Nitrogen dioxide and formaldehyde measurements from the GEOstationary Coastal and Air Pollution Events (GEO-CAPE) Airborne Simulator over Houston, Texas

Abstract

The GEOstationary Coastal and Air Pollution Events (GEO-CAPE) Airborne Simulator (GCAS) was developed in support of NASA's decadal survey GEO-CAPE geostationary satellite mission. GCAS is an airborne pushbroom remote-sensing instrument, consisting of two channels which make hyperspectral measurements in the ultraviolet/visible (optimized for air quality observations) and the visible-near infrared (optimized for ocean color observations). The GCAS instrument participated in its first intensive field campaign during the Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) campaign in Texas in September 2013. During this campaign, the instrument flew on a King Air B-200 aircraft during 21 flights on 11 days to make air quality observations over Houston, Texas. We present GCAS trace gas retrievals of nitrogen dioxide (NO 2 ) and formaldehyde (CH 2 O), and compare these results with trace gas columns derived from coincident in situ profile measurements of NO 2 and CH 2 O made by instruments on a P-3B aircraft, and with NO 2 observations from ground-based Pandora spectrometers operating in direct-sun and scattered light modes. GCAS tropospheric column measurements correlate well spatially and temporally with columns estimated from the P-3B measurements for both NO 2 (r 2 = 0.89) and CH 2 O (r 2 = 0.54) and with Pandora direct-sun (r 2 = 0.85) and scattered light (r 2 = 0.94) observed NO 2 columns. Coincident GCAS columns agree in magnitude with NO 2 and CH 2 O P-3B-observed columns to within 10 % but are larger than scattered lightPublished by Copernicus Publications on behalf of the European Geosciences Union. 5942 C. R. Nowlan et al.: GCAS measurements of NO 2 and CH 2 O Pandora tropospheric NO 2 columns by 33 % and direct-sun Pandora NO 2 columns by 50 %.

Validation of tropospheric NO<sub>2</sub> column measurements of GOME-2A and OMI using MAX-DOAS and direct sun network observations

“…In total, 15 Pandora direct sun instruments delivering at least 3 months of data have been considered here. They are listed in Table 3 with an indication of their location, ownership, availability (see also (Choi et al, 2019), the largest discrepancies being found in Texas (Nowlan et al, 2018). Good agreement of a few percent between Pandora and GeoTASO has been reported by Judd et al (2019), while differences increase when resampling the comparisons for larger simulated pixel sizes, up to about 40% bias for 10 18x18km², similar to the bias found with OMI (50%).…”

“…As already discussed in Section 5.1, for direct sun stations this could be related to issues with the determination of 5 stratospheric columns in the satellite algorithm. UHMT is a peculiar site, where several studies performed during the DISCOVER-AQ 2013 Texas campaign (Nowlan et al, 2018;Choi et al, 2019) suggested that those Pandora NO2 measurements tend to be too low. Finally, some sites (e.g.…”

Assessment of NO<sub>2</sub> observations during DISCOVER-AQ and KORUS-AQ field campaigns

“…Elsewhere, Pandora data agree with aircraft measurements to within 20% on average, although much larger differences are observed for individual sites. A larger discrepancy for Pandora data in TX is also reported by Nowlan et al (2018), who used various NO 2 measurements to evaluate Geo-TASO NO 2 retrievals. Reasons for such exceptionally large differences could include strong gradients in the NO 2 field that are missed by aircraft spirals, errors in Pandora retrievals, or both.…”

“…A number of validation studies of space-based tropospheric NO 2 columns have been conducted using independent NO 2 observations from airborne in situ mixing ratio measurements (e.g., Boersma et al, 2008;Bucsela et al, 2008;Hains et al, 2010;Lamsal et al, 2014), ground-based total (e.g., Pandora instrument (Herman et al, 2009)) and tropospheric (e.g., MAX-DOAS instrument (e.g., Vlemmix et al, 2010;Irie et al, 2012)) column measurements, and airborne high-resolution DOAS measurements (Lamsal et al, 2017;Nowlan et al, 2018). Most validation studies utilizing in situ/ground-based observations have reported that satellite measurements tend to underestimate tropospheric NO 2 columns, especially over highly polluted areas (e.g., Hains et al, 2010).…”

The Airborne ROmanian Measurements of Aerosols and Trace gases (AROMAT) campaigns

“…They combined ground-based and airborne measurements. DISCOVER-AQ involved the deployment of the Geostationary Trace gas and Aerosol Sensor Optimization instrument (GEOTASO, Leitch et al,2014;Nowlan et al,2016) and of the Geostationary Coastal and Air Pollution Events (GEO-CAPE) Airborne Simulator (GCAS, Kowalewski and Janz,2014;Nowlan et al,2018). In Europe, the two AROMAT campaigns, which took place in Romania in September 2014 and August 2015, demonstrated a suite of new instruments such as the Airborne imaging DOAS instrument for Measurements of Atmospheric Pollution (AirMAP, Schönhardt et al,2015;Meier et al,2017), the NO 2 sonde (Sluis et al, 2010), and the Small Whiskbroom Imager for atmospheric compositioN monitorinG (SWING, Merlaud et al,2018).…”