On-line Monitoring of Atmospheric Inorganic Gases and Aerosols in the Southeastern and Northwestern U.S.

Importance of the topic

Aerosol particles and their gaseous precursors play a critical role in regional and global climate through radiative forcing and cloud microphysics. Inorganic ions such as sulfate, nitrate and ammonium influence particle hygroscopicity and cloud interactions. Detailed, time-resolved chemical speciation of both gas and aerosol phases is therefore essential to understand atmospheric processes, determine aerosol sources, and assess anthropogenic impacts on air quality and climate.

Objectives and study overview

This study aimed to evaluate the inorganic composition of atmospheric gases and aerosols in two distinct regions of the United States: rural Southeastern Alabama (summer 2013) and urban Northwestern Oregon (autumn 2013). Using an on-line monitoring system (MARGA), the project compared regional differences in anion and cation concentrations, assessed gas-aerosol partitioning for key species (SO2/SO42–, NH3/NH4+, HNO3/NO3–) and inferred aerosol sources such as coal-fired power plants, vehicle emissions, sea salt and mineral dust.

Methodology and instrumentation

Ambient air was sampled through sequential PM10 and PM2.5 cyclones at both sites. Gaseous and particulate fractions were separated by a rotating annular denuder. The gas stream was absorbed in an oxidizing aqueous solution and analyzed for anions and cations by ion chromatography with conductivity detection; lithium bromide served as internal standard. Aerosols were collected in a steam-jet aerosol collector, condensed, and similarly quantified. Columns and eluents were optimized for anion and cation resolution, ensuring rapid, continuous measurements with 1-hour time resolution.

Main results and discussion

In Alabama, aerosol sulfate concentrations were elevated, reflecting influence from nearby coal-fired power plants, and aerosol acidity persisted despite measurement of multiple cations. The ammonium equivalent did not fully neutralize sulfate and nitrate, indicating the presence of excess H+ and possible contributions from other cations. In contrast, Portland aerosols were nearly neutralized by ammonium, with nitrate primarily existing as NH4NO3. Diurnal cycles revealed active partitioning of nitric acid in Alabama (daytime gas-to-particle uptake) and predominantly particle-phase nitrate in Portland. Detailed cation analysis during nitrate peaks identified sea salt contributions in Portland (Na+ and Cl– correlated) and mineral dust influence in Alabama (elevated Ca2+, K+ without Cl–).

Benefits and practical applications

The MARGA platform provides high-resolution, simultaneous gas and aerosol speciation, enabling real-time source identification and process understanding. Such data support air quality management, climate modeling, and regulatory compliance by distinguishing anthropogenic from natural aerosol contributions and by clarifying the chemical mechanisms of aerosol formation.

Future trends and opportunities

Advances will focus on integrating back-trajectory analysis and thermodynamic modeling to refine source attribution and partitioning dynamics. Expanding sensor networks and coupling with emerging data assimilation and machine learning tools will enhance spatial coverage and predictive capability. Continuous multiphase ion monitoring can be extended to organic acids and trace metals, further improving the characterization of complex aerosol systems.

Conclusion

This comparative field study demonstrated significant regional differences in inorganic aerosol chemistry and gas-particle partitioning. The MARGA system effectively resolved key ions in both gas and particulate phases, revealing distinct source signatures and diurnal behavior in rural versus urban environments. These insights contribute to improved understanding of aerosol climate effects and support targeted air quality interventions.

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