scholarly journals The impacts of marine-emitted halogens on OH radical in East Asia during summer

2021 ◽  
Author(s):  
Shidong Fan ◽  
Ying Li
Keyword(s):  
East Asia ◽  
Atmospheric Chemistry ◽  
Southern China ◽  
Process Analysis ◽  
Oh Radical ◽  
Key Species ◽  
Air Chemistry ◽  
Inorganic Iodine ◽  
The Impact ◽  
Halogen Species

Abstract. Relationships between oceanic emissions and air chemistry are intricate and still not fully understood. For regional air chemistry, a better understanding of marine halogen emission on hydroxyl (OH) radical is crucial. OH radical is a key species in atmospheric chemistry because it can oxidize almost all trace species in the atmosphere. In the marine atmosphere, OH level could be significantly affected by the halogen species emitted from the ocean. However, due to the complicated interactions of halogens with OH through different pathways, it is not well understood how halogens influence OH and even great uncertain in the signs of net effect. Therefore, in this study, we aim to quantify the impact of marine-emitted halogens (including Cl, Br, and I) through different pathways on OH in the high OH season by using WRF-CMAQ model with process analysis and state-of-the-art halogen chemistry in the East Asia Seas. Results show a very complicated response of OH production rate (POH) to marine halogen emissions. The monthly POH is generally decreased over the ocean with maxima of about 10–15 % in the Philippine Sea, but is increased in many nearshore areas with maxima of about 7–9 % in the Bohai Sea. In the coastal areas of southern China, the monthly POH could also decrease 3–5 % in the Greater Bay Area, but with a daytime hourly maximum decrease over 30 %. Analysis to the individual reactions using integrated reaction rate (IRR) show that the net change of POH is controlled by the competitions of three main pathways through different halogen species. Sea spray aerosols (SSA) and inorganic iodine gases are the main species to influence the strengths of these three pathways and therefore have the most significant impacts on POH. Both of these two types of species decrease POH through physical processes, while generally increase POH through chemical processes. In the ocean atmosphere, the controlling species are inorganic iodine gases and the complicated iodine chemistry determines the basic pattern of ΔPOH, while over the continent, SSA is the controlling species and the SSA extinction effect leads to the negative ΔPOH in the southern China. Our results indicate that marine-emitted halogen species have notable impacts over the ocean and have potential impact on the coastal atmospheric oxidation. The identified main (previously known or unknown) pathways and their controlling factors from different halogen species to OH radical explains the halogen-induced change of POH East Asia and also can be applied in other circumstances (e.g., different domains, regions, and emission rates).

scholarly journals Importance of reactive halogens in the tropical marine atmosphere: a regional modelling study using WRF-Chem

2019 ◽  
Vol 19 (5) ◽  
pp. 3161-3189 ◽  
Author(s):  
Alba Badia ◽  
Claire E. Reeves ◽  
Alex R. Baker ◽  
Alfonso Saiz-Lopez ◽  
Rainer Volkamer ◽  
...  
Keyword(s):  
Atmospheric Chemistry ◽  
Sea Salt ◽  
Heterogeneous Chemistry ◽  
Marine Atmosphere ◽  
Tropical Ocean ◽  
Mixing Ratios ◽  
Reactive Halogen Species ◽  
Inorganic Iodine ◽  
The Impact ◽  
Halogen Species

Abstract. This study investigates the impact of reactive halogen species (RHS, containing chlorine (Cl), bromine (Br) or iodine (I)) on atmospheric chemistry in the tropical troposphere and explores the sensitivity to uncertainties in the fluxes of RHS to the atmosphere and their chemical processing. To do this, the regional chemistry transport model WRF-Chem has been extended to include Br and I, as well as Cl chemistry for the first time, including heterogeneous recycling reactions involving sea-salt aerosol and other particles, reactions of Br and Cl with volatile organic compounds (VOCs), along with oceanic emissions of halocarbons, VOCs and inorganic iodine. The study focuses on the tropical east Pacific using field observations from the Tropical Ocean tRoposphere Exchange of Reactive halogen species and Oxygenated VOC (TORERO) campaign (January–February 2012) to evaluate the model performance. Including all the new processes, the model does a reasonable job reproducing the observed mixing ratios of bromine oxide (BrO) and iodine oxide (IO), albeit with some discrepancies, some of which can be attributed to difficulties in the model's ability to reproduce the observed halocarbons. This is somewhat expected given the large uncertainties in the air–sea fluxes of the halocarbons in a region where there are few observations of their seawater concentrations. We see a considerable impact on the inorganic bromine (Bry) partitioning when heterogeneous chemistry is included, with a greater proportion of the Bry in active forms such as BrO, HOBr and dihalogens. Including debromination of sea salt increases BrO slightly throughout the free troposphere, but in the tropical marine boundary layer, where the sea-salt particles are plentiful and relatively acidic, debromination leads to overestimation of the observed BrO. However, it should be noted that the modelled BrO was extremely sensitive to the inclusion of reactions between Br and the oxygenated VOCs (OVOCs), which convert Br to HBr, a far less reactive form of Bry. Excluding these reactions leads to modelled BrO mixing ratios greater than observed. The reactions between Br and aldehydes were found to be particularly important, despite the model underestimating the amount of aldehydes observed in the atmosphere. There are only small changes to the inorganic iodine (Iy) partitioning and IO when the heterogeneous reactions, primarily on sea salt, are included. Our model results show that tropospheric Ox loss due to halogens ranges between 25 % and 60 %. Uncertainties in the heterogeneous chemistry accounted for a small proportion of this range (25 % to 31 %). This range is in good agreement with other estimates from state-of-the-art atmospheric chemistry models. The upper bound is found when reactions between Br and Cl with VOCs are not included and, consequently, Ox loss by BrOx, ClOx and IOx cycles is high (60 %). With the inclusion of halogens in the troposphere, O3 is reduced by 7 ppbv on average. However, when reactions between Br and Cl with VOCs are not included, O3 is much lower than observed. Therefore, the tropospheric Ox budget is highly sensitive to the inclusion of halogen reactions with VOCs and to the uncertainties in current understanding of these reactions and the abundance of VOCs in the remote marine atmosphere.

scholarly journals Impacts of climate change and emissions on atmospheric oxidized nitrogen deposition over East Asia

2018 ◽  
Author(s):  
Junxi Zhang ◽  
Yang Gao ◽  
L. Ruby Leung ◽  
Kun Luo ◽  
Huan Liu ◽  
...  
Keyword(s):  
Climate Change ◽  
East Asia ◽  
East China Sea ◽  
Atmospheric Chemistry ◽  
East China ◽  
Anthropogenic Emission ◽  
China Sea ◽  
Rcp 8.5 ◽  
Emission Changes ◽  
The Impact

Abstract. A multi-model ensemble of Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) simulations are used to study the atmospheric oxidized nitrogen (NOy) deposition over East Asia under climate and emission changes projected for the future. Both dry and wet NOy deposition shows significant decreases in the 2100s under RCP 4.5 and RCP 8.5, primarily due to large anthropogenic emission reduction over both land and sea. However, in the near future of the 2030s, both dry and wet NOy deposition increases significantly due to continued increase in emissions. The individual effect of climate or emission changes on dry and wet NOy deposition is also investigated. The impact of climate change on dry NOy deposition is relatively minor, but the effect on wet deposition, primarily caused by changes in precipitation, is much higher. For example, over the East China Sea, wet NOy deposition increases significantly in summer due to climate change by the end of this century under RCP 8.5, which may subsequently enhance marine primary production. Over the coastal seas of China, as the transport of NOy from land becomes weaker due to the decrease of anthropogenic emissions, the effect of ship emission and lightning emission becomes more important. On average, seasonal mean total NOy deposition is projected to be enhanced by 24–48 % and 3 %–37 % over Yellow Sea and East China Sea, respectively, by the end of this century. Therefore, continued control of both anthropogenic emission over land and ship emissions may reduce NOy deposition to the Chinese coastal seas.

scholarly journals Evaluating the influences of biomass burning during 2006 BASE-ASIA: a regional chemical transport modeling

2012 ◽  
Vol 12 (9) ◽  
pp. 3837-3855 ◽  
Author(s):  
J. S. Fu ◽  
N. C. Hsu ◽  
Y. Gao ◽  
K. Huang ◽  
C. Li ◽  
...  
Keyword(s):  
Southeast Asia ◽  
East Asia ◽  
Long Range ◽  
Biomass Burning ◽  
Cross Sections ◽  
Southern China ◽  
Source Region ◽  
Long Range Transport ◽  
Range Transport ◽  
The Impact

Abstract. To evaluate the impact of biomass burning from Southeast Asia to East Asia, this study conducted numerical simulations during NASA's 2006 Biomass-burning Aerosols in South-East Asia: Smoke Impact Assessment (BASE-ASIA). Two typical episode periods (27–28 March and 13–14 April) were examined. Two emission inventories, FLAMBE and GFED, were used in the simulations. The influences during two episodes in the source region (Southeast Asia) contributed to the surface CO, O3 and PM2.5 concentrations as high as 400 ppbv, 20 ppbv and 80 μg m−3, respectively. The perturbations with and without biomass burning of the above three species during the intense episodes were in the range of 10 to 60%, 10 to 20% and 30 to 70%, respectively. The impact due to long-range transport could spread over the southeastern parts of East Asia and could reach about 160 to 360 ppbv, 8 to 18 ppbv and 8 to 64 μg m−3 on CO, O3 and PM2.5, respectively; the percentage impact could reach 20 to 50% on CO, 10 to 30% on O3, and as high as 70% on PM2.5. In March, the impact of biomass burning mainly concentrated in Southeast Asia and southern China, while in April the impact becomes slightly broader and even could go up to the Yangtze River Delta region. Two cross-sections at 15° N and 20° N were used to compare the vertical flux of biomass burning. In the source region (Southeast Asia), CO, O3 and PM2.5 concentrations had a strong upward transport from surface to high altitudes. The eastward transport becomes strong from 2 to 8 km in the free troposphere. The subsidence process during the long-range transport contributed 60 to 70%, 20 to 50%, and 80% on CO, O3 and PM2.5, respectively to surface in the downwind area. The study reveals the significant impact of Southeastern Asia biomass burning on the air quality in both local and downwind areas, particularly during biomass burning episodes. This modeling study might provide constraints of lower limit. An additional study is underway for an active biomass burning year to obtain an upper limit and climate effects.

scholarly journals Evaluating the influences of biomass burning during 2006 BASE-ASIA: a regional chemical transport modeling

2011 ◽  
Vol 11 (12) ◽  
pp. 32205-32243 ◽  
Author(s):  
J. S. Fu ◽  
N. C. Hsu ◽  
Y. Gao ◽  
K. Huang ◽  
C. Li ◽  
...  
Keyword(s):  
Southeast Asia ◽  
East Asia ◽  
Long Range ◽  
Biomass Burning ◽  
Cross Sections ◽  
Southern China ◽  
Source Region ◽  
Long Range Transport ◽  
Range Transport ◽  
The Impact

Abstract. To evaluate the impact of biomass burning from Southeast Asia to East Asia, this study conducted numerical simulations during NASA's 2006 Biomass-burning Aerosols in South-East Asia: Smoke Impact Assessment (BASE-ASIA). Two typical episode periods (27–28 March and 13–14 April) were examined. Two emission inventories, FLAMBE and GFED, were used in the simulations. The influences during two episodes in the source region (Southeast Asia) contributed to the surface CO, O3 and PM2.5 concentrations as high as 400 ppbv, 20 ppbv and 80 μg m−3, respectively. The perturbations with and without biomass burning of the above three species during the intense episodes were in the range of 10 to 60%, 10 to 20% and 30 to 70%, respectively. The impact due to long-range transport could spread over the southeastern parts of East Asia and could reach about 160 to 360 ppbv, 8 to 18 ppbv and 8 to 64 μg m−3 on CO, O3 and PM2.5, respectively; the percentage impact could reach 20 to 50% on CO, 10 to 30% on O3, and as high as 70% on PM2.5. In March, the impact of biomass burning was mainly concentrated in Southeast Asia and Southern China, while in April the impact becomes slightly broader, potentially including the Yangtze River Delta region. Two cross-sections at 15° N and 20° N were used to compare the vertical flux of biomass burning. In the source region (Southeast Asia), CO, O3 and PM2.5 concentrations had a strong upward transport from surface to high altitudes. The eastward transport becomes strong from 2 to 8 km in the free troposphere. The subsidence process during the long-range transport contributed 60 to 70%, 20 to 50%, and 80% to CO, O3 and PM2.5, respectively to surface in the downwind area. The study reveals the significant impact of Southeastern Asia biomass burning on the air quality in both local and downwind areas, particularly during biomass burning episodes. This modeling study might provide lower limit constraints. An additional study is underway for an active biomass burning year to obtain an upper limit and climate effects.

scholarly journals Diurnal Cycle of a Heavy Rainfall Corridor over East Asia

2017 ◽  
Vol 145 (8) ◽  
pp. 3365-3389 ◽  
Author(s):  
Guixing Chen ◽  
Weiming Sha ◽  
Toshiki Iwasaki ◽  
Zhiping Wen
Keyword(s):  
East Asia ◽  
Heavy Rainfall ◽  
Southern China ◽  
Central China ◽  
Moist Convection ◽  
Low Level ◽  
Effective Discharge ◽  
Upward Transport ◽  
Convection Initiation ◽  
The Impact

Moist convection occurred repeatedly in the midnight-to-morning hours of 11–16 June 1998 and yielded excessive rainfall in a narrow latitudinal corridor over East Asia, causing severe flood. Numerical experiments and composite analyses of a 5-day period are performed to examine the mechanisms governing nocturnal convection. Both simulations and observations show that a train of MCSs concurrently developed along a quasi-stationary mei-yu front and coincided with the impact of a monsoon surge on a frontogenetic zone at night. This process was regulated primarily by a nocturnal low-level jet (NLLJ) in the southwesterly monsoon that formed over southern China and extended to central China. In particular, the NLLJ acted as a mechanism of moisture transport over the plains. At its northern terminus, the NLLJ led to a zonal band of elevated conditionally unstable air where strong low-level ascent overcame small convective inhibition, triggering new convection in three preferred plains. An analysis of convective instability shows that the low-tropospheric intrusion of moist monsoon air generated CAPE of ~1000 J kg−1 prior to convection initiation, whereas free-atmospheric forcing was much weaker. The NLLJ-related horizontal advection accounted for most of the instability precondition at 100–175 J kg−1 h−1. At the convective stage, instability generation by the upward transport of moisture increased to ~100 J kg−1 h−1, suggesting that ascending inflow caused feedback in convection growth. The convection dissipated in late morning with decaying NLLJ and moisture at elevated layers. It is concluded that the diurnally varying summer monsoon acted as an effective discharge of available moist energy from southern to central China, generating the morning-peak heavy rainfall corridor.

scholarly journals Importance of reactive halogens in the tropical marine atmosphere: A regional modelling study using WRF-Chem

2017 ◽  
Author(s):  
Alba Badia ◽  
Claire E. Reeves ◽  
Alex R. Baker ◽  
Alfonso Saiz-Lopez ◽  
Rainer Volkamer ◽  
...  
Keyword(s):  
Atmospheric Chemistry ◽  
Marine Boundary Layer ◽  
Transport Model ◽  
Model Performance ◽  
Sea Salt ◽  
Heterogeneous Reactions ◽  
Regional Modelling ◽  
Mixing Ratios ◽  
Inorganic Iodine ◽  
The Impact

Abstract. This study investigates the impact of halogens on atmospheric chemistry in the tropical troposphere and explores the sensitivity of this to uncertainties in the fluxes of halogens to the atmosphere and the chemical processing. To do this the regional chemistry transport model WRF-Chem has been extended, for the first time, to include halogen chemistry (bromine, chlorine and iodine chemistry), including heterogeneous recycling reactions involving sea-salt aerosol and other particles, reactions of Br with volatile organic compounds (VOCs), along with oceanic emissions of halocarbons, VOCs and inorganic iodine. The study focuses on the tropical East Pacific using field observations from the TORERO campaign (January–February 2012) to evaluate the model performance. Including all the new processes, the model does a reasonable job reproducing the observed mixing ratios of BrO and IO, albeit with some discrepancies, some of which can be attributed to difficulties in the model's ability to reproduce the observed halocarbons. This is somewhat expected given the large uncertainties in the air-sea fluxes of the halocarbons in a region where there are few observations of seawater concentrations. We see a considerable impact on the Bry partitioning when heterogeneous chemistry is included, with a greater proportion of the Bry in active forms such as BrO, HOBr and dihalogens. Including debromination of sea-salt increases BrO slightly throughout the free troposphere, but in the tropical marine boundary layer, where the sea-salt particles are plentiful and relatively acidic, debromination leads to overestimation of the observed BrO. However, it should be noted that the modelled BrO was extremely sensitive to the inclusion of reactions between Br and the VOCs, which convert Br to HBr, a far less reactive form of Bry. Excluding these reactions leads to modelled BrO mixing ratios greater than observed. The reactions between Br and aldehydes were found to be particularly important, despite the model underestimating the amount of aldehydes observed in the atmosphere. There are only small changes to Iy partitioning and IO when the heterogeneous reactions, primarly on sea-salt, are included. Our model results show that the tropospheric Ox loss due to halogens is 31 %. This loss is mostly due to I (16 %) and Br (14 %) and it is in good agreement with other estimates from state-of-the-art atmospheric chemistry models.

Study of the Impact of Summer Monsoon Circulation on Spatial Distribution of Aerosols in East Asia Based on Numerical Simulations

2011 ◽  
Vol 50 (11) ◽  
pp. 2270-2282 ◽  
Author(s):  
Libin Yan ◽  
Xiaodong Liu ◽  
Ping Yang ◽  
Zhi-Yong Yin ◽  
Gerald R. North
Keyword(s):  
Spatial Distribution ◽  
East Asia ◽  
Summer Monsoon ◽  
Southern China ◽  
Asian Summer Monsoon ◽  
Department Of Energy ◽  
Monsoon Circulation ◽  
Aerosol Model ◽  
Asian Summer ◽  
The Impact

AbstractThe regional coupled climate–chemistry/aerosol model (RegCM3) is used to investigate the difference in the spatial distribution of aerosol optical depth (AOD) between a strong summer monsoon year (SSMY; July 2003) and a weak summer monsoon year (WSMY; July 2002) under the actual- and same-emission scenarios. It is shown that the intensity of the Asian summer monsoon is primarily responsible for the AOD spatial distribution anomaly in midsummer over East Asia. Specifically, the AOD over southern China, upwind of the Asian summer monsoon, is greater in WSMY than in SSMY, but the opposite is observed for the AOD downwind over northern China and the Korean Peninsula. The AOD spatial distribution patterns simulated on the basis of the actual emission inventories for the SSMY and WSMY do not substantially differ from their counterparts that are based on the same emission inventory, confirming that the monsoon circulation, rather than local emissions or dry and wet deposition processes, is the predominant factor determining the regional AOD distribution. These modeling results are consistent with the analyses based on the Moderate Resolution Imaging Spectroradiometer (MODIS) products, NCAR–Department of Energy wind fields, and air parcel movements according to the 7-day trajectories of air parcels determined by the Hybrid Single-Particle Lagrangian Integrated Trajectory model.

scholarly journals Influence of meteorology and anthropogenic pollution on chemical flux divergence of the NO-NO<sub>2</sub>-O<sub>3</sub> triad above and within a natural grassland canopy

2014 ◽  
Vol 11 (7) ◽  
pp. 10737-10777
Author(s):  
D. Plake ◽  
M. Sörgel ◽  
P. Stella ◽  
A. Held ◽  
I. Trebs
Keyword(s):  
Atmospheric Chemistry ◽  
Aerodynamic Resistance ◽  
Trace Gas ◽  
Flux Divergence ◽  
Data Set ◽  
Chemical Flux ◽  
Key Species ◽  
The Impact ◽  
First Time ◽  
Natural Grassland

Abstract. The detailed understanding of surface–atmosphere exchange of reactive trace gas species is a crucial precondition for reliable modeling of processes in atmospheric chemistry. Plant canopies significantly impact the atmospheric budget of trace gases. In the past, many studies focused on taller forest canopies or crops, where the bulk plant material is concentrated in the uppermost canopy layer. However, within grasslands, a land-cover class that globally covers vast terrestrial areas, the canopy structure is fundamentally different, as the main biomass is concentrated in the lowest canopy part. This has obvious implications for aerodynamic in-canopy transport, and consequently also impacts on global budgets of key species in atmospheric chemistry such as nitric oxide (NO), nitrogen dioxide (NO2) and ozone (O3). This study presents for the first time a~comprehensive data set of directly measured in-canopy transport times and aerodynamic resistances, chemical timescales, Damköhler numbers, trace gas and micrometeorological measurements for a natural grassland canopy (canopy height = 0.6 m). Special attention is paid to the impact of contrasting meteorological and air chemical conditions on in-canopy transport and chemical flux divergence. Our results show that the grassland canopy is decoupled throughout the day. In the lower canopy, the measured transport times are fastest during nighttime, which is due to convection during nighttime and stable stratification during daytime in this layer. The inverse was found in the layers above. During periods of low wind speed and high NOx (NO+NO2) levels, the effect of canopy decoupling on trace gas transport was found especially distinct. The aerodynamic resistance in the lower canopy (0.04–0.2 m) was around 1000 s m−1, thus as high as values from literature representing the lowest meter of an Amazonian rain forest canopy. The aerodynamic resistance representing the bulk canopy was found to be more than 3–4 times higher as in forests. Calculated Damköhler numbers (ratio of transport and chemical timescales) suggested a strong flux divergence for the NO-NO2-O3 triad within the canopy during daytime. At that time, the timescale of NO2 plant uptake ranged from 90 to 160 s and was the fastest relevant timescale, i.e. faster than the reaction of NO and O3. Thus, our results clearly reveal that grassland canopies of similar structure have a strong potential to retain soil emitted NO by uptake of NO2 by the plants. Furthermore, a photo-chemical O3 production above the canopy was observed, which resulted from a~surplus of NO2 from the NO-NO2-O3 photostationary state. The O3 production was one order of magnitude higher during high NOx than during low NOx periods and resulted in an O3 flux underestimation, which was observed for the first time.

scholarly journals Influence of aromatics on tropospheric gas-phase composition

2020 ◽  
Author(s):  
Domenico Taraborrelli ◽  
David Cabrera-Perez ◽  
Sara Bacer ◽  
Sergey Gromov ◽  
Jos Lelieveld ◽  
...  
Keyword(s):  
East Asia ◽  
Gas Phase ◽  
Atmospheric Chemistry ◽  
General Circulation ◽  
Anthropogenic Activities ◽  
Regional Scale ◽  
Circulation Model ◽  
Significant Loss ◽  
Tropospheric Chemistry ◽  
The Impact

Abstract. Aromatics contribute a significant fraction to organic compounds in the troposphere and are mainly emitted by anthropogenic activities and biomass burning. Their oxidation in lab experiments is known to lead to the formation of ozone and aerosol precursors. However, their overall impact on tropospheric composition is uncertain as it depends on transport, multiphase chemistry, and removal processes of the oxidation intermediates. Representation of aromatics in global atmospheric models has been either neglected or highly simplified. Here, we present an assessment of their impact on the gas-phase chemistry, using the general circulation model EMAC (ECHAM5/MESSy Atmospheric Chemistry). We employ a comprehensive kinetic model to represent the oxidation of the following monocyclic aromatics: benzene, toluene, xylenes, phenol, styrene, ethylbenzene, trimethylbenzenes, benzaldehyde, and lumped higher aromatics that contain more than 9 C atoms. Significant regional changes are identified for several species. For instance, glyoxal increases by 130 % in Europe and 260 % in East Asia, respectively. Large increases in HCHO are also predicted in these regions. In general, the influence of aromatics is particularly evident in areas with high concentrations of NOx, with increases up to 12 % in O3 and 17 % in OH. On a global scale, the estimated net changes are minor when aromatic compounds are included in our model. For instance, the tropospheric burden of CO increases by about 6 %, while the burdens of OH, O3, and NOx (NO + NO2) decrease between 3 % and 9 %. The global mean changes are small, partially because of compensating effects between high- and low-NOx regions. The largest change is predicted for the important aerosol precursor glyoxal, which increases globally by 36 %. In contrast to other studies, the net change in tropospheric ozone is predicted to be negative, −3 % globally. This change is larger in the northern hemisphere where global models usually show positive biases. We find that the reaction with phenoxy radicals is a significant loss for ozone, of the order of 200–300 Tg/yr, which is similar to the estimated ozone loss due to bromine chemistry. Although the net global impact of aromatics is limited, our results indicate that aromatics can strongly influence tropospheric chemistry on a regional scale, most significantly in East Asia. An analysis of the main model uncertainties related to oxidation and emissions suggests that the impact of aromatics may even be significantly larger.

scholarly journals Influence of meteorology and anthropogenic pollution on chemical flux divergence of the NO–NO<sub>2</sub>–O<sub>3</sub> triad above and within a natural grassland canopy

2015 ◽  
Vol 12 (4) ◽  
pp. 945-959 ◽  
Author(s):  
D. Plake ◽  
M. Sörgel ◽  
P. Stella ◽  
A. Held ◽  
I. Trebs
Keyword(s):  
Atmospheric Chemistry ◽  
Trace Gases ◽  
Aerodynamic Resistance ◽  
Trace Gas ◽  
Flux Divergence ◽  
Canopy Layer ◽  
Chemical Flux ◽  
Key Species ◽  
The Impact ◽  
Natural Grassland

Abstract. The detailed understanding of surface–atmosphere exchange fluxes of reactive trace gases is a crucial precondition for reliable modelling of processes in atmospheric chemistry. Plant canopies significantly impact the atmospheric budget of trace gases. In the past, many studies focused on taller forest canopies or crops, where the bulk plant material is concentrated in the uppermost canopy layer. However, within grasslands, a land-cover class that globally covers vast terrestrial areas, the canopy structure is fundamentally different, as the main biomass is concentrated in the lowest part of the canopy. This has obvious implications for aerodynamic in-canopy transport, and consequently also impacts on global budgets of key species in atmospheric chemistry such as nitric oxide (NO), nitrogen dioxide (NO2) and ozone (O3). This study presents for the first time a comprehensive data set of directly measured in-canopy transport times and aerodynamic resistances, chemical timescales, Damköhler numbers, trace gas and micrometeorological measurements for a natural grassland canopy (canopy height = 0.6 m). Special attention is paid to the impact of contrasting meteorological and air chemical conditions on in-canopy transport and chemical flux divergence. Our results show that the grassland canopy is decoupled throughout the day. In the lowermost canopy layer, the measured transport times are fastest during nighttime, which is due to convection during nighttime and a stable stratification during daytime in this layer. The inverse was found in the layers above. During periods of low wind speed and high NOx (NO+NO2) levels, the effect of canopy decoupling on trace gas transport was found to be especially distinct. The aerodynamic resistance in the lowermost canopy layer (0.04–0.2 m) was around 1000 s m−1, which is as high as values determined previously for the lowest metre of an Amazonian rain forest canopy. The aerodynamic resistance representing the bulk canopy was found to be more than 3–4 times higher than in forests. Calculated Damköhler numbers (ratio of transport and chemical timescales) suggest a strong flux divergence for the NO–NO2–O3 triad within the canopy during daytime. During that time, the timescale of NO2 uptake by plants ranged from 90 to 160 s and was the fastest relevant timescale, i.e. faster than the reaction of NO and O3. Thus, our results reveal that grassland canopies of similar structure exhibit a strong potential to retain soil-emitted NO due to oxidation and subsequent uptake of NO2 by plants. Furthermore, photo-chemical O3 production was observed above the canopy, which was attributed to a deviation from the NO–NO2–O3 photostationary state by a surplus of NO2 due to oxidation of NO, by e.g. peroxy radicals. The O3 production was one order of magnitude higher during high NOx than during low NOx periods and resulted in an underestimation of the O3 deposition flux measured with the EC method.

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