Terrain effect on atmospheric process in seasonal ozone variation over the Sichuan Basin, Southwest China

Terrain effect is challenging for understanding atmospheric environment changes under complex topography. This study targets the Sichuan Basin (SCB), a deep basin isolated by plateaus and mountains in Southwest China, by employing WRF-Chem with integrated process rates (IPR) analysis to characterize the terrain-driven seasonal variations of tropospheric ozone (O3) with atmospheric physical and chemical processes. Results show that the basin terrain exerts reversed impacts on regional air quality changes by aggravating summertime and alleviating wintertime near-surface O3 with the relative contributions oscillating seasonally between -40% and 40% in SCB. Similarly, a seasonal shift of vertical O3 structures is dominated by summertime positive and wintertime negative changes in the lower troposphere induced by basin terrain. The key contributions of atmospheric process to near-surface O3 are identified with vertical and horizontal transport, which is dominated by basin terrain with intensifying seasonal and diurnal variations. With the existence of basin, the daytime O3 productions at the near-surface layer are elevated in months of warm seasons (April and July) but inhibited in the cold seasons (October and January), presenting a seasonal transition of primary factor from meteorology to aerosol-radiation forcing on photochemical reactions. Driven by plateau-basin thermodynamic forcing, horizontal O3 transport between the SCB and eastern TP is enhanced by mountain-plains solenoid (MPS), and even nocturnal O3-rich layers contribute to the impacts of vertical exchange on near-surface O3 levels. The terrain effects of deep basin under the interaction of Asian monsoons and westerlies could jointly change atmospheric physical and chemical processes to construct the seasonal and diurnal O3 evolution patterns over the SCB region.

Ozone pollution, water deficit stress and time drive poplar phyllospheric bacterial community structure

Ground-level ozone (O₃) pollution often rise in the summer and coincide with drought stress, which alters the relationships between trees and associated microbial communities in a manner that can have pronounced effects on associated biological activity and ecosystem integrity. Discerning the responses of phyllosphere microbial communities to O₃ and water deficit could highlight the ability of plant-microbe interactions to either exacerbate or mitigate the effects of these stressors. Accordingly, this study was designed as the first report to specifically interrogate the impacts of elevated O₃ and water deficit stress on phyllospheric bacterial community composition and diversity in hybrid poplar saplings. Significant reductions in phyllospheric bacterial alpha diversity indices were observed, with clear evidence of significant time × water deficit stress interactions. The combination of elevated O₃ and water deficit stress shifted in the bacterial community composition over sampling time, resulted in significant increases in the relative abundance of the dominant Gammaproteobacteria phyla together with reductions in Betaproteobacteria. An increased prevalence of Gammaproteobacteria may represent a potential diagnostic dysbiosis-related biosignature associated with poplar disease risk. Significant positive correlations were observed between both Betaproteobacteria abundance and diversity indices and key foliar photosynthetic traits and isoprene emissions, whereas these parameters were negatively correlated with Gammaproteobacteria abundance. These findings suggest that the photosynthesis-related properties in plant leaves are closely related to the makeup of the phyllosphere bacterial community. These data provide novel insight into how plant-associated microbes can help maintain plant health and the stability of the local ecosystem in O₃-polluted and dried environments.

Temporal characteristics of carbon dioxide and ozone over a rural-cropland area in the Yangtze River Delta of eastern China

In this study, rural atmospheric carbon dioxide (CO₂) and ozone (O₃) were measured from January 2015 to December 2018 to investigate characteristics of greenhouse gases in eastern China. Results showed that the annual average CO₂ (O₃) concentration in 2018 decreased by 2% (increased by 19%) when compared with that in 2015. CO₂ concentrations exhibited monthly variability, peaking in February (443.7 ppm) and reaching their lowest levels in July (363.0 ppm); whereas, monthly O₃ showed a bimodal pattern with peaks in June (51.3 ppb) and September (34.5 ppb). Regarding the diurnal variation, the maximum CO₂ (O₃) concentration occurred at nighttime (in the daytime) and a minimum CO₂ (O₃) in the daytime (at nighttime). As demonstrated by correlation analysis, CO₂ and O₃ variations were partly modulated by NOx and PM_2.5. Furthermore, CO₂ showed significant positive correlations with relative humidity in winter, while O₃ showed strong positive correlations with temperature in spring. CO₂ was accumulated from local sources under calm conditions (< 2 m s^-1) and derived from remote sources at high wind speeds (> 4 m s^-1), while O₃ concentrations were peaking at medium wind speeds of 2-4 m s^-1. CO₂ was found to derive from long-distance (short-distance transport) transport in spring (the other three seasons), whereas O₃ is mainly from long-distance (short-distance) transport in winter (the other three seasons). This work sheds light on the temporal characteristics of CO₂ and O₃, which has important implications for implementing practices to mitigate source emissions over cropland areas.

Effect of bromine and iodine chemistry on tropospheric ozone over Asia-Pacific using the CMAQ model

Recent studies have focused on the chemistry of tropospheric halogen species which are able to deplete tropospheric ozone (O₃). In this study, the effect of bromine and iodine chemistry on tropospheric O₃ within the annual cycle in Asia-Pacific is investigated using the CMAQ model with the newly embedded bromine and iodine chemistry and a blended and customized emission inventory considering marine halogen emission. Results indicate that the vertical profiles of bromine and iodine species show distinct features over land/ocean and daytime/nighttime, related to natural and anthropogenic emission distributions and photochemical reactions. The halogen-mediated O₃ loss has a strong seasonal cycle, and reaches a maximum of -15.9 ppbv (-44.3%) over the ocean and -13.4 ppbv (-38.9%) over continental Asia among the four seasons. Changes in solar radiation, dominant wind direction, and nearshore chlorophyll-a accumulation all contribute to these seasonal differences. Based on the distances to the nearest coastline, the onshore and offshore features of tropospheric O₃ loss caused by bromine and iodine chemistry are studied. Across a coastline-centric 400-km-wide belt from onshore to offshore, averaged maximum gradient of O₃ loss reaches 1.1 ppbv/100 km at surface level, while planetary boundary layer (PBL) column mean of O₃ loss is more moderate, being approximately 0.7 ppbv/100 km. Relative high halogen can be found over Tibetan Plateau (TP) and the largest O₃ loss (approximately 4-5 ppbv) in the PBL can be found between the western boundary of the domain and the TP. Halogens originating from marine sources can potentially affect O₃ concentration transported from the stratosphere over the TP region. As part of efforts to improve our understanding of the effect of bromine and iodine chemistry on tropospheric O₃, we call for more models and monitoring studies on halogen chemistry and be considered further in air pollution prevention and control policy.

Response of isoprene emission from poplar saplings to ozone pollution and nitrogen deposition depends on leaf position along the vertical canopy profile

We investigated isoprene (ISO) emission and gas exchange in leaves from different positions along the vertical canopy profile of poplar saplings (Populus euramericana cv. '74/76'). For a growing season, plants were subjected to four N treatments, control (NC, no N addition), low N (LN, 50 kg N ha^-1year^-1), middle N (MN, 100 kg N ha^-1year^-1), high N (HN, 200 kg N ha^-1year^-1) and three O₃ treatments (CF, charcoal-filtered ambient air; NF, non-filtered ambient air; NF + O₃, NF + 40 ppb O₃). Our results showed the effects of O₃ and/or N on standardized ISO rate (ISO_rate) and photosynthetic parameters differed along with the leaf position, with larger negative effects of O₃ and positive effects of N on ISO_rate and photosynthetic parameters in the older leaves. Expanded young leaves were insensitive to both treatments even at very high O₃ concentration (67 ppb as 10-h average) and HN treatment. Significant O₃ × N interactions were only found in middle and lower leaves, where ISO_rate declined by O₃ just when N was limited (NC and LN). With increasing light-saturated photosynthesis and chlorophyll content, ISO_rate was reduced in the upper leaves but on the contrary increased in middle and lower leaves. The responses of ISO_rate to AOT40 (accumulated exposure to hourly O₃ concentrations > 40 ppb) and POD_Y (accumulative stomatal uptake of O₃ > Y nmol O₃ m^-2 PLA s^-1) were not significant in upper leaves, but ISO_rate significantly decreased with increasing AOT40 or POD_Y under limited N supply in middle leaves but at all N levels in lower leaves. Overall, ISO_rate changed along the vertical canopy profile in response to combined O₃ and N exposure, a behavior that should be incorporated into multi-layer canopy models. Our results are relevant for modelling regional isoprene emissions under current and future O₃ pollution and N deposition scenarios.

Ozone exposure, nitrogen addition and moderate drought dynamically interact to affect isoprene emission in poplar

Ozone (O₃) pollution can induce changes in plant growth and metabolism, and in turn, affects isoprene emission (ISO), but the extent of these effects may be modified by co-occurring soil water and nitrogen (N) availability. To date, however, much less is known about the combined effects of two of these factors on isoprene emission from plants. We investigated for the first time the combined effects of O₃ exposure (CF, charcoal-filtered air; EO₃, non-filtered air plus 40 ppb of O₃), N addition (N0, no additional N; N50, 50 kg ha^-1 year^-1 of N) and moderate drought (WW, well-watered; WR, 40% of WW irrigation) on photosynthetic carbon assimilation and ISO emission in hybrid poplar at both leaf- and plant-level over time. Consistent with leaf-level photosynthesis (Pn_leaf) and ISO (ISO_leaf) responses, plant-level ISO (ISO_plant) responses to O₃, N addition and moderate drought were more marked after long exposure (September) than short exposure duration (July). EO₃ significantly decreased ISO_leaf and Pn_leaf, while WR and N50 significantly increased them. Although O₃ and water interacted significantly to affect Pn_leaf over the exposure duration, neither N50 nor WR mitigated the negative effects of EO₃ on ISO_leaf. When ISO was scaled up to the plant level, the WR-induced increase in ISO_leaf under EO₃ was offset by a reduction in total leaf area. By contrast, effects of EO₃ on ISO_plant were not changed by N addition. Our results highlight that the dynamic effects on ISO emission change over the exposure duration depending on involved co-occurring factors and evaluation scales.

No significant interactions between nitrogen stimulation and ozone inhibition of isoprene emission in Cathay poplar

Isoprene emission from plants subject to a combination of ozone (O₃) and nitrogen (N) has never been investigated. Cathay poplar (Populus cathayana) saplings were exposed to O₃ (CF, charcoal-filtered air, NF, non-filtered ambient air and E-O₃, non-filtered air +40ppb) and N treatments (N0, 0kgNha^-1year^-1, N50, 50kgNha^-1year^-1 and N100, 100kgNha^-1year^-1) for 96days. Increasing O₃ exposure decreased isoprene emission (11.5% in NF and 57.9% in E-O₃), as well as light-saturated photosynthetic rate (A_sat) and chlorophyll content, while N load increased isoprene emission (19.6% in N50 and 33.4% in N100) as well as A_sat and chlorophyll content. Although O₃ and N interacted significantly in A_sat, N did not mitigate the negative effects of O₃ on isoprene emission, i.e. the combined effects were additive and did not interact. These results warrant more research on the combined effects of co-existing global change factors on future isoprene emission and atmospheric chemical processes.

Characteristics of surface O₃ over Qinghai Lake area in Northeast Tibetan Plateau, China

Surface O3 was monitored continuously during Aug. 12, 2010 to Jul. 21, 2011 at a high elevation site (3,200 m above sea level) in Qinghai Lake area (36°58'37″N, 99°53'56″E) in Northeast Tibetan Plateau, China. Daily average O3 ranged from 21.8 ppbv to 65.3 ppbv with an annual average of 41.0 ppbv. Seasonal average of O3 followed a decreasing order of summer>autumn>spring>winter. Diurnal variations of O3 showed low concentrations during daytime and high concentrations during late night and early morning. An intensive campaign was also conducted during Aug. 13-31, 2010 to investigate correlations between meteorological or chemical conditions and O3. It was found that O3 was poorly correlated with solar radiation due to the insufficient NOx in the ambient air, thus limiting O3 formation under strong solar radiation. In contrast, high O3 levels always coincided with strong winds, suggesting that stratospheric O3 and long range transport might be the main sources of O3 in this rural area. Back-trajectory analysis supported this hypothesis and further indicated the transport of air masses from northwest, northeast and southeast directions.

Identification and characterization of MOR-CP, a cysteine protease induced by ozone and developmental senescence in maize (Zea mays L.) leaves

Among the different classes of endoproteases, cysteine proteases are consistently associated with senescence, defense signaling pathways and cellular responses to abiotic stresses. The objectives of this work were to study the effects of various concentrations of ozone on gene expression and enzymatic activity for papain-like cysteine proteases (PLCPs), in the leaves of maize plants grown under field conditions. Leaves from ranks 12 and 10 (cob leaf) were harvested regularly over a long-term artificial ozone fumigation experiment (50 d). Tissues were tested for transcriptional and activity changes concerning cysteine proteases, using qRT-PCR for the newly identified ozone-responsive PLCP gene (Mor-CP) and synthetic oligopeptide Boc-Val-Leu-Lys-AMC as a PLCP-specific substrate, respectively. Results showed that developmental senescence induced a significant and progressive rise in CP activity, only in the older leaves 10 and had no effect on Mor-CP gene expression levels. On the other hand, ozone dramatically enhanced Mor-CP mRNA levels and global PLCP enzymatic activity in leaves 12 and 10, particularly toward the end of the treatment. Ozone impact was more pronounced in the older leaves 10. Together, these observations concurred to conclude that ozone stress enhances natural senescence processes, such as those related to proteolysis.