Dry deposition of aerosols from the atmosphere is an important but poorly understood and inadequately modeled process in atmospheric systems for climate and air quality. Comparisons of currently used aerosol dry deposition models to a compendia of published field measurement studies in various landscapes show very poor agreement over a wide range of particle sizes. In this study, we develop and test a new aerosol dry deposition model that is a modification of the current model in the Community Multiscale Air Quality (CMAQ) model. The new model agrees much better with measured dry deposition velocities across particle sizes. The key innovation is the addition of a second inertial impaction term for microscale obstacles such as leaf hairs, microscale ridges, and needleleaf edge effects. The most significant effect of the new model is to increase the mass dry deposition of the accumulation mode aerosols in CMAQ. Accumulation mode mass dry deposition velocities increase by almost an order of magnitude in forested areas with lesser increases for shorter vegetation. Peak PM2.5 concentrations are reduced in some forested areas by up to 40% in CMAQ simulations. Over the continuous United States, the new model reduced PM2.5 by an average of 16% for July 2018 at the Air Quality System monitoring sites. For summer 2018 simulations, bias and error of PM2.5 concentrations are significantly reduced, especially in forested areas.

The U.S. Climate Reference Network (USCRN) has been engaged in ground-based soil water and soil temperature measurements since 2009. As a nationwide climate network, the network stations are distributed across vast complex terrains. Due to the expansive distribution of the network and the related variability in soil properties, obtaining site-specific calibrations for sensors is a significant and costly endeavor. Presented here are three commercial-grade electromagnetic sensors, with built-in thermistors to measure both soil water and soil temperature, including the SoilVUE10 Time Domain Reflectometry (TDR) probe (hereafter called SP, for SoilVUE Probe) (Campbell Scientific, Inc., Logan, UT), the 50 MHz coaxial impedance dielectric sensor (model HydraProbe (hereafter called HP), Stevens Water Monitoring Systems, Inc., Portland, OR), and the TDR-315L Acclima Probe (hereafter called AP) sensor (model TDR-315L, Acclima, Inc., Meridian, ID), which were evaluated in a nonconductive loam soil in Oak Ridge, Tennessee, USA from 2021 to 2022. The manufacturer-supplied calibration equation for loam soils was successfully used in this study. Measurements of volumetric water content by SP were much lower than gravimetric measurements in the top 20-cm soil horizon, where soil water showed relatively large spatial variability. Study results highlight that the SP may be an important alternative to reduce soil disturbances that usually ensue when HP and AP sensors are installed; however, in-situ calibrations are essential for the SP for xeric soil water conditions.

Terrestrial-aquatic interfaces such as salt marshes, mangroves, and similar wetlands provide an optimum natural environment for the sequestration and long-term storage of carbon (C) from the atmosphere, commonly known as coastal blue carbon. There are over 4 million acres of salt marsh in the US and over half of these are along the east coast of the US. Due to anthropogenic activities, this area presents the greatest nitrogen (N) pollution problem in coastal ecosystems in the U.S. as part of atmospheric N deposition, runoff, and riverine export. Ammonia (NH3) is the most abundant alkaline gas in the atmosphere. Agricultural intensification is the primary anthropogenic source of NH3 leading to a doubling of reactive nitrogen (Nr) entering the biosphere. Despite this, there are limited atmospheric measurements of NH3 concentrations in coastal areas along the east coast. The objective of this study is to advance our process-level understanding of NH3 air-surface exchange over a tidal salt marsh at the Saint Jones Reserve (DE), which is part of the National Estuarine Research Reserve System (NERRs). Continuous and high temporal resolution measurements of atmospheric NH3 concentrations were measured using a cavity ring-down spectrometer, reporting 30 min concentration averages. These high temporal resolution measurements allowed the estimation of the average diurnal cycle of NH3 fluxes using a new analytical methodology. Micrometeorological measurements were also measured using the eddy covariance system operated concurrently above the tidal marsh at the research site, which is part of the AmeriFlux network (US-StJ). This pilot study represents one of the few atmospheric measurements of NH3 over a tidal salt marsh in the eastern U.S. Such measurements are important to characterize the processes that influence the exchanges of NH3 between the atmosphere and the aquatic surface and provide baseline data to form more reliable parameterizations to simulate NH3 deposition and emissions in tidal salt marshes using surface-atmosphere transfer models.

The western United States experienced a record-breaking wildfire season in 2020. This study quantifies the contribution of wildfire emissions to the exceedances of health-based National Ambient Air Quality Standard (NAAQS) for fine particles (PM2.5) by comparing two CMAQ simulations, with and without wildfire emissions. During August to October 2020, western wildfires contributed 23% of surface PM2.5 in the contiguous US (CONUS), with a larger contribution in Pacific Coast (43%) and Mountain Region (42%). Consequently, wildfires were the primary contributor to the 3,720 observed exceedances. The wildfire influence peaked on September 14th, 2020, when 273 exceedances were recorded and wildfire emissions contributed 41%, 81%, and 72% to surface PM2.5 concentrations in the CONUS, Pacific Coast, and Mountain regions, respectively. Our finding highlights the predominating influence of wildfires on air quality, and potentially human health, that is expected to grow with increasing fire activities, while anthropogenic emissions decrease.