Two Decades of Coastal Background NMHCs: Trends and Implications for Atmospheric Chemistry
What can two decades of air monitoring from a quiet Gulf Coast outpost tell us about the shifting chemistry of our atmosphere?
Ph.D. students Morshad Ahmed (Graduation Summer 2024) and Mateen Ahmad (Graduation Spring 2024), advised by Bernhard Rappenglueck, have recently completed an important data analysis project that awaited their expertise to fully mature. Their work was recently published (Ahmed et al., 2025).
The Texas Commission on Environmental Quality (TCEQ) operates a network of Automatic Gas Chromatographs (Auto-GCs) in Houston, measuring non-methane hydrocarbons (NMHCs) emitted from various sources, particularly Houston’s petrochemical industry. NMHCs significantly influence atmospheric chemistry, interacting with radicals such as hydroxyl (OH), ozone (O3), the nitrate radical (NO3), and halogens to produce organic radicals (RO2). These radicals further react with nitrogen oxides (NOx) to form secondary species, notably ozone (O3), thus critically impacting Houston’s air quality.
However, the primary focus of this analysis was background air to evaluate potential changes in large-scale emissions and their impacts on atmospheric oxidation capacity. Due to the financial implications, long-term background NMHC measurements are rare. While most Houston Auto-GC sites are located near the heavily industrialized Ship Channel, the Lake Jackson site, positioned southwest of Houston and halfway to the Gulf Coast, provides a unique opportunity for such analysis. This site reached a significant milestone of 20 years of service in 2023. Although it has not yet reached the World Meteorological Organization’s (WMO) 30-year benchmark requirement, the team chose to proceed with the analysis, building on preliminary updates previously presented by Dr. Rappenglueck at numerous international conferences, including AMS, AGU, EGU, ASAAQ, and WMO meetings.
In their research, Morshad and Mateen applied the Positive Matrix Factorization (PMF) source apportionment method—previously untested for atmospheric background air. They identified two primary sources: a biogenic source predominantly driven by isoprene, and another source largely composed of C2-C5 alkanes. A closer analysis of wintertime data, reflecting emission sources most clearly, yielded important findings: ethane levels have increased annually by 1.05 ± 0.51% since approximately 2009, correlating with heightened oil and gas extraction activities. Conversely, acetylene (-1.67 ± 0.51%/year) and benzene (-2.43 ± 0.14%/year) concentrations have consistently declined, likely due to advancements in gasoline technology. The less pronounced decrease in acetylene compared to benzene is attributed to biomass burning emissions.
To assess atmospheric oxidation capacity, the study employed the established metric of Propylene Equivalent concentration. Results indicated that during summer, isoprene contributes between 80–90% of this measure, with a statistically significant upward trend (0.45 ppbC/year). Additional analyses confirmed this increase is predominantly driven by rising ambient temperatures, boosting isoprene emissions by approximately 20 ± 1.6%.
Following their graduation, Morshad Ahmed now serves as Senior Technical Project Manager at the Air Quality Division of the Texas Commission on Environmental Quality (TCEQ) in Austin, TX, and Mateen Ahmad is currently an Assistant Professor of Mathematics at the University of Lahore in Pakistan.
Citation: Ahmed M., Ahmad M., Rappenglueck B. (2025): Twenty Years (2004-2023) observation of non-methane hydrocarbons in a subtropical coastal environment – Indications of increased isoprene emissions, Atmospheric Environment, 343, 120993, doi.org/10.1016/j.atmosenv.2024.120993