Abstract
Airborne and ground-based Pandora spectrometer NO2 column measurements were collected during the 2018 Long Island Sound Tropospheric Ozone Study (LISTOS) in the New York City/Long Island Sound region, which coincided with early observations from the Sentinel-5P TROPOspheric Monitoring Instrument (TROPOMI) instrument. Both airborne- and ground-based measurements are used to evaluate the TROPOMI NO2 Tropospheric Vertical Column (TrVC) product v1.2 in this region, which has high spatial and temporal heterogeneity in NO2. First, airborne and Pandora TrVCs are compared to evaluate the uncertainty of the airborne TrVC and establish the spatial representativeness of the Pandora observations. The 171 coincidences between Pandora and airborne TrVCs are found to be highly correlated (r2 =0.92 and slope of 1.03), with the largest individual differences being associated with high temporal and/or spatial variability. These reference measurements (Pandora and airborne) are complementary with respect to temporal coverage and spatial representativity. Pandora spectrometers can provide continuous long-term measurements but may lack areal representativity when operated in direct-sun mode. Airborne spectrometers are typically only deployed for short periods of time, but their observations are more spatially representative of the satellite measurements with the added capability of retrieving at subpixel resolutions of 250m×250m over the entire TROPOMI pixels they overfly. Thus, airborne data are more correlated with TROPOMI measurements (r2 = 0.96) than Pandora measurements are with TROPOMI (r2 = 0.84). The largest outliers between TROPOMI and the reference measurements appear to stem from too spatially coarse a priori surface reflectivity (0.5°) over bright urban scenes. In this work, this results during cloud-free scenes that, at times, are affected by errors in the TROPOMI cloud pressure retrieval impacting the calculation of tropospheric air mass factors. This factor causes a high bias in TROPOMI TrVCs of 4%–11%. Excluding these cloud-impacted points, TROPOMI has an overall low bias of 19%–33% during the LISTOS timeframe of June–September 2018. Part of this low bias is caused by coarse a priori profile input from the TM5-MP model; replacing these profiles with those from a 12 km North American Model–Community Multiscale Air Quality (NAMCMAQ) analysis results in a 12%–14% increase in the TrVCs. Even with this improvement, the TROPOMI-NAMCMAQ TrVCs have a 7%–19% low bias, indicating needed improvement in a priori assumptions in the air mass factor calculation. Future work should explore additional impacts of a priori inputs to further assess the remaining low biases in TROPOMI using these datasets.
Highlights
Nitrogen dioxide (NO2) is an air pollutant emitted naturally through soil emissions and lightning, as well as anthropogenically as a combustion product from sources such as mobile vehicles, powerplants, and industrial processes
These types of measurements have been used in the past to evaluate the Ozone Monitoring Instrument (OMI) Tropospheric Vertical Column (TrVC) product, though this was shown to be challenging in polluted areas as spatial variability in NO2 can result in sampling mismatches between the small spatial scale measurements from the ground-based spectrometers and the > 300 km2 pixels from OMI (Lamsal et al, 2014; Reed et al, 2015; Goldberg et al, 2017; Judd et al, 2019)
The airborne spectrometer observed a distinct very polluted plume over New York City (NYC) and over the 48 min period between the airborne and TROPOspheric Monitoring Instrument (TROPOMI) observations, and the Pandora spectrometer located at CCNY observed a 50 % decrease in NO2 total vertical column, leading to a large difference between the airborne and TROPOMI TrVCs when the temporal window is extended to ± 60 min (Maria Tzortziou, personal communication, 8 August 2020)
Summary
Nitrogen dioxide (NO2) is an air pollutant emitted naturally through soil emissions and lightning, as well as anthropogenically as a combustion product from sources such as mobile vehicles, powerplants, and industrial processes. Additional in-depth studies in recent literature have been mostly confined to ground-based column measurements from multiaxis differential optical absorption spectroscopy (MAX-DOAS) and/or direct-sun column measurements (e.g., from Pandora spectrometers) (e.g., Griffin et al, 2019; Zhao et al, 2020; Ialongo et al, 2020, Wang et al, 2020) These types of measurements have been used in the past to evaluate the OMI Tropospheric Vertical Column (TrVC) product, though this was shown to be challenging in polluted areas as spatial variability in NO2 can result in sampling mismatches between the small spatial scale measurements from the ground-based spectrometers and the > 300 km pixels from OMI (Lamsal et al, 2014; Reed et al, 2015; Goldberg et al, 2017; Judd et al, 2019). Together these results demonstrate TROPOMI’s capability for observing the spatial distribution of NO2 in heterogeneous environments and demonstrate approaches for resolving apparent differences associated with linking observations from different measurement strategies
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