Long-term trends in inorganic aerosol chemical composition and chemistry at an urban and rural site in the northeastern US

Atmospheric nitrate and sulfate are major inorganic particulate matter components that impact human and ecosystem health and air quality. Over the last several decades, emissions of the precursor gases, nitrogen oxides (NOx = NO + NO2) and sulfur dioxide (SO2), have dramatically decreased in the US in response to federal regulations. However, the response in concentrations of particulate nitrate (pNO3) and sulfate (pSO4) have not followed predictions due to complex non-linear chemistry feedbacks that may differ amongst environments (i.e., urban vs. rural). In this study, we explored the long-term response of particle chemistry for urban and rural environments in southern New England, a region historically impacted by NOx and SO2 emissions. Particulate matter (PM10) samples collected via the same method from 2005 to 2015 at urban and rural locations in Rhode Island were analyzed for their major inorganic components, and air mass trajectories and statistical analysis were used to identify source regions over time. Our results indicated a significant urban-rural aerosol chemical composition gradient for sampling locations within 40 km. Over time, as anthropogenic influences have decreased, the relative contribution of marine and crustal sources has increased greatly, impacting fine and coarse particle chemistry in recent years. Total mass concentrations of chemical species, particularly anthropogenic pSO4 and particulate ammonium (pNH4), have shown dramatic decreases over the ten years at both the urban and rural sites; however, pNO3 concentration increased by 95 % and 57 % in the urban and rural sites, respectively, despite significant NOx emission reductions. Our results demonstrate that changes in chemical mechanisms due to the decrease in SO2 emissions contributed to decreases in pNH4, along with enhanced pNO3 concentration. Furthermore, the change in SO2 emissions has significantly impacted the atmospheric lifetime and transport distance of pNH4, favoring more localized contributions in recent years.

Automated summary

Atmospheric nitrate and sulfate, major inorganic components of particulate matter, have a significant impact on human health, ecosystem health, and air quality. This study investigated the long-term response of particle chemistry in urban and rural environments in southern New England, an area historically affected by nitrogen oxides (NOx) and sulfur dioxide (SO2) emissions. The results showed that there was a significant urban-rural aerosol chemical composition gradient within a 40 km radius. As anthropogenic influences decreased, the relative contribution of marine and crustal sources increased, affecting the chemistry of fine and coarse particles. Total mass concentrations of chemical species, particularly anthropogenic sulfate and particulate ammonium, decreased dramatically over the ten-year period at both urban and rural sites. However, nitrate concentration increased despite significant reductions in NOx emissions, indicating changes in chemical mechanisms. Furthermore, the decrease in SO2 emissions significantly impacted the atmospheric lifetime and transport distance of particulate ammonium.