Fluxes of nitrogen oxides above a subtropical forest canopy in China

Dry deposition of Nitrogen (N) in forests is commonly estimated from inferential method and/or throughfall measurements, with inevitable uncertainty. In this study, we applied an aerodynamic gradient method to directly measure the nitrogen oxides (NO_x) flux above the canopy of a subtropical forest in southeastern China for two consecutive years. The flux and transfer velocity generally reached the maximum absolute values in the midday, with the largest diurnal maximum of absolute flux values observed in the winter of 2015 and that of transfer velocity in the autumn of 2015. The annual average transfer velocity was -0.79 and -0.38 cm s^-1 in 2015 and 2016, respectively. Although the net downward NO_x fluxes predominated for both years, upward flux (net emission) of NO_x was observed during spring months, which reflected the possible bi-directional exchange balanced by soil-atmosphere and foliage-atmosphere exchanges. The NO_x concentration seemed to be the most important factor controlling the NO_x exchange above canopy, and could mainly explain the seasonal variation of N deposition. The linear regression between the NO_x flux and concertation was explored, and it was observed that the deposition of NO_x was offset by possible underlayer emission of NO_x when the ambient NO_x concentration below1.7 ppbv and 1.9 ppbv at night and in the day, respectively. The average dry deposition of NO_x for the two years was 6.28 ± 0.06 kg N ha^-1 a^-1, >40% of which might be uptake by the canopy, estimated by comparing the wet/throughfall deposition measurement of nitrate with the observation of NO_x flux. This indicated the importance of stomatal uptake of NO_x in nitrogen budget in subtropical forests.

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.

Characteristics and influence factors of NO2 exchange flux between the atmosphere and P. nigra

Nitrogen dioxide (NO₂) is an important substance in atmospheric photochemical processes and can also be absorbed by plants. NO₂ fluxes between the atmosphere and P. nigra seedlings were investigated by a double dynamic chambers method in Beijing from June 15 to September 3, 2017. The range of NO₂ exchange fluxes between P. nigra seedlings and the atmosphere was from -14.6 to 0.8 nmol/(m²·sec) (the positive data represent NO₂ emission from trees, while the negative values indicate absorption). Under ambient concentrations, the mean NO₂ flux during the fast-growing stage (Jun. 15-Aug. 4) was -3.0 nmol/(m²·sec), greater than the flux of -1.5 nmol/(m²·sec) during the later growth stage (Aug. 8-Sept. 3). The daily exchange fluxes of NO₂ obviously fluctuated. The fluxes were largest in the morning and decreased gradually over time. Additionally, the NO₂ fluxes were larger under high light intensities than under low light intensities during the whole growth period. The effects of temperature on NO₂ fluxes were different under two growth periods. The NO₂ exchange fluxes were larger in a range of temperatures close to 44°C in the fast-growing stage, whereas there were no evident differences in NO₂ exchange fluxes under widely differing temperatures in the later growth stage. Under polluted conditions, the uptake ability of NO₂ was weakened. Additionally, the compensation point of NO₂ was 5.6 ppb in the fast-growing stage, whereas it was 1.4 ppb in the later growth stage. The deposition velocities of NO₂ were between 0.3 and 2.4 mm/sec.

Limited effect of ozone reductions on the 20-year photosynthesis trend at Harvard forest

Ozone (O₃ ) damage to leaves can reduce plant photosynthesis, which suggests that declines in ambient O₃ concentrations ([O₃ ]) in the United States may have helped increase gross primary production (GPP) in recent decades. Here, we assess the effect of long-term changes in ambient [O₃ ] using 20 years of observations at Harvard forest. Using artificial neural networks, we found that the effect of the inclusion of [O₃ ] as a predictor was slight, and independent of O₃ concentrations, which suggests limited high-frequency O₃ inhibition of GPP at this site. Simulations with a terrestrial biosphere model, however, suggest an average long-term O₃ inhibition of 10.4% for 1992-2011. A decline of [O₃ ] over the measurement period resulted in moderate predicted GPP trends of 0.02-0.04 μmol C m^-2  s^-1  yr^-1 , which is negligible relative to the total observed GPP trend of 0.41 μmol C m^-2  s^-1  yr^-1 . A similar conclusion is achieved with the widely used AOT40 metric. Combined, our results suggest that ozone reductions at Harvard forest are unlikely to have had a large impact on the photosynthesis trend over the past 20 years. Such limited effects are mainly related to the slow responses of photosynthesis to changes in [O₃ ]. Furthermore, we estimate that 40% of photosynthesis happens in the shade, where stomatal conductance and thus [O₃ ] deposition is lower than for sunlit leaves. This portion of GPP remains unaffected by [O₃ ], thus helping to buffer the changes of total photosynthesis due to varied [O₃ ]. Our analyses suggest that current ozone reductions, although significant, cannot substantially alleviate the damages to forest ecosystems.

Highly functionalized organic nitrates in the southeast United States: Contribution to secondary organic aerosol and reactive nitrogen budgets

Speciated particle-phase organic nitrates (pONs) were quantified using online chemical ionization MS during June and July of 2013 in rural Alabama as part of the Southern Oxidant and Aerosol Study. A large fraction of pONs is highly functionalized, possessing between six and eight oxygen atoms within each carbon number group, and is not the common first generation alkyl nitrates previously reported. Using calibrations for isoprene hydroxynitrates and the measured molecular compositions, we estimate that pONs account for 3% and 8% of total submicrometer organic aerosol mass, on average, during the day and night, respectively. Each of the isoprene- and monoterpenes-derived groups exhibited a strong diel trend consistent with the emission patterns of likely biogenic hydrocarbon precursors. An observationally constrained diel box model can replicate the observed pON assuming that pONs (i) are produced in the gas phase and rapidly establish gas-particle equilibrium and (ii) have a short particle-phase lifetime (∼2-4 h). Such dynamic behavior has significant implications for the production and phase partitioning of pONs, organic aerosol mass, and reactive nitrogen speciation in a forested environment.

The Role of Plant-Microbe Interactions and Their Exploitation for Phytoremediation of Air Pollutants

Since air pollution has been linked to a plethora of human health problems, strategies to improve air quality are indispensable. Despite the complexity in composition of air pollution, phytoremediation was shown to be effective in cleaning air. Plants are known to scavenge significant amounts of air pollutants on their aboveground plant parts. Leaf fall and runoff lead to transfer of (part of) the adsorbed pollutants to the soil and rhizosphere below. After uptake in the roots and leaves, plants can metabolize, sequestrate and/or excrete air pollutants. In addition, plant-associated microorganisms play an important role by degrading, detoxifying or sequestrating the pollutants and by promoting plant growth. In this review, an overview of the available knowledge about the role and potential of plant-microbe interactions to improve indoor and outdoor air quality is provided. Most importantly, common air pollutants (particulate matter, volatile organic compounds and inorganic air pollutants) and their toxicity are described. For each of these pollutant types, a concise overview of the specific contributions of the plant and its microbiome is presented. To conclude, the state of the art and its related future challenges are presented.

Fluxes of oxidised and reduced nitrogen above a mixed coniferous forest exposed to various nitrogen emission sources

Concentrations of nitrogen gases (NH(3), NO(2), NO, HONO and HNO(3)) and particles (pNH(4) and pNO(3)) were measured over a mixed coniferous forest impacted by high nitrogen loads. Nitrogen dioxide (NO(2)) represented the main nitrogen form, followed by nitric oxide (NO) and ammonia (NH(3)). A combination of gradient method (NH(3) and NO(x)) and resistance modelling techniques (HNO(3), HONO, pNH(4) and pNO(3)) was used to calculate dry deposition of nitrogen compounds. Net flux of NH(3) amounted to -64 ng N m(-2) s(-1) over the measuring period. Net fluxes of NO(x) were upward (8.5 ng N m(-2) s(-1)) with highest emission in the morning. Fluxes of other gases or aerosols substantially contributed to dry deposition. Total nitrogen deposition was estimated at -48 kg N ha(-1) yr(-1) and consisted for almost 80% of NH(x). Comparison of throughfall nitrogen with total deposition suggested substantial uptake of reduced N (+/-15 kg N ha(-1) yr(-1)) within the canopy.