Nitrogenous air pollutants and ozone exposure in the central Sierra Nevada and White Mountains of California - Distribution and evaluation of ecological risks

Estimates of biomass reductions of ozone sensitive herbaceous plants in California

Exposure to tropospheric ozone pollution impairs photosynthesis and growth in plants and this can have consequences for ecosystems. However, exposure-response research in the United States (U.S.) has historically focused on trees and crops, and less attention has been given to non-crop herbaceous species. We combined U.S. Environmental Protection Agency ozone monitoring data from the entirety of 2016 with published exposure-response relationships from controlled exposure experiments for twenty herbaceous plant species occurring in California. The U.S. Department of Agriculture PLANTS database was used to identify county-level occurrence data of these plant species. A kriged ozone exposure surface for 2016 was generated using data from monitoring stations in California and surrounding states, using Accumulated Ozone exposure over a Threshold of 40 ppb (AOT40) as an exposure metric. County-wide ozone exposure estimations were then combined with published exposure response functions for focal plants, and maps were created to estimate ozone-induced growth losses in the counties where the plants occur. Plant species had estimated annual growth losses from <1 % to >20 % based on exposure levels and sensitivity. Of the 20 species, 17 had predicted biomass loss >5 % in at least one county, emphasizing the vulnerability of herbaceous species at recent ozone concentrations. Butte, Nevada, Plumas, San Luis Obispo, and Shasta Counties, an area of about 31,652 km2, had the highest number of species (6) with >10 % estimated biomass loss, the loss threshold for European critical levels. White clover (Trifolium repens L.) was one of the most affected species with more than an estimated 10 % annual estimated growth loss over 59 % of the state. Overall, these estimated growth losses demonstrate potential for shifts in plant communities and negative effects on ecosystems. This study addresses critical policy needs for risk assessments on herbaceous species in a single year of ozone exposure.

Nitrogen addition changed the relationships of fine root respiration and biomass with key physiological traits in ozone-stressed poplars

Increasing ozone (O₃) and nitrogen (N) addition may have contradictory effects on plant photosynthesis and growth. However, it remains unclear whether these effects on aboveground parts further change the root resource management strategy and the relationships of fine root respiration and biomass with other physiological traits. In this study, an open-top chamber experiment was conducted to investigate the effects of O₃ alone and in combination with nitrogen (N) addition on root production and fine root respiration of poplar clone 107 (Populus × euramericana cv. '74/76'). Saplings were grown with (100 kg ha^-1 year^-1) or without (+0 kg ha^-1 year^-1) N addition under two O₃ regimes (non-filtered ambient air or non-filtered ambient air + 60 ppb of O₃). After about two to three months of treatment, elevated O₃ significantly decreased fine root biomass and starch content but increased fine root respiration, which occurred in tandem with inhibited leaf light-saturated photosynthetic rate (A_sat). Nitrogen addition did not change fine root respiration or biomass, neither did it alter the effect of elevated O₃ on the fine root traits. However, N addition weakened the relationships of fine root respiration and biomass with A_sat, fine root starch and N concentrations. No significant relationships of fine root biomass and respiration with soil mineralized N were observed under elevated O₃ or N addition. These results imply that changed relationships of plant fine root traits under global changes should be considered into earth system process models to project more accurately future carbon cycle.

Performances of a system for free-air ozone concentration elevation with poplar plantation under increased nitrogen deposition

The increasing emission of nitrogen oxides exerts large impacts on vegetation by raising surface ozone (O3) concentrations and enhancing atmospheric nitrogen (N) deposition. We established a free-air O3 concentration elevation and enhanced N deposition system (O3-N-FACE) in Beijing, China, to investigate long-term effects of elevated O3 and N deposition on poplar plantation. Eight square plots with a side length of 16 m were randomly allocated to elevated O3 (E-O3) and ambient air (AA) treatments. Ozone generated by electric discharge in pure oxygen is mixed with clean and dry air, and released from small holes on the tubes installed above the plant canopy at a rate controlled to keep O3 concentration in E-O3 plots by 50% higher than that in AA plots. Each O3 treatment plot consisted of four subplots with a factorial combination of 2 lines of poplar clones and 2 levels of N deposition rate. In enhanced N deposition subplots, we sprayed urea solution on the plantation floor at a rate of 60 kg ha-1 year-1. We hereby present the system performances during the growing seasons of 2018 and 2019: the first 2 years of experiment. The mean daytime O3 concentrations of E-O3 plots were 38% and 31% higher than AA plots in 2018 and 2019, respectively. And, in 2019, the accumulated O3 exposure over 40 ppb (AOT40) in E-O3 plots was 70% higher than that in AA plots. The hourly mean O3 concentrations in E-O3 plots were within 20% of the target for 83% of time on average across the four E-O3 plots. Within the E-O3 plots, spatial distribution of the hourly O3 concentration exhibited the maximum deviation at 24% in 2019. We concluded that performance of this system is better than other similar facilities for trees and suitable for a long-term experiment of enhanced O3 and N.

Assessing the intra-urban variability of nitrogen oxides and ozone across a highly heterogeneous urban area

High-resolution air quality maps are critical towards assessing and understanding exposures to elevated air pollution in dense urban areas. However, these surfaces are rarely available in low- and middle-income countries that suffer from some of the highest air pollution levels worldwide. In this study, we make use of land use regressions (LURs) to generate annual and seasonal, high-resolution nitrogen dioxide (NO₂), nitrogen oxides (NO_x), and ozone (O₃) exposure surfaces for the Greater Beirut Area (GBA) in Lebanon. NO₂, NO_x and O₃ concentrations were monitored using passive samplers that were deployed at 55 pre-defined monitoring locations. The average annual concentrations of NO₂, NO_x, and O₃ across the GBA were 36.0, 89.7, and 26.9 ppb, respectively. Overall, the performance of the generated models was appropriate, with low biases, high model robustness, and acceptable R² values that ranged between 0.66 and 0.73 for NO₂, 0.56 and 0.60 for NO_x, and 0.54 and 0.65 for O₃. Traffic-related emissions as well as the operation of a fossil-fuel power plant were found to be the main contributors to the measured NO₂ and NO_x levels in the GBA, whereas they acted as sinks for O₃ concentrations. No seasonally significant differences were found for the NO₂ and NO_x pollution surfaces; as their seasonal and annual models were largely similar (Pearson's r > 0.85 for both pollutants). On the other hand, seasonal O₃ pollution surfaces were significantly different. The model results showed that around 99% of the population of the GBA were exposed to NO₂ levels that exceeded the World Health Organization defined annual standard.

Spatiotemporal variations and risk assessment of ambient air O3, PM10 and PM2.5 in a coastal city of China

Rapid industrialization and urbanization has created significant air pollution problems that have recently begin to impact the lives and health of human beings in China. This study systematically investigated the spatiotemporal variations and the associated health risks of ambient O₃, PM₁₀ and PM_2.5 between 2016 and 2019. The relationships between the target air pollutants and meteorological conditions were further analyzed using the Spearman rank correlation coefficient method. The results demonstrated that the annual mean concentrations of PM₁₀ and PM_2.5 experienced a decreasing trend overall, and PM_2.5 significantly decreased from 1.54 μg/m³ in 2016 to 1.48 μg/m³ in 2019. In contrast, the annual mean concentrations of O₃ were nearly constant during the study period with a slight increasing trend. The pollutants exhibited different seasonal variations and cyclical diurnal variations. The most highest O₃ pollution was seen in spring and summer, while spring and winter were the seasons with the most PM₁₀ and PM_2.5 pollution. The highest concentrations of O₃ appeared in periods of strong solar radiation intensity and photochemical reactions. The highest concentrations of PM₁₀ and PM_2.5 appeared at commuting time. The pollutant concentrations were significantly affected by meteorological conditions. Finally, the non-carcinogenic risks from exposure to O₃, PM₁₀ and PM_2.5 were at an acceptable level (HI < 0.96) and O₃ accounted for ~50% of the total non-carcinogenic risks. However, PM_2.5 posed highly carcinogenic risks (2.5 × 10^-4 < CR < 1.6 × 10^-1) and O₃ exposure showed high potential ecological impacts on vegetation (AOT40: 23.3 ppm-h; W126: 29.0 ppm-h).

Association between exposure to airborne pollutants and COVID-19 in Los Angeles, United States with ensemble-based dynamic emission model

This study aims to find the association between short-term exposure to air pollutants, such as particulate matters and ground-level ozone, and SARS-CoV-2 confirmed cases. Generalized linear models (GLM), a typical choice for ecological modeling, have well-established limitations. These limitations include apriori assumptions, inability to handle multicollinearity, and considering differential effects as the fixed effect. We propose an Ensemble-based Dynamic Emission Model (EDEM) to address these limitations. EDEM is developed at the intersection of network science and ensemble learning, i.e., a specialized approach of machine learning. Generalized Additive Model (GAM), i.e., a variant of GLM, and EDEM are tested in Los Angeles and Ventura counties of California, which is one of the biggest SARS-CoV-2 clusters in the US. GAM depicts that a 1 μg/m³, 1 μg/m³, and 1 ppm increase (lag 0-7) in PM 2.5, PM 10, and O3 is associated with 4.51% (CI: 7.01 to -2.00) decrease, 1.62% (CI: 2.23 to -1.022) decrease, and 4.66% (CI: 0.85 to 8.47) increase in daily SARS-CoV-2 cases, respectively. Subsequent increment in lag resulted in the negative association between pollutants and SARS-CoV-2 cases. EDEM results in an R2 score of 90.96% and 79.16% on training and testing datasets, respectively. EDEM confirmed the negative association between particulates and SARS-CoV-2 cases; whereas, the O3 depicts a positive association; however, the positive association observed through GAM is not statistically significant. In addition, the county-level analysis of pollutant concentration interactions suggests that increased emissions from other counties positively affect SARS-CoV-2 cases in adjoining counties as well. The results reiterate the significance of uniformly adhering to air pollution mitigation strategies, especially related to ground-level ozone.

The contribution of wildland fire emissions to deposition in the U S: implications for tree growth and survival in the Northwest

Ecosystems require access to key nutrients like nitrogen (N) and sulfur (S) to sustain growth and healthy function. However, excessive deposition can also damage ecosystems through nutrient imbalances, leading to changes in productivity and shifts in ecosystem structure. While wildland fires are a known source of atmospheric N and S, little has been done to examine the implications of wildland fire deposition for vulnerable ecosystems. We combine wildland fire emission estimates, atmospheric chemistry modeling, and forest inventory data to (a) quantify the contribution of wildland fire emissions to N and S deposition across the U S, and (b) assess the subsequent impacts on tree growth and survival rates in areas where impacts are likely meaningful based on the relative contribution of fire to total deposition. We estimate that wildland fires contributed 0.2 kg N ha-1 yr-1 and 0.04 kg S ha-1 yr-1 on average across the U S during 2008-2012, with maxima up to 1.4 kg N ha-1 yr-1 and 0.6 kg S ha-1 yr-1 in the Northwest representing over ~30% of total deposition in some areas. Based on these fluxes, exceedances of S critical loads as a result of wildland fires are minimal, but exceedances for N may affect the survival and growth rates of 16 tree species across 4.2 million hectares, with the most concentrated impacts occurring in Oregon, northern California, and Idaho. Understanding the broader environmental impacts of wildland fires in the U S will inform future decision making related to both fire management and ecosystem services conservation.

An integrated system for rapid assessment of ecological quality based on remote sensing data

Ecological quality assessment (EQA) is important for regional socio-economic development and its sustainability. To assess the status of land ecological quality more precisely, an ecological quality assessment system with 11 indicators of ecological stability, ecosystem service function, and habitat stress was established using the analytic hierarchy process for Guangdong Province, a highly urbanized region of China. Remotely sensed data were mainly used to quantify the 11 indicators and acquire regional EQA graphs at high spatial resolution. In addition, we used the spatial autocorrelation measure Moran's I to capture dynamic signatures of spatial organization of ecological quality in the study area. The results show that the ecological quality of the study area is heterogeneous spatially but relatively consistent in different regions. Significant positive spatial autocorrelation for EQI in Guangdong was revealed by global Moran's I. Potential ecological hot spot or cold spot were detected based on the spatial clustering patterns that were obtained by local Moran's I. Lands with low ecological quality is mainly distributed in economically developed areas such as the Pearl River Delta and coastal cities in eastern and western Guangdong, while those with high ecological quality are mostly situated in northern mountainous areas that have lush vegetation. The low assessment scores for Guangdong, especially for the Pearl River Delta, are highly correlated with large populations and degrees of industrial agglomeration; this is mainly because urbanization and economic development jeopardize the environment. The presented case study can facilitate information provision and targeted strategy making for environmental protection. This study provides a helpful approach to assess and to analyze the ecological status in the future research. In contrast with methods that employ a single metric and limited data, the assessment system proposed in this study expands the potential application of the remotely sensed data and enriches the methodological system for EQAs.

The higher availability of nutrients increases the production but decreases the quality of pollen grains in Juniperus communis L

Next to global warming, excessive deposition of nitrogen has an alarming environmental impact on forest ecology, especially within dioecious species. Resource availability affects seed quality and can affect the distribution of plant species. Lower seed productivity can also be a result of limited pollen availability or lower pollen quality. A few studies have assessed the effect of nutrient availability on the quantity and quality of pollen grains produced. Therefore, rooted shoots of dioecious, male Juniperus communis L. grown in different nutritional conditions were used to assess the impact on productivity and quality attributes of produced pollen grains (pollen volume, morphology, germination, and chemical composition). The results indicated that nutrient availability impacts pollen grain development. Male plants growing in nutrient-rich environments appear to compensate for the lower quality of produced pollen grains by producing a higher number of male cones and thus a greater quantity of pollen. In contrast, the opposite was observed in plants growing in nutrient-poor environments. The availability of nitrogen and other nutrients will probably continue to impact soils in the foreseeable future due to anthropogenic activity and can be one of the drivers that can impact the reproduction and distribution of plants.