Estimation of OH in urban plumes using TROPOMI-inferred NO₂ ∕ CO

Abstract

Abstract. A new method is presented for estimating urban hydroxyl radical (OH) concentrations using the downwind decay of the ratio of nitrogen dioxide over carbon monoxide column-mixing ratios (XNO2/XCO) retrieved from the Tropospheric Monitoring Instrument (TROPOMI). The method makes use of plumes simulated by the Weather Research and Forecast model (WRF-Chem) using passive-tracer transport, instead of the encoded chemistry, in combination with auxiliary input variables such as Copernicus Atmospheric Monitoring Service (CAMS) OH, Emission Database for Global Atmospheric Research v4.3.2 (EDGAR) NOx and CO emissions, and National Center for Environmental Protection (NCEP)-based meteorological data. NO2 and CO mixing ratios from the CAMS reanalysis are used as initial and lateral boundary conditions. WRF overestimates NO2 plumes close to the center of the city by 15 % to 30 % in summer and 40 % to 50 % in winter compared to TROPOMI observations over Riyadh. WRF-simulated CO plumes differ by 10 % with TROPOMI in both seasons. The differences between WRF and TROPOMI are used to optimize the OH concentration, NOx, CO emissions and their backgrounds using an iterative least-squares method. To estimate OH, WRF is optimized using (a) TROPOMI XNO2/XCO and (b) TROPOMI-derived XNO2 only. For summer, both the NO2/CO ratio optimization and the XNO2 optimization increase the prior OH from CAMS by 32 ± 5.3 % and 28.3 ± 3.9 %, respectively. EDGAR NOx and CO emissions over Riyadh are increased by 42.1 ± 8.4 % and 101 ± 21 %, respectively, in summer. In winter, the optimization method doubles the CO emissions while increasing OH by ∼ 52 ± 14 % and reducing NOx emissions by 15.5 ± 4.1 %. TROPOMI-derived OH concentrations and the pre-existing exponentially modified Gaussian function fit (EMG) method differ by 10 % in summer and winter, confirming that urban OH concentrations can be reliably estimated using the TROPOMI-observed NO2/CO ratio. Additionally, our method can be applied to a single TROPOMI overpass, allowing one to analyze day-to-day variability in OH, NOx and CO emission.

From CO2 emissions to atmospheric NO2 mixing ratios: simulating chemical processes and their impacts on TROPOMI retrievals over the Middle East

“…The anticorrelation between O3 and NOx for the Saudi Arabia region was also observed via in-situ measurements in the area during 2007 (Butenhoff et al, 2015). Lama et al (2022)…”

Observing Downwind Structures of Urban HCHO Plumes From Space: Implications to Non‐Methane Volatile Organic Compound Emissions

“…Our attempt starts with Riyadh (Saudi Arabia), one of the largest cities on the Arabian Peninsula and is usually considered as an ideal place for satellite detection of urban plumes (e.g., NO 2 and CO) due to its isolated location, large emission, and frequent clear sky conditions (Beirle et al., 2011; Lama et al., 2022; Valin et al., 2013). It is also an optimal spot to observe the HCHO urban plume as it is surrounded by desert and has low biogenic VOC emissions.…”

Interannual changes in atmospheric oxidation over forests determined from space

“…Machine learning (ML) approaches have been developed, trained on model output, that use satellite-based inputs to diagnose tropospheric OH patterns over tropical oceans (17,18) and urban regions (19,20). Space-borne NO 2 measurements have also been used to infer urban OH based on the downwind plume evolution (21)(22)(23)(24). While these approaches have provided insights into OH variability, they have been limited to oceanic or urban regions and either rely on prior model assumptions to train an ML algorithm or require specific urban outflow conditions.…”