Regime shift in secondary inorganic aerosol formation and nitrogen deposition in the rural US

Secondary inorganic aerosols (SIA) play an important role in air pollution and climate change, and their formation modulates atmospheric deposition of reactive nitrogen (Nr; including oxidized and reduced nitrogen), impacting the nitrogen cycle. Large-scale and long-term analyses of SIA formation based on model simulations have significant uncertainties. Here, we improve constraints on SIA formation using decade-long in-situ observations of aerosol composition and gaseous precursors from multiple monitoring networks across the US. We reveal a shift in the formation regime of SIA in the rural US between 2011 – 2020, making rural areas less sensitive to changes in ammonia (NH3) concentrations and shortening the effective atmospheric lifetime of reduced forms of Nr. This leads to potential increases in Nr deposition near NH3 emission hotspots, with ecosystem impacts warranting further investigation. NH3, a critical but not directly regulated precursor of fine particulate matter (PM2.5) in the US, has been increasingly scrutinized to decrease PM2.5. Our findings, however, show controlling NH3 became significantly less effective for mitigating PM2.5 in the rural US. We highlight the need for more collocated aerosol and precursor observations for better characterization of SIA formation in urban areas and regions increasingly impacted by wildfires and dust.

Methods
This dataset contains the following data:
1. Integrated observations from several aerosol composition and gaseous precursor monitoring networks and meteorological stations between 2011 and 2020. Observations of the chemical composition and gaseous precursors of aerosols are from: 1) the Clean Air Status and Trends Network (CASTNET), the Interagency Monitoring of Protected Visual Environments (IMPROVE) network, the US Environmental Protection Agency’s (EPA’s) PM2.5 Chemical Speciation Monitoring Network (CSN), and the Ammonia Monitoring Network (AMoN). Meteorological observations are from the NOAA Integrated Surface Database (ISD).

To integrate the monitoring networks, we first identify the spatial window for collocation determination by comparing observations from CASTNET, IMPROVE, and EPA CSN sites as well as temperature (T) and relative humidity (RH) observations from CASTNET and ISD located within 10, 25, 50, and 100 km of each other. For aerosol observations, no significant difference was found with different spatial windows. However, T and RH from CASTNET and ISD significantly differ when a spatial window of 100 km is used. Therefore, a spatial window of 50 km is selected for observation integration. With this spatial window, we find 68 AMoN sites with at least CASTNET and ISD sites located within 50 km. Combining observations from these three networks provides all the inputs needed for aerosol thermodynamic modeling. All observations are averaged biweekly to match the start and end dates of AMoN observations since it has the lowest sampling frequency.

2. Aerosol thermodynamic simulations based on the integrated observations using the ISORROPIA-II model. ISORROPIA-II is a full thermodynamic model for inorganic aerosol formation, and we use it to simulate aerosol properties and sensitivities of SIA formation to precursors. The model is run in the “forward mode” to simulate partitionings of total ammonium (NH4T=NH4- + NH3) and total nitrate (NO3T=NO3- + HNO3). Although ISORROPIA-II has been validated with observations from intensive field campaigns, using it with biweekly averaged observations from monitoring networks has not been tested before and requires careful evaluation. We conducted nine case studies to investigate the impacts of measurement biases and low temporal resolutions.

3. Time series of reactive nitrogen deposition at different distances to NH3 hotspots. Dividing the contiguous US into four zones according to their distances to the nearest NH3 emission hotspot (<50 km, 50 - 150 km, 150 - 300 km, and >300 km), we analyze the trend of annual Nr total deposition from the Total Deposition Estimates Using the Measurement Model Fusion (TDep MMF) between 2010 and 2019. The TDep MMF combines wet deposition observations from the National Trends Network (NTN), ambient air monitoring data from CASTNET, and simulations from the EPA's Air Quality Time Series (EQUATES) project. The areas of the 95th percentile of NH3 emission rates across the U.S. based on the 2017 EQUATES NH3 emissions are considered NH3 emission hotspots, except for sporadic locations with just one 12 km × 12 km grid. We also considered the hotspots defined as the areas of the 95th percentile of NH3 column amounts from the Infrared Atmospheric Sounding Interferometer (IASI) satellite NH3 observations (v2.2r) across the US.