Characterization of Errors in Satellite-Based HCHO/NO2 Tropospheric Column Ratios With Respect to Chemistry, Column-to-PBL Translation, Spatial Representation, and Retrieval Uncertainties

• Published in: Atmospheric chemistry and physics 2023-02, Vol.23 (3), p.1963-1986
• Main Authors: Souri, Amir H., Johnson, Matthew S., Wolfe, Glenn M., Crawford, James H., Fried, Alan, Wisthaler, Armin, Brune, William H., Blake, Donald R., Weinheimer, Andrew J., Verhoelst, Tijl, Compernolle, Steven, Pinardi, Gaia, Vigouroux, Corrine, Langerock, Bavo, Choi, Sungyeon, Lamsal, Lok, Zhu, Lei, Sun, Shuai, Cohen, Ronald C., Min, Kyung-Eun, Cho, Changmin, Philip, Sajeev, Liu, Xiong, Chance, Kelly
• Format: Article
• Language: English
• Subjects: Air quality; Aircraft; Algorithms; Anthropogenic factors; Atmospheric chemistry; Boundary layers; Chemistry; Chemistry and Materials (General); Confidence intervals; Error analysis; Errors; Exact solutions; Gaussian distribution; Monitoring; Monitoring instruments; Mountains; Nitrogen; Nitrogen compounds; Nitrogen dioxide; Nitrogen oxides; Normal distribution; Organic compounds; Ozone; Ozone monitoring; Ozone production; Photochemicals; Physics (General); Planetary boundary layer; Rational functions; Ratios; Remote sensing; Representations; Resolution; Retrieval; Satellites; Sensors; Spatial data; Standard error; Statistical analysis; Troposphere; Uncertainty; Urban areas; Vegetation; VOCs; Volatile organic compounds
• ISSN: 1680-7316; 1680-7324
• DOI: 10.5194/acp-23-1963-2023

The availability of formaldehyde (HCHO) (a proxy for volatile organic compound reactivity) and nitrogen dioxide (NO2) (a proxy for nitrogen oxides) tropospheric columns from ultraviolet–visible (UV–Vis) satellites has motivated many to use their ratios to gain some insights into the near-surface ozone sensitivity. Strong emphasis has been placed on the challenges that come with transforming what is being observed in the tropospheric column to what is actually in the planetary boundary layer (PBL) and near the surface; however, little attention has been paid to other sources of error such as chemistry, spatial representation, and retrieval uncertainties. Here we leverage a wide spectrum of tools and data to quantify those errors carefully. Concerning the chemistry error, a well-characterized box model constrained by more than 500 h of aircraft data from NASA's air quality campaigns is used to simulate the ratio of the chemical loss of HO2 + RO2 (LROx) to the chemical loss of NOx (LNOx). Subsequently, we challenge the predictive power of HCHO/NO2 ratios (FNRs), which are commonly applied in current research, in detecting the underlying ozone regimes by comparing them to LROx/LNOx. FNRs show a strongly linear (R2=0.94) relationship with LROx/LNOx, but only on the logarithmic scale. Following the baseline (i.e., ln(LROx/LNOx) = −1.0 ± 0.2) with the model and mechanism (CB06, r2) used for segregating NOx-sensitive from VOC-sensitive regimes, we observe a broad range of FNR thresholds ranging from 1 to 4. The transitioning ratios strictly follow a Gaussian distribution with a mean and standard deviation of 1.8 and 0.4, respectively. This implies that the FNR has an inherent 20 % standard error (1σ) resulting from not accurately describing the ROx–HOx cycle. We calculate high ozone production rates (PO3) dominated by large HCHO × NO2 concentration levels, a new proxy for the abundance of ozone precursors. The relationship between PO3 and HCHO × NO2 becomes more pronounced when moving towards NOx-sensitive regions due to nonlinear chemistry; our results indicate that there is fruitful information in the HCHO × NO2 metric that has not been utilized in ozone studies. The vast amount of vertical information on HCHO and NO2 concentrations from the air quality campaigns enables us to parameterize the vertical shapes of FNRs using a second-order rational function permitting an analytical solution for an altitude adjustment factor to partition the tropospheric columns in