Abstract. The measurement of OH reactivity, the inverse of the OH lifetime, provides a powerful tool to investigate atmospheric photochemistry. A new airborne OH reactivity instrument was designed and deployed for the first time on the NASA DC-8 aircraft during the second phase of Intercontinental Chemical Transport Experiment-B (INTEX-B) campaign, which was focused on the Asian pollution outflow over Pacific Ocean and was based in Hawaii and Alaska. The OH reactivity was measured by adding OH, generated by photolyzing water vapor with 185 nm UV light in a moveable wand, to the flow of ambient air in a flow tube and measuring the OH signal with laser induced fluorescence. As the wand was pulled back away from the OH detector, the OH signal decay was recorded; the slope of −Δln(signal)/Δ time was the OH reactivity. The overall absolute uncertainty at the 2σ confidence levels is about 1 s−1 at low altitudes (for decay about 6 s−1), and 0.7 s−1 at high altitudes (for decay about 2 s−1). From the median vertical profile obtained in the second phase of INTEX-B, the measured OH reactivity (4.0±1.0 s−1) is higher than the OH reactivity calculated from assuming that OH was in steady state (3.3&amp;amp;plusmn0.8 s−1), and even higher than the OH reactivity that was calculated from the total measurements of all OH reactants (1.6±0.4 s−1). Model calculations show that the missing OH reactivity is consistent with the over-predicted OH and under-predicted HCHO in the boundary layer and lower troposphere. The over-predicted OH and under-predicted HCHO suggest that the missing OH sinks are most likely related to some highly reactive VOCs that have HCHO as an oxidation product.

J. Mao

X. Ren

W. H. Brune

J. R. Olson

J. H. Crawford

A. Fried

L. G. Huey

R. C. Cohen

B. Heikes

H. B. Singh

D. R. Blake

G. W. Sachse

G. S. Diskin

S. R. Hall

R. E. Shetter

Atmospheric chemistry and physics

Atmos. Chem. Phys.

Airborne measurement of OH reactivity during INTEX-B

INTEX-B期间OH反应性的机载测量

Abstract. Gas-phase oxidation pathways and products of anthropogenic volatile organic compounds (VOCs), mainly aromatics, are the subject of intensive research, with attention paid to their contributions to secondary organic aerosol (SOA) formation and potentially new particle formation (NPF) in the urban atmosphere. In this study, a series of OH-initiated oxidation experiments of trimethylbenzene (TMB, C9H12) including 1,2,4-TMB, 1,3,5-TMB, 1,2,3-TMB, and 1,2,4-(methyl-D3)-TMBs (C9H9D3) were investigated in an oxidation flow reactor (OFR) in the absence and presence of NOx. Products were measured using a suite of state-of-the-art instruments, i.e. a nitrate-based chemical ionization–atmospheric pressure interface time-of-flight mass spectrometer (nitrate CI-APi-TOF), an iodide-adduct chemical ionization time-of-flight mass spectrometer (iodide CI-TOF) equipped with a Filter Inlet for Gases and AEROsols (FIGAERO), and a Vocus proton-transfer-reaction mass spectrometer (Vocus PTR). A large number of C9 products with 1–11 oxygen atoms and C18 products presumably formed from dimerization of C9 peroxy radicals were observed, hinting at the extensive existence of autoxidation and accretion reaction pathways in the OH-initiated oxidation reactions of TMBs. Oxidation products of 1,2,4-(methyl-D3)-TMBs with deuterium atoms in different methyl substituents were then used as a molecular basis to propose potential autoxidation reaction pathways. Accretion of C9 peroxy radicals is the most significant for aromatics with meta-substituents and the least for aromatics with ortho-substituents if the number and size of substituted groups are identical. The presence of NOx would suppress the formation of highly oxygenated molecules (HOMs) of C18 and enhance the formation of organonitrates and even dinitrate organic compounds. Our results show that the oxidation products of TMB are much more diverse and could be more oxygenated than the current mechanisms predict.

Yuwei Wang

Archit Mehra

Jordan E. Krechmer

Gan Yang

Xiaoyu Hu

Yiqun Lu

Andrew Lambe

Manjula Canagaratna

Jianmin Chen

Douglas Worsnop

Hugh Coe

Lin Wang

Oxygenated products formed from OH-initiated reactions of trimethylbenzene: autoxidation and accretion

Abstract. Highly oxygenated organic molecules (HOMs) can participate in new particle formation (NPF) and enhance growth of newly formed particles partially because of their low volatility. Previous studies have shown formation of HOMs via autoxidation reactions of RO2 intermediates generated by OH-initiated oxidation of anthropogenic volatile organic compounds (VOCs). It was also suggested that multi-generation OH oxidation could be an important source for aromatics-derived HOMs. However, our understanding of the generation of aromatics-derived HOMs is still insufficient, especially of their formation mechanisms, which determine molar yields of HOMs and are essential to the establishment of global chemical box models related to HOMs. In this study, with a potential aerosol mass oxidation flow reactor (PAM OFR), two series of OH-initiated oxidation experiments of 1,3,5-trimethylbenzene (1,3,5-TMB) were conducted to investigate the formation of aromatics-derived HOMs. In the first series, the evolution of oxidation products of 1,3,5-TMB in an OH exposure range of (0.5–5.0) × 1010 molecules cm−3 s, equivalent to an OH exposure of 0.7–6.9 h at an OH concentration ([OH]) of 2×106 molecules cm−3, was investigated by a nitrate-based chemical ionization mass spectrometer and a Vocus proton-transfer-reaction mass spectrometer, indicating significant secondary OH chemistry during the aging of stabilized first-generation oxygenated products within an intraday OH exposure and formation of various HOMs with lower double-bond equivalence (DBE). In addition, organonitrates, formed after the introduction of NOx into the reaction systems, further confirmed the existence of such secondary reactions. The second series of experiments was conducted with same residence time but much lower [OH], which also shows the generation of multi-generation HOMs with an [OH] as low as 1.06×107 molecules cm−3 for 53 s, i.e., an OH exposure of around 5.86×108 molecules cm−3 s. Our study suggests the important role of secondary OH chemistry in the oxidation of aromatics if these oxygenated products survived long enough in the ambient atmosphere and elucidates detailed formation mechanisms of certain HOM products.

Chuang Li

Ying Zhang

Yueyang Li

Xueyan Yang

Yizhen Wu

Lei Yao

Hefeng Zhang

Secondary reactions of aromatics-derived oxygenated organic molecules lead to plentiful highly oxygenated organic molecules within an intraday OH exposure

芳香族衍生的含氧化合物在日内OH暴露下的二次反应导致大量高度氧化的有机分子生成

Abstract. Oil and natural gas are important for energy supply around the world. The exploring, drilling, transportation and processing in oil and gas regions can release a lot of volatile organic compounds (VOCs). To understand the VOC levels, compositions and sources in such regions, an oil and gas station in northwest China was chosen as the research site and 57 VOCs designated as the photochemical precursors were continuously measured for an entire year (September 2014–August 2015) using an online monitoring system. The average concentration of total VOCs was 297 ± 372 ppbv and the main contributor was alkanes, accounting for 87.5 % of the total VOCs. According to the propylene-equivalent concentration and maximum incremental reactivity methods, alkanes were identified as the most important VOC groups for the ozone formation potential. Positive matrix factorization (PMF) analysis showed that the annual average contributions from natural gas, fuel evaporation, combustion sources, oil refining processes and asphalt (anthropogenic and natural sources) to the total VOCs were 62.6 ± 3.04, 21.5 ± .99, 10.9 ± 1.57, 3.8 ± 0.50 and 1.3 ± 0.69 %, respectively. The five identified VOC sources exhibited various diurnal patterns due to their different emission patterns and the impact of meteorological parameters. Potential source contribution function (PSCF) and concentration-weighted trajectory (CWT) models based on backward trajectory analysis indicated that the five identified sources had similar geographic origins. Raster analysis based on CWT analysis indicated that the local emissions contributed 48.4–74.6 % to the total VOCs. Based on the high-resolution observation data, this study clearly described and analyzed the temporal variation in VOC emission characteristics at a typical oil and gas field, which exhibited different VOC levels, compositions and origins compared with those in urban and industrial areas.

Huang Zheng

Shaofei Kong

Xinli Xing

Yao Mao

Tianpeng Hu

Yang Ding

Gang Li

Dantong Liu

Shuanglin Li

Shihua Qi

Atmospheric Chemistry and Physics

Monitoring of volatile organic compounds (VOCs) from an oil and gas station in northwest China for 1 year

中国西北地区某加油站挥发性有机化合物(VOCs)一年监测

Abstract. The interconnection of local downslope flows of different intensities with the turbulent characteristics and thermal structure of the atmospheric boundary layer (ABL) is investigated through observations. Measurements are carried out in a relatively flat area 2 km away from the steep slopes of the Sierra de Guadarrama (central Iberian Peninsula). A total of 40 thermally driven downslope events are selected from an observational database spanning the summer 2017 period by using an objective and systematic algorithm that accounts for a weak synoptic forcing and local downslope wind direction. We subsequently classify the downslope events into weak, moderate and intense categories, according to their maximum 6 m wind speed. This classification enables us to contrast their main differences regarding the driving mechanisms, associated ABL turbulence and thermal structure, and the major dynamical characteristics. We find that the strongest downslope flows (U &gt; 3.5 m s−1) develop when soil moisture is low ( &lt; 0.07 m3 m−3) and the synoptic wind not so weak (3.5 m s−1 &lt; V850 &lt; 6 m s−1) and roughly parallel to the direction of the downslope flow. The latter adds an important dynamical input, which induces an early flow advection from the nearby steep slope, when the local thermal profile is not stable yet. Consequently, turbulence driven by the bulk shear increases up to friction velocity (u*) ≃ 1 m s−1, preventing the development of the surface-based thermal inversion and giving rise to the so-called weakly stable boundary layer. On the contrary, when the dynamical input is absent, buoyancy acceleration drives the formation of a katabatic flow, which is weak (U &lt; 1.5 m s−1) and generally manifested in the form of a shallow jet below 3 m. The relative flatness of the area favours the formation of very stable boundary layers marked by very weak turbulence (u* &lt; 0.1 m s−1). In between, moderate downslope flows show intermediate characteristics, depending on the strength of the dynamical input and the occasional interaction with down-basin winds. On the other hand, by inspecting individual weak and intense events, we further explore the impact of downslope flows on CO2 variability. By relating the dynamics of the distinct turbulent regimes to the CO2 budget, we are able to estimate the contribution of the different terms. For the intense event, indeed, we infer a horizontal transport of 67 ppm in 3 h driven by the strong downslope advection.

Jon Ander Arrillaga

Carlos Yagüe

Carlos Román-Cascón

Mariano Sastre

Maria Antonia Jiménez

Gregorio Maqueda

Jordi Vilà-Guerau de Arellano

From weak to intense downslope winds: origin, interaction with boundary-layer turbulence and impact on CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; variability

从弱到强下坡风：起源、与边界层湍流的相互作用及其对CO&lt;sub&gt;2&lt;/sub&gt;变异性的影响

Abstract. Water vapour in the atmosphere is the source of a major climate feedback mechanism and potential increases in the availability of water vapour could have important consequences for mean and extreme precipitation. Future precipitation changes further depend on how the hydrological cycle responds to different drivers of climate change, such as greenhouse gases and aerosols. Currently, neither the total anthropogenic influence on the hydrological cycle nor that from individual drivers is constrained sufficiently to make solid projections. We investigate how integrated water vapour (IWV) responds to different drivers of climate change. Results from 11 global climate models have been used, based on simulations where CO2, methane, solar irradiance, black carbon (BC), and sulfate have been perturbed separately. While the global-mean IWV is usually assumed to increase by ∼7 % per kelvin of surface temperature change, we find that the feedback response of IWV differs somewhat between drivers. Fast responses, which include the initial radiative effect and rapid adjustments to an external forcing, amplify these differences. The resulting net changes in IWV range from 6.4±0.9 % K−1 for sulfate to 9.8±2 % K−1 for BC. We further calculate the relationship between global changes in IWV and precipitation, which can be characterized by quantifying changes in atmospheric water vapour lifetime. Global climate models simulate a substantial increase in the lifetime, from 8.2±0.5 to 9.9±0.7 d between 1986–2005 and 2081–2100 under a high-emission scenario, and we discuss to what extent the water vapour lifetime provides additional information compared to analysis of IWV and precipitation separately. We conclude that water vapour lifetime changes are an important indicator of changes in precipitation patterns and that BC is particularly efficient in prolonging the mean time, and therefore likely the distance, between evaporation and precipitation.

Øivind Hodnebrog

Gunnar Myhre

Bjørn H. Samset

Kari Alterskjær

Timothy Andrews

Olivier Boucher

Gregory Faluvegi

Dagmar Fläschner

Piers M. Forster

Matthew Kasoar

Alf Kirkevåg

Jean-Francois Lamarque

Dirk Olivié

Thomas B. Richardson

Dilshad Shawki

Drew Shindell

Keith P. Shine

Philip Stier

Toshihiko Takemura

Apostolos Voulgarakis

Duncan Watson-Parris

Water vapour adjustments and responses differ between climate drivers

气候驱动因素下的水汽调整与响应存在差异

Abstract. Despite the need for researchers to understand terrestrial biospheric carbon fluxes to account for carbon cycle feedbacks and predict future CO2 concentrations, knowledge of these fluxes at the regional scale remains poor. This is particularly true in mountainous areas, where complex meteorology and lack of observations lead to large uncertainties in carbon fluxes. Yet mountainous regions are often where significant forest cover and biomass are found – i.e., areas that have the potential to serve as carbon sinks. As CO2 observations are carried out in mountainous areas, it is imperative that they are properly interpreted to yield information about carbon fluxes. In this paper, we present CO2 observations at three sites in the mountains of the western US, along with atmospheric simulations that attempt to extract information about biospheric carbon fluxes from the CO2 observations, with emphasis on the observed and simulated diurnal cycles of CO2. We show that atmospheric models can systematically simulate the wrong diurnal cycle and significantly misinterpret the CO2 observations, due to erroneous atmospheric flows as a result of terrain that is misrepresented in the model. This problem depends on the selected vertical level in the model and is exacerbated as the spatial resolution is degraded, and our results indicate that a fine grid spacing of ∼ 4 km or less may be needed to simulate a realistic diurnal cycle of CO2 for sites on top of the steep mountains examined here in the American Rockies. In the absence of higher resolution models, we recommend coarse-scale models to focus on assimilating afternoon CO2 observations on mountaintop sites over the continent to avoid misrepresentations of nocturnal transport and influence.

John C. Lin

Derek V. Mallia

Dien Wu

Britton B. Stephens

How can mountaintop CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; observations be used to constrain regional carbon fluxes?

如何利用山顶二氧化碳观测来约束区域碳通量？

Abstract. Nitrous acid (HONO) fluxes were measured above an agricultural field site near Paris during different seasons. Above bare soil, different crops were measured using the aerodynamic gradient (AG) method. Two LOPAPs (LOng Path Absorption Photometer) were used to determine the HONO gradients between two heights. During daytime mainly positive HONO fluxes were observed, which showed strong correlation with the product of the NO2 concentration and the long wavelength UV light intensity, expressed by the photolysis frequency J(NO2). These results are consistent with HONO formation by photosensitized heterogeneous conversion of NO2 on soil surfaces as observed in recent laboratory studies. An additional influence of the soil temperature on the HONO flux can be explained by the temperature-dependent HONO adsorption on the soil surface. A parameterization of the HONO flux at this location with NO2 concentration, J(NO2), soil temperature and humidity fits reasonably well all flux observations at this location.

Sebastian Laufs

Mathieu Cazaunau

Patrick Stella

Ralf Kurtenbach

Pierre Cellier

Abdelwahid Mellouki

Benjamin Loubet

Jörg Kleffmann

Diurnal fluxes of HONO above a crop rotation

作物轮作上空HONO的日间通量

Abstract. Volatile organic compounds (VOCs) play important roles in the tropospheric atmosphere. In this study, VOCs were measured at an urban site in Guangzhou, one of the megacities in the Pearl River Delta (PRD), using a gas chromatograph–mass spectrometer/flame ionization detection (GC–MS/FID) and a proton transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS). Diurnal profile analyses show that stronger chemical removal by OH radicals for more reactive hydrocarbons occurs during the daytime, which is used to estimate the daytime average OH radical concentration. In comparison, diurnal profiles of oxygenated volatile organic compounds (OVOCs) indicate evidence of contributions from secondary formation. Detailed source analyses of OVOCs, using a photochemical age-based parameterization method, suggest important contributions from both primary emissions and secondary formation for measured OVOCs. During the campaign, around 1700 ions were detected in PTR-ToF-MS mass spectra, among which there were 462 ions with noticeable concentrations. VOC signals from these ions are quantified based on the sensitivities of available VOC species. OVOC-related ions dominated PTR-ToF-MS mass spectra, with an average contribution of 73 % ± 9 %. Combining measurements from PTR-ToF-MS and GC–MS/FID, OVOCs contribute 57 % ± 10 % to the total concentration of VOCs. Using concurrent measurements of OH reactivity, OVOCs measured by PTR-ToF-MS contribute greatly to the OH reactivity (19 % ± 10 %). In comparison, hydrocarbons account for 21 % ± 11 % of OH reactivity. Adding up the contributions from inorganic gases (48 % ± 15 %), ∼ 11 % (range of 0 %–19 %) of the OH reactivity remains `missing”, which is well within the combined uncertainties between the measured and calculated OH reactivity. Our results demonstrate the important roles of OVOCs in the emission and evolution budget of VOCs in the urban atmosphere.

Caihong Wu

Chaomin Wang

Sihang Wang

Wenjie Wang

Bin Yuan

Jipeng Qi

Baolin Wang

Hongli Wang

Chen Wang

Wei Song

Xinming Wang

Weiwei Hu

Shengrong Lou

Chenshuo Ye

Yuwen Peng

Zelong Wang

Yibo Huangfu

Yan Xie

Manni Zhu

Junyu Zheng

Xuemei Wang

Bin Jiang

Zhanyi Zhang

Min Shao

Measurement report: Important contributions of oxygenated compounds to emissions and chemistry of volatile organic compounds in urban air

测量报告：含氧化合物对城市空气中挥发性有机化合物排放与化学过程的重要贡献

Abstract. The effect of vapor-wall deposition on secondary organic aerosol (SOA) formation has gained significant attention; however, uncertainties in experimentally derived SOA mass yields due to uncertainties in particle-wall deposition remain. Different approaches have been used to correct for particle-wall deposition in SOA formation studies, each having its own set of assumptions in determining the particle-wall loss rate. In volatile and intermediate-volatility organic compound (VOC and IVOC) systems in which SOA formation is governed by kinetically limited growth, the effect of vapor-wall deposition on SOA mass yields can be constrained by using high surface area concentrations of seed aerosol to promote the condensation of SOA-forming vapors onto seed aerosol instead of the chamber walls. However, under such high seed aerosol levels, the presence of significant coagulation may complicate the particle-wall deposition correction. Here, we present a model framework that accounts for coagulation in chamber studies in which high seed aerosol surface area concentrations are used. For the α-pinene ozonolysis system, we find that after accounting for coagulation, SOA mass yields remain approximately constant when high seed aerosol surface area concentrations ( ≥  8000 µm2 cm−3) are used, consistent with our prior study (Nah et al., 2016) showing that α-pinene ozonolysis SOA formation is governed by quasi-equilibrium growth. In addition, we systematically assess the uncertainties in the calculated SOA mass concentrations and yields between four different particle-wall loss correction methods over the series of α-pinene ozonolysis experiments. At low seed aerosol surface area concentrations (< 3000 µm2 cm−3), the SOA mass yields at peak SOA growth obtained from the particle-wall loss correction methods agree within 14 %. However, at high seed aerosol surface area concentrations ( ≥  8000 µm2 cm−3), the SOA mass yields at peak SOA growth obtained from different particle-wall loss correction methods can differ by as much as 58 %. These differences arise from assumptions made in the particle-wall loss correction regarding the first-order particle-wall loss rate. This study highlights the importance of accounting for particle-wall deposition accurately during SOA formation chamber experiments and assessing the uncertainties associated with the application of the particle-wall deposition correction method when comparing and using SOA mass yields measured in different studies.

Theodora Nah

Renee C. McVay

Jeffrey R. Pierce

John H. Seinfeld

Nga L. Ng

Constraining uncertainties in particle-wall deposition correction during SOA formation in chamber experiments

约束室内实验中SOA形成期间颗粒物壁面沉积校正的不确定性

ATMOSPHERIC CHEMISTRY AND PHYSICS (ACP) 是一本非营利性国际科学期刊，致力于出版和公开讨论调查地球大气及其潜在化学和物理过程的高质量研究。它涵盖的海拔范围从陆地和海洋表面直至涡轮顶，包括对流层、平流层和中间层。主要学科领域包括大气建模、实地测量、遥感以及气体、气溶胶、云和降水、同位素、辐射、动力学的实验室研究，生物圈相互作用和水圈相互作用期刊范围侧重于对大气科学具有一般意义的研究，而不是主要具有当地或技术兴趣的调查。

Atmospheric Chemistry and Physics (ACP) is a non-profit international scientific journal dedicated to publishing and discussing high-quality research on the Earth's atmosphere and its potential chemical and physical processes. The scope of the journal ranges from the land and ocean surfaces to the tropopause, covering the troposphere, stratosphere, and mesosphere. Major subject areas include atmospheric modeling, field measurements, remote sensing, as well as laboratory studies on gases, aerosols, clouds and precipitation, isotopes, radiation, and dynamics, along with interactions in the biosphere and hydrosphere. The journal focuses on research that has general significance for atmospheric science rather than investigations primarily of local or technical interest.

《Atmospheric Chemistry and Physics》（ACP）是一本非营利性国际科学期刊，专注于发布和讨论关于地球大气及其潜在化学和物理过程的高质量研究。期刊涵盖的研究范围从陆地和海洋表面到涡轮顶，包括对流层、平流层和中间层。主要学科领域包括大气建模、实地测量、遥感技术，以及气体、气溶胶、云和降水、同位素、辐射、动力学的实验室研究，生物圈和水圈的相互作用。该期刊的重点是对大气科学具有普遍意义的研究，而非仅限于地方性或技术性兴趣的调查。

期刊	Atmospheric Chemistry and Physics
ISSN	1680-7316, 1680-7324
h-index	253
Journal Impact	5.30
国际分区	METEOROLOGY & ATMOSPHERIC SCIENCES(Q1)
国内分区	地球科学(1区)地球科学气象与大气科学(1区)地球科学环境科学(2区)

Atmospheric Chemistry and Physics

Airborne measurement of OH reactivity during INTEX-B

Oxygenated products formed from OH-initiated reactions of trimethylbenzene: autoxidation and accretion

Secondary reactions of aromatics-derived oxygenated organic molecules lead to plentiful highly oxygenated organic molecules within an intraday OH exposure

Monitoring of volatile organic compounds (VOCs) from an oil and gas station in northwest China for 1 year

From weak to intense downslope winds: origin, interaction with boundary-layer turbulence and impact on CO&lt;sub&gt;2&lt;/sub&gt; variability

Water vapour adjustments and responses differ between climate drivers

How can mountaintop CO&lt;sub&gt;2&lt;/sub&gt; observations be used to constrain regional carbon fluxes?

Diurnal fluxes of HONO above a crop rotation

Measurement report: Important contributions of oxygenated compounds to emissions and chemistry of volatile organic compounds in urban air

Constraining uncertainties in particle-wall deposition correction during SOA formation in chamber experiments

Atmospheric Chemistry and Physics

Airborne measurement of OH reactivity during INTEX-B

Oxygenated products formed from OH-initiated reactions of trimethylbenzene: autoxidation and accretion

Secondary reactions of aromatics-derived oxygenated organic molecules lead to plentiful highly oxygenated organic molecules within an intraday OH exposure

Monitoring of volatile organic compounds (VOCs) from an oil and gas station in northwest China for 1 year

From weak to intense downslope winds: origin, interaction with boundary-layer turbulence and impact on CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; variability

Water vapour adjustments and responses differ between climate drivers

How can mountaintop CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; observations be used to constrain regional carbon fluxes?

Diurnal fluxes of HONO above a crop rotation

Measurement report: Important contributions of oxygenated compounds to emissions and chemistry of volatile organic compounds in urban air

Constraining uncertainties in particle-wall deposition correction during SOA formation in chamber experiments

From weak to intense downslope winds: origin, interaction with boundary-layer turbulence and impact on CO<sub>2</sub> variability

How can mountaintop CO<sub>2</sub> observations be used to constrain regional carbon fluxes?