left panel compares 1-minute measurements from CAMS and ISAF. The right panel compares GEOS-Chem and CAMS HCHO. Here and elsewhere for model-observation comparisons, HCHO observations along the flight tracks are averaged onto the GEOS-Chem grids (0.25°×0.3125°, 47 vertical layers) and time steps (10 minutes). HCHO data points are colored by atmospheric pressure. Slopes and intercepts of reduced major axis (RMA) regressions are shown along with the correlation coefficient (r), sample size (N), RMA regression line (in blue), and 1:1 line.

**Figure 3**
Mean temperature, base isoprene emissions, and HCHO columns in the GEOS-Chem model for the SEAC⁴RS period (5 August–25 September 2013). The top panel shows the midday (1200–1300 local time) surface air temperature from the GEOS-FP assimilated meteorological data. The middle panel shows the MEGAN 2.1 base isoprene emissions from Guenther et al. [2012] for standard conditions (air temperature=303 K, photosynthetic photon flux density=200 μmol m⁻² s⁻¹ for sunlit leaves and 50 μmol m⁻² s⁻¹ for shaded leaves.) The bottom panel shows the GEOS-Chem HCHO columns computed with MEGAN 2.1 isoprene emissions and sampled at 1330 local time, under OMI-SAO schedule, and filtered by OMI-SAO quality flags and cloud conditions.

**Figure 4**
Frequency distribution of mixed layer depths over the Southeast US during the SEAC⁴RS period (5 August–25 September 2013). Observations by aerosol lidar aboard the aircraft [DIAL-HSRL Mixed Layer Heights README, 2014] are compared to the local GEOS-FP data used to drive GEOS-Chem, before and after the 40% downward correction. The frequency distributions are constructed from 1-minute average data along the aircraft flight tracks over the Southeast US (box in Figure 1) for the 1200–1700 local time window.

**Figure 5**
HCHO vertical column densities over the Southeast US averaged over the SEAC⁴RS period (5 August–25 September 2013). The bottom panels show six retrievals from four satellites (OMI, GOME2A, GOME2B and OMPS) and three different groups (Table 1). The top panels show (1) GEOS-Chem model results sampled on the OMI schedule with filtering by OMI-SAO quality flags and cloud conditions, and increased by 10% to correct for the bias relative to CAMS aircraft measurements; and (2) columns derived from the CAMS aircraft measurements (same as bottom left panel of Figure 1 but on a different color scale). The black rectangle represents the Southeast US domain (same as in Figure 1). Colorbar is a logarithmic scale.

**Figure 6**
Air mass factor differences between retrievals. The left panel shows mean scattering weights (w) and shape factors (S) for HCHO retrievals over the Southeast US during the SEAC⁴RS period, and the right panel shows the product of the two from which the *AMF* is derived by vertical integration using Equation (3). Values are shown for the OMI-SAO, OMI-BIRA, GOME2B-BIRA, and OMPS-PCA retrievals. Mean *AMF* values are given in legend. Also shown is the observed HCHO shape factor (black) from the mean CAMS profile in Figure 1.

**Figure 7**
Daily variability of HCHO vertical column densities over the Southeast US during SEAC⁴RS. The top panel shows daily HCHO columns averaged over the Southeast US (box in Figure 5) for the OMI-SAO and OMI-BIRA retrievals. GEOS-Chem columns (black) are sampled following the OMI viewing geometry with filtering by OMI-SAO quality flags and cloud conditions, and scaled up by 10% on the basis of comparison with CAMS aircraft columns. The bottom panel shows the local midday (1200–1300 local time) surface air temperature over the Southeast US domain from the GEOS-FP assimilated meteorological data. Also shown for each data set is the temporal correlation coefficient (r) with temperature.

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References

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Observing atmospheric formaldehyde (HCHO) from space: validation and intercomparison of six retrievals from four satellites (OMI, GOME2A, GOME2B, OMPS) with SEAC⁴RS aircraft observations over the Southeast US

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Observing atmospheric formaldehyde (HCHO) from space: validation and intercomparison of six retrievals from four satellites (OMI, GOME2A, GOME2B, OMPS) with SEAC⁴RS aircraft observations over the Southeast US

Authors

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Abstract

Figures

References

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