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1. Urban aerosol chemistry at a land–water transition site during summer – Part 1: Impact of agricultural and industrial ammonia emissions

   https://doi.org/10.5194/acp-21-13051-2021  

   Balasus, Nicholas ; Battaglia Jr., Michael A. ; Ball, Katherine ; Caicedo, Vanessa ; Delgado, Ruben ; Carlton, Annmarie G. ; Hennigan, Christopher J. ( January 2021 , Atmospheric Chemistry and Physics)

   Abstract. This study characterizes the impact of the Chesapeake Bay and associated meteorological phenomena on aerosol chemistry during the second Ozone Water-Land Environmental Transition Study (OWLETS-2) field campaign, which took place from 4 June to 5 July 2018. Measurements of inorganic PM2.5 composition, gas-phase ammonia (NH3), and an array of meteorological parameters were undertaken at Hart-Miller Island (HMI), a land–water transition site just east of downtown Baltimore on the Chesapeake Bay. The observations at HMI were characterized by abnormally high NH3 concentrations (maximum of 19.3 µg m−3, average of 3.83 µg m−3), which were more than a factor of 3 higher than NH3 levels measured at the closest atmospheric Ammonia Monitoring Network (AMoN) site (approximately 45 km away). While sulfate concentrations at HMI agreed quite well with those measured at a regulatory monitoring station 45 km away, aerosol ammonium and nitrate concentrations were significantly higher, due to the ammonia-rich conditions that resulted from the elevated NH3. The high NH3 concentrations were largely due to regional agricultural emissions, including dairy farms in southeastern Pennsylvania and poultry operations in the Delmarva Peninsula (Delaware–Maryland–Virginia). Reduced NH3 deposition during transport over the Chesapeake Bay likely contributed to enhanced concentrations at HMI compared to the more inland AMoN site. Several peak NH3 events weremore »recorded, including the maximum NH3 observed during OWLETS-2, that appear to originate from a cluster of industrial sources near downtown Baltimore. Such events were all associated with nighttime emissions and advection to HMI under low wind speeds (< 1 m s−1) and stable atmospheric conditions. Our results demonstrate the importance of industrial sources, including several that are not represented in the emissions inventory, on urban air quality. Together with our companion paper, which examines aerosol liquid water and pH during OWLETS-2, we highlight unique processes affecting urban air quality of coastal cities that are distinct from continental locations.« less
2. Urban aerosol chemistry at a land-water transition site during summer – Part 2: Aerosol pH and liquid water content

   Battaglia Jr., M. A. ( January 2021 , Atmospheric chemistry and physics discussion)

   Particle acidity (aerosol pH) is an important driver of atmospheric chemical processes and the resulting effects on human and environmental health. Understanding the factors that control aerosol pH is critical when enacting control strategies targeting specific outcomes. This study characterizes aerosol pH at a land-water transition site near Baltimore, MD during summer 2018 as part of the second Ozone Water-Land Environmental Transition Study (OWLETS-2) field campaign. Inorganic fine mode aerosol composition, gas-phase NH3 measurements, and all relevant meteorological parameters were used to characterize the effects of temperature, aerosol liquid water (ALW), and composition on predictions of aerosol pH. Temperature, the factor linked to the control of NH3 partitioning, was found to have the most significant effect on aerosol pH during OWLETS-2. Overall, pH varied with temperature at a rate of −0.047 K−1 across all observations, though the sensitivity was −0.085 K−1 for temperatures > 293 K. ALW had a minor effect on pH, except at the lowest ALW levels (< 1 µg m−3) which caused a significant increase in aerosol acidity (decrease in pH). Aerosol pH was generally insensitive to composition (SO42− , SO42−:NH4+ , Tot-NH3 = NH3 + NH4+), consistent with recent studies in other locations. In a companion paper, the sources of episodic NH3 events (95th percentile concentrations, NH3 > 7.96 µg m−3)more »during the study are analyzed; aerosol pH was higher by only ~0.1–0.2 pH units during these events compared to the study mean. A case study was analyzed to characterize the response of aerosol pH to nonvolatile cations (NVCs) during a period strongly influenced by primary Chesapeake Bay emissions. Depending on the method used, aerosol pH was estimated to be either weakly (~0.1 pH unit change based on NH3 partitioning calculation) or strongly (~1.4 pH unit change based on ISORROPIA thermodynamic model predictions) affected by NVCs. The case study suggests a strong pH gradient with size during the event and underscores the need to evaluate assumptions of aerosol mixing state applied to pH calculations. Unique features of this study, including the urban land-water transition site and the strong influence of NH3 emissions from both agricultural and industrial sources, add to the understanding of aerosol pH and its controlling factors in diverse environments.« less
3. Urban aerosol chemistry at a land–water transition site during summer – Part 2: Aerosol pH and liquid water content

   https://doi.org/10.5194/acp-21-18271-2021  

   Battaglia Jr., Michael A. ; Balasus, Nicholas ; Ball, Katherine ; Caicedo, Vanessa ; Delgado, Ruben ; Carlton, Annmarie G. ; Hennigan, Christopher J. ( January 2021 , Atmospheric Chemistry and Physics)

   Abstract. Particle acidity (aerosol pH) is an important driver of atmospheric chemical processes and the resulting effects on human and environmentalhealth. Understanding the factors that control aerosol pH is critical when enacting control strategies targeting specific outcomes. This studycharacterizes aerosol pH at a land–water transition site near Baltimore, MD, during summer 2018 as part of the second Ozone Water-Land EnvironmentalTransition Study (OWLETS-2) field campaign. Inorganic fine-mode aerosol composition, gas-phase NH3 measurements, and all relevantmeteorological parameters were used to characterize the effects of temperature, aerosol liquid water (ALW), and composition on predictions ofaerosol pH. Temperature, the factor linked to the control of NH3 partitioning, was found to have the most significant effect on aerosol pHduring OWLETS-2. Overall, pH varied with temperature at a rate of −0.047 K−1 across all observations, though the sensitivity was−0.085 K−1 for temperatures > 293 K. ALW had a minor effect on pH, except at the lowest ALW levels(< 1 µg m−3), which caused a significant increase in aerosol acidity (decrease in pH). Aerosol pH was generally insensitive tocomposition (SO42-, SO42-:NH4+, total NH3 (Tot-NH3) = NH3 + NH4+), consistentwith recent studies in other locations. In a companion paper, the sources of episodic NH3 events (95th percentile concentrations,NH3 > 7.96 µg m−3) during the study are analyzed; aerosol pH was higher by only ∼ 0.1–0.2 pHmore »unitsduring these events compared to the study mean. A case study was analyzed to characterize the response of aerosol pH to nonvolatile cations (NVCs)during a period strongly influenced by primary Chesapeake Bay emissions. Depending on the method used, aerosol pH was estimated to be either weakly(∼ 0.1 pH unit change based on NH3 partitioning calculation) or strongly (∼ 1.4 pH unit change based onISORROPIA thermodynamic model predictions) affected by NVCs. The case study suggests a strong pH gradient with size during the event and underscores the need to evaluate assumptions of aerosol mixing state applied to pH calculations. Unique features of this study, including the urban land–water transition site and the strong influence of NH3 emissions from both agricultural and industrial sources, add to the understanding of aerosol pH and its controlling factors in diverseenvironments.« less
4. Modeling the sources and transport processes during extreme ammonia episodes in the US Corn Belt

   https://doi.org/doi.org/10.1029/2019JD031207  

   Hu, C ; Griffis, T.J. ; Baker, J.M. ; Wood, J.D. ; Millet, D.B. ; Lee, X. ( January 2020 , Journal of geophysical research)

   Atmospheric ammonia (NH3) is the primary form of reactive nitrogen (Nr) and a precursor ofammonium (NH4+) aerosols. Ammonia has been linked to adverse impacts on human health, the loss ofecosystem biodiversity, and plays a key role in aerosol radiative forcing. The midwestern United States is themajor NH3source in North America because of dense livestock operations and the high use of syntheticnitrogen fertilizers. Here, we combine tall‐tower (100 m) observations in Minnesota and Weather Researchand Forecasting model coupled with Chemistry (WRF‐Chem) modeling to investigate high and low NH3emission episodes within the U.S. Corn Belt to improve our understanding of the distribution of emissionsources and transport processes. We examined observations and performed model simulations for cases inFebruary through November of 2017 and 2018. The results showed the following: (1) Peak emissions inNovember 2017 were enhanced by above‐normal air temperatures, implying aQ10(i.e., the change in NH3emissions for a temperature increase of 10°C) of 2.5 for emissions. (2) The intensive livestock emissionsrom northern Iowa, approximately 400 km away from the tall tower, accounted for 17.6% of theabundance in tall‐tower NH3mixing ratios. (3) Ammonia mixing ratios in the innermost domain 3frequently (i.e., 336 hr, 48% of November 2017) exceeded 5.3 ppb, an important air qualitymore »health standard.(4) In November 2017, simulated NH3net ecosystem exchange (the difference between NH3emissionsand dry deposition) accounted for 60–65% of gross NH3emissions for agricultural areas and was2.8–3.1 times the emissions of forested areas. (5) We estimated a mean annual NH3net ecosystem exchangeof 1.60 ± 0.06 nmol · m−2·s−1for agricultural lands and−0.07 ± 0.02 nmol · m−2·s−1for forested lands.These results imply that future warmer fall temperatures will enhance agricultural NH3emissions, increasethe frequency of dangerous NH3episodes, and enhance dry NH3deposition in adjacent forested lands.« less
5. Long-term assessment of floodplain reconnection as a stream restoration approach for managing nitrogen in ground and surface waters

   https://doi.org/10.1007/s11252-021-01199-z  

   Mayer, Paul M. ; Pennino, Michael J. ; Newcomer-Johnson, Tammy A. ; Kaushal, Sujay S. ( January 2022 , Urban Ecosystems)

   Stream restoration is a popular approach for managing nitrogen (N) in degraded, flashy urban streams. Here, we investigated the long-term effects of stream restoration involving floodplain reconnection on riparian and in-stream N transport and transformation in an urban stream in the Chesapeake Bay watershed. We examined relationships between hydrology, chemistry, and biology using a Before/After-Control/Impact (BACI) study design to determine how hydrologic flashiness, nitrate (NO₃⁻) concentrations (mg/L), and N flux, both NO₃⁻and total N (kg/yr), changed after the restoration and floodplain hydrologic reconnection to its stream channel. We examined two independent surface water and groundwater data sets (EPA and USGS) collected from 2002–2012 at our study sites in the Minebank Run watershed. Restoration was completed during 2004 and 2005. Afterward, the monthly hydrologic flashiness index, based on mean monthly discharge, decreased over time from 2002 and 2008. However, from 2008–2012 hydrologic flashiness returned to pre-restoration levels. Based on the EPA data set, NO₃⁻concentration in groundwater and surface water was significantly less after restoration while the control site showed no change. DOC and NO₃⁻were negatively related before and after restoration suggesting C limitation of N transformations. Long-term trends in surface water NO₃⁻concentrations based on USGS surface water data showed downwardmore »trends after restoration at both the restored and control sites, whereas specific conductance showed no trend. Comparisons of NO₃⁻concentrations with Cl⁻concentrations and specific conductance in both ground and surface waters suggested that NO₃⁻reduction after restoration was not due to dilution or load reductions from the watershed. Modeled NO₃⁻flux decreased post restoration over time but the rate of decrease was reduced likely due to failure of restoration features that facilitated N transformations. Groundwater NO₃⁻concentrations varied among stream features suggesting that some engineered features may be functionally better at creating optimal conditions for N retention. However, some engineered features eroded and failed post restoration thereby reducing efficacy of the stream restoration to reduce flashiness and NO₃⁻flux. N management via stream restoration will be most effective where flashiness can be reduced and DOC made available for denitrifiers. Stream restoration may be an important component of holistic watershed management including stormwater management and nutrient source control if stream restoration and floodplain reconnection can be done in a manner to resist the erosive effects of large storm events that can degrade streams to pre-restoration conditions. Long-term evolution of water quality functions in response to degradation of restored stream channels and floodplains from urban stressors and storms over time warrants further study, however.

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