Gaseous Nitrophenols sources and their contribution to HONO formation in an urban area

Nitrophenols (NPs) are compounds that comprise of hydroxyl (-OH) and nitro (-NO₂) functional groups attached to at least one aromatic ring. NPs have significant impacts on human health, climate, and atmospheric chemistry. Despite numerous measurements of particulate NPs, little is still known about their gaseous atmospheric sources, chemistry, and fate. In this study, four gaseous NPs - 2,4-dinitrophenol (2,4-DNP), 4-nitrophenol (4-NP), 2-nitrophenol (2-NP), and 2-Methyl-4-nitrophenol (2-Me-4-NP) were continuously monitored during late spring in an urban area of Houston, Texas. Among the four NPs, 4-NP showed the highest abundance, followed by 2-Me-4-NP, 2-NP, and 2,4-DNP, with average concentrations of 0.47 ± 0.12 ppt, 0.41 ± 0.16 ppt, and 0.27 ± 0.09 ppt, respectively. Utilizing the Positive Matrix Factorization (PMF) model, seven sources: industrial NPs, secondary formation, phenol sources, acetonitrile source, natural gas/crude oil, traffic, and petrochemical industries/oil refineries were identified with NPs’ contributions to each factor of 83.3%, 6.6%, 3.3%, 3.2%, 2.0%, 0.9%, and 0.7%, respectively. A zero-dimensional Atmospheric Chemistry (AtChem2) box model was used to simulate the observed 2-NP and 2,4-DNP. The model revealed a 50.0% and 70.0% contribution from JNO₂, aligning with measured 2-NP and 2,4-DNP, respectively. This resulted in a nitrous acid (HONO) production of 7.5 ± 2.5 ppt/h between 06:00 and 18:00 Central Standard Time (CST) from both NPs. An extrapolation including other known NPs suggests a maximum HONO formation of 13.8 ppt/h, still magnitudes lower than other known HONO formation processes. Nevertheless, it represents a non-negligible fraction and should be considered in areas with substantial primary NPs emissions, and the corresponding reaction mechanisms should be included in any such model. Combining PMF analysis with a photochemical box model provides identification of NPs sources and their atmospheric impact on HONO formation, offering policymakers insights for implementing effective control measures.