Background and Objectives
Carbon monoxide (CO) is a significant atmospheric pollutant that substantially impacts air quality and atmospheric chemistry. This gas is primarily produced from incomplete combustion of fossil fuels in industrial activities, transportation, and heating systems, as well as from natural sources such as biomass burning and methane oxidation. Carbon monoxide plays a key role in the oxidative capacity of the atmosphere through its reaction with the hydroxyl radical (OH), thereby indirectly affecting the global budget of greenhouse gases such as methane and tropospheric ozone. The relatively long lifetime of CO in the troposphere (several weeks to months) makes this gas a suitable tracer for studying atmospheric transport processes of pollution at regional and global scales. Despite extensive global studies on CO distribution using satellite sensors, regional studies in the Middle East, particularly Iran, are very limited and have mostly focused on the lower troposphere with low vertical resolution. This research was conducted with the aim of investigating the vertical distribution of CO in the upper troposphere over Iran, identifying altitude patterns of concentration, and understanding the vertical transport mechanisms of this pollutant during the period 2010 to 2020.
Materials and Methods
In this study, Level 2 data from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) Version 5.3 onboard the SCISAT satellite were utilized. This instrument measures vertical profiles of atmospheric gases from the middle troposphere to the thermosphere using the solar occultation technique with high spectral resolution (0.02{\mathrm{\ cm}}^{-1}) in the 2 to 13 micrometer range. From among 70 solar occultation events recorded within a 700 km radius of Yazd city (the geographical center of Iran), 8 representative events from stations including Bam, Ashgabat, Ahvaz, Gachsaran, Firoozkooh, Karaj, Bandar Abbas, and Yazd-Isfahan corridor were selected based on data quality control criteria (\mathrm{QA\ Flag}\geq0.6) and appropriate spatial coverage. Vertical profiles of carbon monoxide volume mixing ratio (VMR) were extracted from altitudes of 5.5 to 120 km with a vertical resolution of one kilometer. Data processing and analysis were performed using GrADS and Origin software.
Results and Discussion
Results showed that the highest CO concentrations were observed at Bandar Abbas station with a value of 105 ppbv at an altitude of 13.2 km, and at Gachsaran with a value of 92 ppbv at an altitude of 14.1 km. The Firoozkooh and Karaj stations in the north showed moderate concentrations (65 to 75 ppbv), while Ahvaz and Ashgabat stations in the west and east exhibited similar values (70 to 80 ppbv). The Bam station had limited applicability for tropospheric analysis due to valid data starting from altitudes above the tropopause. All profiles showed a significant decrease in CO concentration from 15 km altitude upward, indicating the transition from the troposphere to the stratosphere. The average volume mixing ratio in the altitude range of 8 to 12 km was found to be 92 ppbv. Validation of results with the study by Rinsland et al. (2007) showed good agreement (R^2=0.89, \mathrm{RMSE}=6.2\mathrm{\ ppbv}). The high CO concentrations at Bandar Abbas and Gachsaran were attributed to extensive industrial activities including oil and gas refineries, steel industries, gas flares, and port transportation. The main mechanism for CO transport to high altitudes is deep convection caused by intense surface heating in southern regions of Iran, which transports surface pollutants to the upper troposphere. The results of this research emphasize the necessity of continuous monitoring of atmospheric pollutants and development of studies on vertical transport mechanisms under Iran’s specific climatic conditions.