Satellite-based aerosol optical depth estimates over the continental U.S. during the 2020 wildfire season: Roles of smoke and land cover.

Wildfires produce smoke that can affect an area >1000 times the burn extent, with far-reaching human health, ecologic, and economic impacts. Accurately estimating aerosol load within smoke plumes is therefore crucial for understanding and mitigating these impacts. We evaluated the effectiveness of the latest Collection 6.1 MODIS Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm in estimating aerosol optical depth (AOD) across the U.S. during the historic 2020 wildfire season. We compared satellite-based MAIAC AOD to ground-based AERONET AOD measurements during no-, light-, medium-, and heavy-smoke conditions identified using the Hazard Mapping System Fire and Smoke Product. This smoke product consists of maximum extent smoke polygons digitized by analysts using visible band imagery and classified according to smoke density. We also examined the strength of the correlations between satellite- and ground-based AOD for major land cover types under various smoke density levels. MAIAC performed well in estimating AOD during smoke-affected conditions. Correlations between MAIAC and AERONET AOD were strong for medium- (r = 0.91) and heavy-smoke (r = 0.90) density, and MAIAC estimates of AOD showed little bias relative to ground-based AERONET measurements (normalized mean bias = 3 % for medium, 5 % for heavy smoke). During two high AOD, heavy smoke episodes, MAIAC underestimated ground-based AERONET AOD under mixed aerosol (i.e., smoke and dust; median bias = -0.08) and overestimated AOD under smoke-dominated (median bias = 0.02) aerosol. MAIAC most overestimated ground-based AERONET AOD over barren land (mean NMB = 48 %). Our findings indicate that MODIS MAIAC can provide robust estimates of AOD as smoke density increases in coming years. Increased frequency of mixed aerosol and expansion of developed land could affect the performance of the MAIAC algorithm in the future, however, with implications for evaluating wildfire-associated health and welfare effects and air quality standards.

Source characterization of volatile organic compounds at Carlsbad Caverns National Park.

Carlsbad Caverns National Park (CAVE), located in southeastern New Mexico, experiences elevated ground-level ozone (O3) exceeding the National Ambient Air Quality Standard (NAAQS) of 70 ppbv. It is situated adjacent to the Permian Basin, one of the largest oil and gas (O&G) producing regions in the US. In 2019, the Carlsbad Caverns Air Quality Study (CarCavAQS) was conducted to examine impacts of different sources on ozone precursors, including nitrogen oxides (NOx) and volatile organic compounds (VOCs). Here, we use positive matrix factorization (PMF) analysis of speciated VOCs to characterize VOC sources at CAVE during the study. Seven factors were identified. Three factors composed largely of alkanes and aromatics with different lifetimes were attributed to O&G development and production activities. VOCs in these factors were typical of those emitted by O&G operations. Associated residence time analyses (RTA) indicated their contributions increased in the park during periods of transport from the Permian Basin. These O&G factors were the largest contributor to VOC reactivity with hydroxyl radicals (62%). Two PMF factors were rich in photochemically generated secondary VOCs; one factor contained species with shorter atmospheric lifetimes and one with species with longer lifetimes. RTA of the secondary factors suggested impacts of O&G emissions from regions farther upwind, such as Eagle Ford Shale and Barnett Shale formations. The last two factors were attributed to alkenes likely emitted from vehicles or other combustion sources in the Permian Basin and regional background VOCs, respectively.Implications: Carlsbad Caverns National Park experiences ground-level ozone exceeding the National Ambient Air Quality Standard. Volatile organic compounds are critical precursors to ozone formation. Measurements in the Park identify oil and gas production and development activities as the major contributors to volatile organic compounds. Emissions from the adjacent Permian Basin contributed to increases in primary species that enhanced local ozone formation. Observations of photochemically generated compounds indicate that ozone was also transported from shale formations and basins farther upwind. Therefore, emission reductions of volatile organic compounds from oil and gas activities are important for mitigating elevated O3 in the region.

Complexities in Modeling Organic Aerosol Light Absorption.

Aerosol particles dynamically evolve in the atmosphere by physicochemical interactions with sunlight, trace chemical species, and water. Current modeling approaches fix properties such as aerosol refractive index, introducing spatial and temporal errors in the radiative impacts. Further progress requires a process-level description of the refractive indices as the particles age and experience physicochemical transformations. We present two multivariate modeling approaches of light absorption by brown carbon (BrC). The initial approach was to extend the modeling framework of the refractive index at 589 nm (nD), but that result was insufficient. We developed a second multivariate model using aromatic rings and functional groups to predict the imaginary part of the complex refractive index. This second model agreed better with measured spectral absorption peaks, showing promise for a simplified treatment of BrC optics. In addition to absorption, organic functionalities also alter the water affinity of the molecules, leading to a hygroscopic uptake and increased light absorption, which we show through measurements and modeling.

Wintertime haze and ozone at Dinosaur National Monument.

Dinosaur National Monument (DINO) is located near the northeastern edge of the Uinta Basin and often experiences elevated levels of wintertime ground-level ozone. Previous studies have shown that high ozone mixing ratios in the Uinta Basin are driven by elevated levels of volatile organic compounds (VOCs) and nitrogen oxides (NOx) from regional oil and gas development coupled with temperature inversions and enhanced photochemistry from persistent snow cover. Here, we show that persistent snow cover and temperature inversions, along with abundant ammonia, also lead to wintertime haze in this region. A study was conducted at DINO from November 2018 through May 2020 where ozone, speciated fine and coarse aerosols, inorganic gases, and VOCs were measured. Three National Ambient Air Quality Standards (NAAQS) ozone exceedances were observed in the first winter, and no exceedances were observed in the second winter. In contrast, elevated levels of particulate matter were observed both winters, with 24-h averaged particle light extinction exceeding 100 Mm-1. These haze events were dominated by ammonium nitrate, and particulate organics were highly correlated with ammonium nitrate. Ammonium nitrate formation was limited by nitric acid in winter. As such, reductions in regional NOx emissions should reduce haze levels and improve visibility at DINO in winter. Long-term measurements of particulate matter from nearby Vernal, Utah, suggest that visibility impairment is a persistent issue in the Uinta Basin in winter. From April through October 2019, relatively clean conditions occurred, with average particle extinction of ~10 Mm-1. During this period, ammonium nitrate concentrations were lower by more than an order of magnitude, and contributions from coarse mass and soil to haze levels increased. VOC markers indicated that the high levels of observed pollutants in winter were likely from local sources related to oil and gas extraction activities.Implications: Elevated ground-level ozone and haze levels were observed at Dinosaur National Monument in winter. Haze episodes were dominated by ammonium nitrate, with 24-h averaged particle light extinction exceeding 100 Mm-1, reducing visual range near the surface to ~35 km. Despite elevated ammonium nitrate concentrations, additional gas-phase ammonia was available, such that any increase in NOx emissions in the region is likely to lead to even greater haze levels.

Ammonia Dry Deposition in an Alpine Ecosystem Traced to Agricultural Emission Hotpots.

Elevated reactive nitrogen (Nr) deposition is a concern for alpine ecosystems, and dry NH3 deposition is a key contributor. Understanding how emission hotspots impact downwind ecosystems through dry NH3 deposition provides opportunities for effective mitigation. However, direct NH3 flux measurements with sufficient temporal resolution to quantify such events are rare. Here, we measured NH3 fluxes at Rocky Mountain National Park (RMNP) during two summers and analyzed transport events from upwind agricultural and urban sources in northeastern Colorado. We deployed open-path NH3 sensors on a mobile laboratory and an eddy covariance tower to measure NH3 concentrations and fluxes. Our spatial sampling illustrated an upslope event that transported NH3 emissions from the hotspot to RMNP. Observed NH3 deposition was significantly higher when backtrajectories passed through only the agricultural region (7.9 ng m-2 s-1) versus only the urban area (1.0 ng m-2 s-1) and both urban and agricultural areas (2.7 ng m-2 s-1). Cumulative NH3 fluxes were calculated using observed, bidirectional modeled, and gap-filled fluxes. More than 40% of the total dry NH3 deposition occurred when air masses were traced back to agricultural source regions. More generally, we identified that 10 (25) more national parks in the U.S. are within 100 (200) km of an NH3 hotspot, and more observations are needed to quantify the impacts of these hotspots on dry NH3 deposition in these regions.

Persistent Nonagricultural and Periodic Agricultural Emissions Dominate Sources of Ammonia in Urban Beijing: Evidence from 15N Stable Isotope in Vertical Profiles.

Ammonia (NH3) emission reduction is key to limiting the deadly PM2.5 pollution globally. However, studies of long-term source apportionment of vertical NH3 are relatively limited. On the basis of the one-year measurements of weekly vertical profiles of δ15N-NH3 at 5 heights (2, 15, 102, 180, and 320 m) on a 325-m meteorological tower in urban Beijing, we found that vertical profiles of NH3 concentrations generally remained stable with height. δ15N-NH3 increased obviously as a function of height in cold seasons (with heating) and decreased in warm seasons (with fertilization), indicating a stronger human-induced seasonal variation via regional transport at higher altitudes. Relatively stable δ15N-NH3 near the ground surface suggested the strong local emission. The results of isotopic mixing model (SIAR) indicate that source apportionment using measured δ15N-NH3 only would overestimate the contribution of agricultural emissions to NH3. By using an estimation of initial δ15N-NH3, we found that nonagricultural sources contributed ∼72% of NH3 on average. Our study suggests that (i) both persistent nonagricultural and periodic agricultural emissions drive atmospheric NH3 concentration and its vertical distribution in urban Beijing; and (ii) source apportionment based on measured δ15N-NH3 only likely underestimates fossil fuel source contribution, if the combined NHx isotope effects are not considered.

A review of measurements of air-surface exchange of reactive nitrogen in natural ecosystems across North America.

This review summarizes the state of the science of measurements of dry deposition of reactive nitrogen (Nr) compounds in North America, beginning with current understanding of the importance of dry deposition at the U.S. continental scale followed by a review of micrometeorological flux measurement methods. Measurements of Nr air-surface exchange in natural ecosystems of North America are then summarized, focusing on the U.S. and Canada. Drawing on this synthesis, research needed to address the incompleteness of dry deposition budgets, more fully characterize temporal and geographical variability of fluxes, and better understand air-surface exchange processes is identified. Our assessment points to several data and knowledge gaps that must be addressed to advance dry deposition budgets and air-surface exchange modeling for North American ecosystems. For example, recent studies of particulate (NO3-) and gaseous (NOx, HONO, peroxy nitrates) oxidized N fluxes challenge the fundamental framework of unidirectional flux from the atmosphere to the surface employed in most deposition models. Measurements in forest ecosystems document the importance of in-canopy chemical processes in regulating the net flux between the atmosphere and biosphere, which can result in net loss from the canopy. These results emphasize the need for studies to quantify within- and near-canopy sources and sinks of the full suite of components of the Nr chemical system under study (e.g., NOy or HNO3-NH3-NH4NO3). With respect to specific ecosystems and geographical locations, additional flux measurements are needed particularly in agricultural regions (NH3), coastal zones (NO3- and organic N), and arid ecosystems and along urban to rural gradients (NO2). Measurements that investigate non-stomatal exchange processes (e.g., deposition to wet surfaces) and the biogeochemical drivers of bidirectional exchange (e.g., NH3) are considered high priority. Establishment of long-term sites for process level measurements of reactive chemical fluxes should be viewed as a high priority long-term endeavor of the atmospheric chemistry and ecological communities.

Inorganic nitrogen wet deposition gradients in the Denver-Boulder metropolitan area and Colorado Front Range - Preliminary implications for Rocky Mountain National Park and interpolated deposition maps.

For the first time in the 40-year history of the National Atmospheric Deposition Program/National Trends Network (NADP/NTN), a unique urban-to-rural transect of wet deposition monitoring stations was operated as part of the NTN in 2017 to quantify reactive inorganic nitrogen wet deposition for adjacent urban and rural, montane regions. The transect of NADP stations (sites) was used to collect continuous precipitation depth and weekly wet-deposition samples in the Denver - Boulder, Colorado, urban corridor. Gradients in reactive inorganic nitrogen (Nr) concentrations and wet deposition were identified along the transect, which included Rocky Mountain National Park. Back trajectory modeling and stable isotopes suggested contribution of agricultural ammonia (NH3) to urban Nr wet deposition in Denver, but apportionment of wet-deposited Nr to agricultural versus urban mobile sources was not possible for this study. The results demonstrate the importance of multiple monitoring sites across an urban area in defining fine-scale geographic patterns in atmospheric deposition and its sources. Data from new sites located within 50 km of the urban area demonstrate that the urban influence does not extend as far as the inverse distance weighting would have suggested without such empirical monitoring data. It is important to determine the radius of influence of urban emissions and associated deposition on the interpolated deposition raster, which is constrained by a paucity of monitoring sites east of Denver.

Impact of Front Range sources on reactive nitrogen concentrations and deposition in Rocky Mountain National Park.

Human influenced atmospheric reactive nitrogen (RN) is impacting ecosystems in Rocky Mountain National Park (ROMO). Due to ROMO's protected status as a Class 1 area, these changes are concerning, and improving our understanding of the contributions of different types of RN and their sources is important for reducing impacts in ROMO. In July-August 2014 the most comprehensive measurements (to date) of RN were made in ROMO during the Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ). Measurements included peroxyacetyl nitrate (PAN), C1-C5 alkyl nitrates, and high-time resolution NOx, NOy, and ammonia. A limited set of measurements was extended through October. Co-located measurements of a suite of volatile organic compounds provide information on source types impacting ROMO. Specifically, we use ethane as a tracer of oil and gas operations and tetrachloroethylene (C2Cl4) as an urban tracer to investigate their relationship with RN species and transport patterns. Results of this analysis suggest elevated RN concentrations are associated with emissions from oil and gas operations, which are frequently co-located with agricultural production and livestock feeding areas in the region, and from urban areas. There also are periods where RN at ROMO is impacted by long-range transport. We present an atmospheric RN budget and a nitrogen deposition budget with dry and wet components. Total deposition for the period (7/1-9/30) was estimated at 1.58 kg N/ha, with 87% from wet deposition during this period of above average precipitation. Ammonium wet deposition was the dominant contributor to total nitrogen deposition followed by nitrate wet deposition and total dry deposition. Ammonia was estimated to be the largest contributor to dry deposition followed by nitric acid and PAN (other species included alkyl nitrates, ammonium and nitrate). All three species are challenging to measure routinely, especially at high time resolution.

Quantum Yields of Nitrite (NO2-) from the Photolysis of Nitrate (NO3-) in Ice at 313 nm.

Photochemical reactions of nitrate in snow release reactive nitrogen species via two channels, which produce (1) nitrogen dioxide (NO2) and hydroxyl radical (•OH) and (2) nitrite (NO2-) and oxygen atom (O(3P)). Quantum yields (Φ) for these channels are generally well characterized, except for channel 2 in ice. In this study, we quantify Φ(NO2-) in water ices and examine the impacts of pH and organic scavengers of •OH. Compared to solution results, we find that nitrite quantum yields in ice are more sensitive to pH and that •OH scavengers are less effective, although 2-propanol appears to work well. The temperature dependence (-30 to 25 °C) of Φ(NO2-) in samples containing 2-propanol is well described by a single regression line, ln(Φ(NO2-)) = -(1330 ± 100)(1/T(K)) + (0.09 ± 0.39). At -10 °C, the resulting quantum yield is 4.6 times larger than the previously reported (and recommended) value without an •OH scavenger. Although some reports suggest nitrite is a minor product from nitrate photolysis, based on our current and past results, rates of photoproduction of NO2- and NO2 are similar at room temperature, while NO2- production dominates at lower temperatures in solution and ice.

Quantum Yield of Nitrite from the Photolysis of Aqueous Nitrate above 300 nm.

Photolysis of nitrate (NO3-) produces reactive nitrogen and oxygen species via three different channels, forming: (1) nitrogen dioxide (NO2) and hydroxyl radical (•OH), (2) nitrite (NO2-) and oxygen atom (O(3P)), and (3) peroxynitrite (ONOO-). These photoproducts are important oxidants and reactants in surface waters, atmospheric drops, and snowpacks. While the efficiency of the first channel, to form NO2, is well documented, a large range of values have been reported for the second channel, nitrite, above 300 nm. In part, this disagreement reflects secondary chemistry that can produce or destroy nitrite. In this study, we examine factors that influence nitrite production and find that pH, nitrate concentration, and the presence of an •OH scavenger can be important. We measure an average nitrite quantum yield (Φ(NO2-)) of (1.1 ± 0.2)% (313 nm, 50 µM nitrate, pH ≥ 5), which is at the upper end of past measurements and an order of magnitude above the smallest-and most commonly cited-value reported for this channel. Nitrite production is often considered a very minor channel in nitrate photolysis, but our results indicate it is as important as the NO2 channel. In contrast, at 313 nm we observe no formation of peroxynitrite, corresponding to Φ(ONOO-) < 0.26%.

Aerosol species concentrations and source apportionment of ammonia at Rocky Mountain National Park.

Changes in ecosystem function at Rocky Mountain National Park (RMNP) are occurring because of emissions of nitrogen and sulfate species along the Front Range of the Colorado Rocky Mountains, as well as sources farther east and west. The nitrogen compounds include both oxidized and reduced nitrogen. A year-long monitoring program of various oxidized and reduced nitrogen species was initiated to better understand their origins as well as the complex chemistry occurring during transport from source to receptor. Specifically the goals of the study were to characterize the atmospheric concentrations of nitrogen species in gaseous, particulate, and aqueous phases (precipitation and clouds) along the east and west sides of the Continental Divide; identify the relative contributions to atmospheric nitrogen species in RMNP from within and outside of the state of Colorado; identify the relative contributions to atmospheric nitrogen species in RMNP from emission sources along the Colorado Front Range versus other areas within Colorado; and identify the relative contributions to atmospheric nitrogen species from mobile sources, agricultural activities, and large and small point sources within the state of Colorado. Measured ammonia concentrations are combined with modeled releases of conservative tracers from ammonia source regions around the United States to apportion ammonia to its respective sources, using receptor modeling tools.