Prediction of the Response of a Photoionization Detector to a Complex Gaseous Mixture of Volatile Organic Compounds Produced by α-Pinene Oxidation

Photoionization detectors (PIDs) are lightweight and respond in real time to the concentrations of volatile organic compounds (VOCs), making them suitable for environmental measurements on many platforms. However, the nonselective sensing mechanism of PIDs challenges data interpretation, particularly when exposed to the complex VOC mixtures prevalent in the Earth's atmosphere. Herein, two approaches to this challenge are investigated. In the first, quantum-chemistry calculations are used to estimate photoionization cross sections and ionization potentials of individual species. In the second, machine learning models are trained on these calculated values, as well as empirical PID response factors, and then used for prediction. For both approaches, the resulting information for individual species is used to model the overall PID response to a complex VOC mixture. In complement, laboratory experiments in the Harvard Environmental Chamber are carried out to measure the PID response to the complex molecular mixture produced by α-pinene oxidation under various conditions. The observations show that the measured PID response is 15% to 30% smaller than the PID response modeled by quantum-chemistry calculations of the photoionization cross section for the photo-oxidation experiments and 15% to 20% for the ozonolysis experiments. By comparison, the measured PID response is captured within a 95% confidence interval by the use of machine learning to model the PID response based on the empirical response factor in all experiments. Taken together, the results of this study demonstrate the application of machine learning to augment the performance of a nonselective chemical sensor. The approach can be generalized to other reactive species, oxidants, and reaction mechanisms, thus enhancing the utility and interpretability of PID measurements for studying atmospheric VOCs.

Inferring the diurnal variability of OH radical concentrations over the Amazon from BVOC measurements

The atmospheric oxidation of biogenic volatile organic compounds (BVOC) by OH radicals over tropical rainforests impacts local particle production and the lifetime of globally distributed chemically and radiatively active gases. For the pristine Amazon rainforest during the dry season, we empirically determined the diurnal OH radical variability at the forest-atmosphere interface region between 80 and 325 m from 07:00 to 15:00 LT using BVOC measurements. A dynamic time warping approach was applied showing that median averaged mixing times between 80 to 325 m decrease from 105 to 15 min over this time period. The inferred OH concentrations show evidence for an early morning OH peak (07:00-08:00 LT) and an OH maximum (14:00 LT) reaching 2.2 (0.2, 3.8) × 106 molecules cm-3 controlled by the coupling between BVOC emission fluxes, nocturnal NOx accumulation, convective turbulence, air chemistry and photolysis rates. The results were evaluated with a turbulence resolving transport (DALES), a regional scale (WRF-Chem) and a global (EMAC) atmospheric chemistry model.

Volatile organic compounds and their contribution to ground-level ozone formation in a tropical urban environment

This study aims to determine the trends of volatile organic compound (VOC) concentrations and their potential contribution to O₃ formation. The hourly data (August 2017 to July 2018) for 29 VOCs were obtained from three Malaysian Department of Environment continuous air quality monitoring stations with different urban backgrounds (Shah Alam, Cheras, Seremban). The Ozone Formation Potential (OFP) was calculated based on the individual Maximum Incremental Reactivity (MIR) and VOC concentrations. The results showed that the highest mean total VOC concentrations were recorded at Cheras (148 ± 123 μg m^-3), within the Kuala Lumpur urban environment, followed by Shah Alam (124 ± 116 μg m^-3) and Seremban (86.4 ± 89.2 μg m^-3). VOCs such as n-butane, ethene, ethane and toluene were reported to be the most abundant species at all the selected stations, with overall mean concentrations of 16.6 ± 11.9 μg m^-3, 12.1 ± 13.3 μg m^-3, 10.8 ± 11.9 μg m^-3 and 9.67 ± 9.00 μg m^-3, respectively. Alkenes (51.3-59.1%) and aromatic hydrocarbons (26.4-33.5%) have been identified as the major contributors to O₃ formation in the study areas based on the overall VOC measurements. Relative humidity was found to influence the concentrations of VOCs more than other meteorological parameters. Overall, this study will contribute to further understanding of the distribution of VOCs and their contribution to O₃ formation, particularly in the tropical urban environment.

Chiral monoterpenes reveal forest emission mechanisms and drought responses

Monoterpenes (C₁₀H₁₆) are emitted in large quantities by vegetation to the atmosphere (>100 TgC year^-1), where they readily react with hydroxyl radicals and ozone to form new particles and, hence, clouds, affecting the Earth's radiative budget and, thereby, climate change^1-3. Although most monoterpenes exist in two chiral mirror-image forms termed enantiomers, these (+) and (-) forms are rarely distinguished in measurement or modelling studies^4-6. Therefore, the individual formation pathways of monoterpene enantiomers in plants and their ecological functions are poorly understood. Here we present enantiomerically separated atmospheric monoterpene and isoprene data from an enclosed tropical rainforest ecosystem in the absence of ultraviolet light and atmospheric oxidation chemistry, during a four-month controlled drought and rewetting experiment⁷. Surprisingly, the emitted enantiomers showed distinct diel emission peaks, which responded differently to progressive drying. Isotopic labelling established that vegetation emitted mainly de novo-synthesized (-)-α-pinene, whereas (+)-α-pinene was emitted from storage pools. As drought progressed, the source of (-)-α-pinene emissions shifted to storage pools, favouring cloud formation. Pre-drought mixing ratios of both α-pinene enantiomers correlated better with other monoterpenes than with each other, indicating different enzymatic controls. These results show that enantiomeric distribution is key to understanding the underlying processes driving monoterpene emissions from forest ecosystems and predicting atmospheric feedbacks in response to climate change.

An Overview of the Isoprenoid Emissions From Tropical Plant Species

Terrestrial vegetation is the largest contributor of isoprenoids (a group of biogenic volatile organic compounds (BVOCs)) to the atmosphere. BVOC emission data comes mostly from temperate regions, and less is known about BVOC emissions from tropical vegetation, even though it is estimated to be responsible for >70% of BVOC emissions. This review summarizes the available data and our current understanding of isoprenoid emissions from tropical plant species and the spatial and temporal variation in emissions, which are strongly species-specific and regionally variable. Emission models lacking foliar level data for tropical species need to revise their parameters to account for seasonal and diurnal variation due to differences in dependencies on temperature and light of emissions from plants in other ecosystems. More experimental information and determining how emission capacity varies during foliar development are warranted to account for seasonal variations more explicitly.

Upward trend and formation of surface ozone in the Guanzhong Basin, Northwest China

Increase trend of surface ozone (O₃) was observed in the Guanzhong Basin (GZB) from 2014 to 2020 with growth rates of 3.9-6.4 μg m^-3 yr^-1 for the maximum daily average 8 h (MDA8) O₃ concentrations. To further understand the formation of O₃, investigation of volatile organic compounds (VOCs) was carried out in the summer of 2018. High levels of VOCs were observed in both residential area and industrialized cities. Elevated concentrations of none-methane Hydrocarbon (NMHCs) were observed in rush hours, which indicated dominated roles of traffic activities on the loading of ambient VOCs. In the nighttime, both of NMHCs and oxygenated VOCs (OVOCs) were raised, and the peaks of VOCs kept pace with accumulation of O₃. Wind field indicated that northward and westward air mass, which passed through the remote forest and industrial area in east of the GZB, was responsible to elevated ambient VOCs in the GZB. Traffic emission, fuel evaporation, and solvent using were key contributors to ambient NMHCs, while solvent using and secondary formation dominated the loading of OVOCs. The present study indicated that both local management and regional collaborative control on active VOCs species from typical sources is urgently needed in GZB.

Evaluating the impacts of burning biomass on PM2.5 regional transport under various emission conditions

The fine particulate matter (PM_2.5) emitted by burning biomass has become the main source of pollution in cities; this pollution seriously threatens the ecosystem and inhabitants' health. A major challenge in dealing with this issue is the uncertainty regarding the influence of burning biomass on PM_2.5 regional transport. In this study, Harbin-Changchun Megalopolis is the research area. Using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model combined with satellite data and PM_2.5 monitoring data, we quantitatively analyzed the regional transport of PM_2.5 from burning biomass upwind of each city under different emission conditions. Conditions at burn sites, especially emission intensity and meteorological factors, as well as transport distance play significant roles in the regional transport of PM_2.5. Higher emission intensity aggravated the concentration of downwind PM_2.5, at most 19.7 μg ⋅ m^-3. Shorter transport distance strengthened the impact of biomass burning on downstream PM_2.5 by weakening elimination, which could be up to 96.8 μg ⋅ m^-3. Moreover, meteorological factors at fire points were closely related to the transport of PM_2.5. First, lower planetary boundary layer height could enhance the transport of PM_2.5 from the burning biomass by inhibiting vertical diffusion, and the enhancement could be up to 46.1 μg ⋅ m^-3. Second, compared to strong wind, light wind caused the weaker dilution, enhancing PM_2.5 regional transport by as much as 32.5 μg ⋅ m^-3. Third, relatively humidity at 30%-40% had the strongest effect in facilitating the transport of PM_2.5 from burning biomass. We conclude that comprehensively considering these three factors, namely the emission intensity, transport distance and meteorological factors at burn sites can facilitate the cross-regional development of accurate prediction models and effective pollution control measures.

Monoterpenes from tropical forest and oil palm plantation floor in Malaysian Borneo/Sabah: emission and composition

Regional estimates of VOC fluxes focus largely on emissions from the canopy and omit potential contributions from the forest floor including soil, litter and understorey vegetation. Here, we measured monoterpene emissions every 2 months over 2 years from logged tropical forest and oil palm plantation floor in Malaysian Borneo using static flux chambers. The main emitted monoterpenes were α-pinene, β-pinene and d-limonene. The amount of litter present was the strongest indicator for higher monoterpene fluxes. Mean α-pinene fluxes were around 2.5-3.5 μg C m^-2 h^-1 from the forest floor with occasional fluxes exceeding 100 μg C m^-2 h^-1. Fluxes from the oil palm plantation, where hardly any litter was present, were lower (on average 0.5-2.9 μg C m^-2 h^-1) and only higher when litter was present. All other measured monoterpenes were emitted at lower rates. No seasonal trends could be identified for all monoterpenes and mean fluxes from both forest and plantation floor were ~ 100 times smaller than canopy emission rates reported in the literature. Occasional spikes of higher emissions from the forest floor, however, warrant further investigation in terms of underlying processes and their contribution to regional scale atmospheric fluxes.

Deriving emission fluxes of volatile organic compounds from tower observation in the Pearl River Delta, China

Accurate estimation of speciated emissions of volatile organic compounds (VOCs) is challenging due to the complexity of both species and sources. Evaluation of the bottom-up emission inventory (EI) by atmospheric observation is needed to better understand the VOC emissions and then to control air pollutions caused by VOCs. This study conducts vertical measurements of VOCs between November 3 and 11, 2018 at the Canton Tower in the urban core of Pearl River Delta (PRD), China. A mixed layer gradient (MLG) technique is applied to the tower observation data to derive emission fluxes for individual VOC. The results show that the measured VOCs concentrations at ground level were always higher than those at the heights of 118 m and 488 m. Obvious vertical gradients of concentrations were found for VOC species, such as benzene, toluene and isoprene. The emission flux was estimated to be largest for propane (3.29 mg m^-2 h^-1), followed by toluene (2.55 mg m^-2 h^-1), isoprene (2.24 mg m^-2 h^-1), n-butane (2.10 mg m^-2 h^-1) and iso-pentane (1.73 mg m^-2 h^-1). The total VOC emission fluxes were around 3 times larger than those in the EI, suggesting 1.5-2 times underestimations of ozone formation potential (OFP) and secondary organic aerosol potential (SOAP) by current EI. Substantial underestimations (3-20 times) were found for C₂-C₅ alkanes by current EI. Due to unmeasured input parameters, limited sample size and short sampling period, there are still large uncertainties (40%-117%) in the estimated emission fluxes for individual species. Whereas, this study shows that the tower observation and emission estimation using MLG method could provide useful information for better understanding vertical distributions and emission fluxes of VOCs, and pioneer in assessing the existing emission inventories at species-level and hour-level.

Amazonian biogenic volatile organic compounds under global change

Biogenic volatile organic compounds (BVOCs) play important roles at cellular, foliar, ecosystem and atmospheric levels. The Amazonian rainforest represents one of the major global sources of BVOCs, so its study is essential for understanding BVOC dynamics. It also provides insights into the role of such large and biodiverse forest ecosystem in regional and global atmospheric chemistry and climate. We review the current information on Amazonian BVOCs and identify future research priorities exploring biogenic emissions and drivers, ecological interactions, atmospheric impacts, depositional processes and modifications to BVOC dynamics due to changes in climate and land cover. A feedback loop between Amazonian BVOCs and the trends of climate and land-use changes in Amazonia is then constructed. Satellite observations and model simulation time series demonstrate the validity of the proposed loop showing a combined effect of climate change and deforestation on BVOC emission in Amazonia. A decreasing trend of isoprene during the wet season, most likely due to forest biomass loss, and an increasing trend of the sesquiterpene to isoprene ratio during the dry season suggest increasing temperature stress-induced emissions due to climate change.

Winter VOCs and OVOCs measured with PTR-MS at an urban site of India: Role of emissions, meteorology and photochemical sources

Within the outline of air quality studies at metropolitan city, the mixing ratios of seven selected volatile organic compounds (VOCs) were measured during December 2015 (winter) at an urban site of Pune. The measurement of VOCs was conducted using a proton transfer reaction-quadrupole mass spectrometer (PTR-QMS). The study represents daily variability of ambient VOCs and their various associated emission sources. Diurnal profiles have differed from one VOC to another as the result of their different origins and the influence of different meteorological parameters (i.e. solar radiation, temperature) and planetary boundary layer height (PBL-H). The hourly mixing ratios of Oxygenated-VOCs (OVOCs) and aromatics were in the ranges of 0.6-29 ppbv and 0.13-14 ppbv, respectively with OVOCs accounted for up to 75% of total measured VOCs. The role of long-range transport from the clear Thar Desert and polluted Indo-Gangetic Plain (IGP) was observed during the episodes of 1-15 and 17-31 December 2015, respectively. VOCs showed the strong diurnal variations with peaks during morning and evening hours and lowest in the afternoon. In the evening period, high levels of aromatics coincided with the lowest OVOCs suggests the role of fresh vehicular emissions. Emission ratios of various VOCs as a function of temperature showed the role of different sources including the biogenic and photochemical production as well as the anthropogenic sources, respectively. The higher emission ratio of Δmethanol/Δacetonitrile at the study site suggests the long range transport of biomass burning plumes from the Indo-Gangetic Plain (IGP) during the 17-31, Dec. 2015. In addition to the pattern of emission, the diurnal and day-to-day variations of VOCs were influenced by the local meteorological conditions and depth of planetary boundary layer (PBL-H).

Investigation of emission characteristics of NMVOCs over urban site of western India

This is the first study to characterize the variation and emission of C₂-C₅ non-methane volatile organic compounds (NMVOCs) in a semi-urban site of western India based on measurements during February-December 2015. Anthropogenic NMVOCs show clear seasonal dependence with highest in winter and lowest in monsoon season. Biogenic NMVOCs likes isoprene show highest mixing ratios in the pre-monsoon season. The diurnal variation of NMVOC species can be described by elevated values from night till morning and lower values in the afternoon hours. The elevated levels of NMVOCs during night and early morning hours were caused mainly by weaker winds, temperature inversion and reduced chemical loss. The correlations between NMVOCs, CO and NOx indicate the dominant role of various local emission sources. Use and leakage of liquefied petroleum gas (LPG) contributed to the elevated levels of propane and butanes. Mixing ratios of ethylene, propylene, CO, NOx, etc. show predominant emissions from combustion of fuels in automobiles and industries. The Positive Matrix Factorization (PMF) source apportionments were performed for the seven major emission sectors (i.e. Vehicular exhaust, Mixed industrial emissions, Biomass/Fired brick kilns/Bio-fuel, Petrochem, LPG, Gas evaporation, Biogenic). Emissions from vehicle exhaust and industry-related sources contributed to about 19% and 40% of the NMVOCs, respectively. And the rest (41%) was attributed to the emissions from biogenic sources, LPG, gasoline evaporation and biomass burning. Diurnal and seasonal variations of NMVOCs were controlled by local emissions, meteorology, OH concentrations, long-range transport and planetary boundary layer height. This study provides a good reference for framing environmental policies to improve the air quality in western region of India.

Isoprene photo-oxidation products quantify the effect of pollution on hydroxyl radicals over Amazonia

Nitrogen oxides (NO _x ) emitted from human activities are believed to regulate the atmospheric oxidation capacity of the troposphere. However, observational evidence is limited for the low-to-median NO _x concentrations prevalent outside of polluted regions. Directly measuring oxidation capacity, represented primarily by hydroxyl radicals (OH), is challenging, and the span in NO _x concentrations at a single observation site is often not wide. Concentrations of isoprene and its photo-oxidation products were used to infer the equivalent noontime OH concentrations. The fetch at an observation site in central Amazonia experienced varied contributions from background regional air, urban pollution, and biomass burning. The afternoon concentrations of reactive nitrogen oxides (NO _y ), indicative of NO _x exposure during the preceding few hours, spanned from 0.3 to 3.5 parts per billion. Accompanying the increase of NO _y concentration, the inferred equivalent noontime OH concentrations increased by at least 250% from 0.6 × 10⁶ to 1.6 × 10⁶ cm^-3. The conclusion is that, compared to background conditions of low NO _x concentrations over the Amazon forest, pollution increased NO _x concentrations and amplified OH concentrations, indicating the susceptibility of the atmospheric oxidation capacity over the forest to anthropogenic influence and reinforcing the important role of NO _x in sustaining OH concentrations.

Contribution of biogenic and photochemical sources to ambient VOCs during winter to summer transition at a semi-arid urban site in India

This paper presents the sources and characteristics of ambient volatile organic compounds (VOCs) measured using PTR-TOF-MS instrument in a metropolitan city of India during winter to summer transition period. Mixing ratios of VOCs exhibited strong diurnal, day-to-day and episodic variations. Methanol was the most dominant species with monthly mean values of 18-22 pbbv. The emission ratios of VOCs relative to benzene calculated from nighttime data were used to estimate the relative contributions of vehicle exhaust and other sources. The increasing daytime ratios of oxygenated-VOCs (OVOCs)/benzene and isoprene/benzene from February to March indicates increasing contribution of photo-oxidation and biogenic sources. Daytime fractions of acetone (18%), acetaldehyde (15%) and isoprene (4.5%) to the sum of measured VOCs in March were higher than those in February. Variations of VOCs at lower temperatures (<25 °C) were predominantly controlled by anthropogenic sources. At high temperatures, particularly in the range of 32-40 °C during March, levels of OVOCs and isoprene were influenced by biogenic emissions. The emissions of OVOCs from vehicle exhaust were estimated to be smaller (20-40%) than those from other sources. The contributions of biogenic and secondary sources to OVOCs and isoprene increased by 10-15% from winter to summer. This study provides evidence that the winter-to-summer transition has an impact on sources and composition of VOCs in tropical urban areas.

Airborne observations reveal elevational gradient in tropical forest isoprene emissions

Isoprene dominates global non-methane volatile organic compound emissions, and impacts tropospheric chemistry by influencing oxidants and aerosols. Isoprene emission rates vary over several orders of magnitude for different plants, and characterizing this immense biological chemodiversity is a challenge for estimating isoprene emission from tropical forests. Here we present the isoprene emission estimates from aircraft eddy covariance measurements over the Amazonian forest. We report isoprene emission rates that are three times higher than satellite top-down estimates and 35% higher than model predictions. The results reveal strong correlations between observed isoprene emission rates and terrain elevations, which are confirmed by similar correlations between satellite-derived isoprene emissions and terrain elevations. We propose that the elevational gradient in the Amazonian forest isoprene emission capacity is determined by plant species distributions and can substantially explain isoprene emission variability in tropical forests, and use a model to demonstrate the resulting impacts on regional air quality.

Isoprene emissions are commonly estimated using satellite measurements and model simulations. Here, using eddy covariance, the authors report higher emission rates over the Amazon forest than those estimated with these techniques and a relationship between terrain elevation and isoprene emissions.

Numerical model to quantify biogenic volatile organic compound emissions: The Pearl River Delta region as a case study

This article compiles the actual knowledge of the biogenic volatile organic compound (BVOC) emissions estimated using model methods in the Pearl River Delta (PRD) region, one of the most developed regions in China. The developed history of BVOC emission models is presented briefly and three typical emission models are introduced and compared. The results from local studies related to BVOC emissions have been summarized. Based on this analysis, it is recommended that local researchers conduct BVOC emission studies systematically, from the assessment of model inputs, to compiling regional emission inventories to quantifying the uncertainties and evaluating the model results. Beyond that, more basic researches should be conducted in the future to close the gaps in knowledge on BVOC emission mechanisms, to develop the emission models and to refine the inventory results. This paper can provide a perspective on these aspects in the broad field of research associated with BVOC emissions in the PRD region.

Isoprene photochemistry over the Amazon rainforest

Isoprene photooxidation is a major driver of atmospheric chemistry over forested regions. Isoprene reacts with hydroxyl radicals (OH) and molecular oxygen to produce isoprene peroxy radicals (ISOPOO). These radicals can react with hydroperoxyl radicals (HO2) to dominantly produce hydroxyhydroperoxides (ISOPOOH). They can also react with nitric oxide (NO) to largely produce methyl vinyl ketone (MVK) and methacrolein (MACR). Unimolecular isomerization and bimolecular reactions with organic peroxy radicals are also possible. There is uncertainty about the relative importance of each of these pathways in the atmosphere and possible changes because of anthropogenic pollution. Herein, measurements of ISOPOOH and MVK + MACR concentrations are reported over the central region of the Amazon basin during the wet season. The research site, downwind of an urban region, intercepted both background and polluted air masses during the GoAmazon2014/5 Experiment. Under background conditions, the confidence interval for the ratio of the ISOPOOH concentration to that of MVK + MACR spanned 0.4-0.6. This result implies a ratio of the reaction rate of ISOPOO with HO2 to that with NO of approximately unity. A value of unity is significantly smaller than simulated at present by global chemical transport models for this important, nominally low-NO, forested region of Earth. Under polluted conditions, when the concentrations of reactive nitrogen compounds were high (>1 ppb), ISOPOOH concentrations dropped below the instrumental detection limit (<60 ppt). This abrupt shift in isoprene photooxidation, sparked by human activities, speaks to ongoing and possible future changes in the photochemistry active over the Amazon rainforest.

Removal of Indoor Volatile Organic Compounds via Photocatalytic Oxidation: A Short Review and Prospect

Volatile organic compounds (VOCs) are ubiquitous in indoor environments. Inhalation of VOCs can cause irritation, difficulty breathing, and nausea, and damage the central nervous system as well as other organs. Formaldehyde is a particularly important VOC as it is even a carcinogen. Removal of VOCs is thus critical to control indoor air quality (IAQ). Photocatalytic oxidation has demonstrated feasibility to remove toxic VOCs and formaldehyde from indoor environments. The technique is highly-chemical stable, inexpensive, non-toxic, and capable of removing a wide variety of organics under light irradiation. In this paper, we review and summarize the traditional air cleaning methods and current photocatalytic oxidation approaches in both of VOCs and formaldehyde degradation in indoor environments. Influencing factors such as temperature, relative humidity, deactivation and reactivations of the photocatalyst are discussed. Aspects of the application of the photocatalytic technique to improve the IAQ are suggested.

Investigation of a potential HCHO measurement artifact from ISOPOOH

Recent laboratory experiments have shown that a first generation isoprene oxidation product, ISOPOOH, can decompose to methyl vinyl ketone (MVK) and methacrolein (MACR) on instrument surfaces, leading to overestimates of MVK and MACR concentrations. Formaldehyde (HCHO) was suggested as a decomposition co-product, raising concern that in situ HCHO measurements may also be affected by an ISOPOOH interference. The HCHO measurement artifact from ISOPOOH for the NASA In Situ Airborne Formaldehyde instrument (ISAF) was investigated for the two major ISOPOOH isomers, (1,2)-ISOPOOH and (4,3)-ISOPOOH, under dry and humid conditions. The dry conversion of ISOPOOH to HCHO was 3±2% and 6±4% for (1,2)-ISOPOOH and (4,3)-ISOPOOH, respectively. Under humid (RH= 40-60%) conditions, conversion to HCHO was 6±4% for (1,2)-ISOPOOH and 10±5% for (4,3)-ISOPOOH. The measurement artifact caused by conversion of ISOPOOH to HCHO in the ISAF instrument was estimated for data obtained on the 2013 September 6 flight of the Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC⁴RS) campaign. Prompt ISOPOOH conversion to HCHO was the source for <4% of the observed HCHO, including in the high-isoprene boundary layer. Time-delayed conversion, where previous exposure to ISOPOOH affects measured HCHO later in flight, was conservatively estimated to be < 10% of observed HCHO and is significant only when high ISOPOOH sampling periods immediately precede periods of low HCHO.

Effect of land-use change and management on biogenic volatile organic compound emissions--selecting climate-smart cultivars

Land-use change (LUC) has fundamentally altered the form and function of the terrestrial biosphere. Increasing human population, the drive for higher living standards and the potential challenges of mitigating and adapting to global environmental change mean that further changes in LUC are unavoidable. LUC has direct consequences on climate not only via emissions of greenhouse gases and changing the surface energy balance but also by affecting the emission of biogenic volatile organic compounds (BVOCs). Isoprenoids, which dominate global BVOC emissions, are highly reactive and strongly modify atmospheric composition. The effects of LUC on BVOC emissions and related atmospheric chemistry have been largely ignored so far. However, compared with natural ecosystems, most tree species used in bioenergy plantations are strong BVOC emitters, whereas intensively cultivated crops typically emit less BVOCs. Here, we summarize the current knowledge on LUC-driven BVOC emissions and how these might affect atmospheric composition and climate. We further discuss land management and plant-breeding strategies, which could be taken to move towards climate-friendly BVOC emissions while simultaneously maintaining or improving key ecosystem functions such as crop yield under a changing environment.

Primary and secondary organics in the tropical Amazonian rainforest aerosols: chiral analysis of 2-methyltetraols

This work presents the application of a new method to facilitate the distinction between biologically produced (primary) and atmospherically produced (secondary) organic compounds in ambient aerosols based on their chirality. The compounds chosen for this analysis were the stereomers of 2-methyltetraols, (2R,3S)- and (2S,3R)-methylerythritol, (l- and d-form, respectively), and (2S,3S)- and (2R,3R)-methylthreitol (l- and d-form), shown previously to display some enantiomeric excesses in atmospheric aerosols, thus to have at least a partial biological origin. In this work PM10 aerosol fractions were collected in a remote tropical rainforest environment near Manaus, Brazil, between June 2008 and June 2009 and analysed. Both 2-methylerythritol and 2-methylthreitol displayed a net excess of one enantiomer (either the l- or the d-form) in 60 to 72% of these samples. These net enantiomeric excesses corresponded to compounds entirely biological but accounted for only about 5% of the total 2-methyltetrol mass in all the samples. Further analysis showed that, in addition, a large mass of the racemic fractions (equal mixtures of d- and l-forms) was also biological. Estimating the contribution of secondary reactions from the isomeric ratios measured in the samples (=ratios 2-methylthreitol over 2-methylerythritol), the mass fraction of secondary methyltetrols in these samples was estimated to a maximum of 31% and their primary fraction to a minimum of 69%. Such large primary fractions could have been expected in PM10 aerosols, largely influenced by biological emissions, and would now need to be investigated in finer aerosols. This work demonstrates the effectiveness of chiral and isomeric analyses as the first direct tool to assess the primary and secondary fractions of organic aerosols.

Atmospheric aerosols in Amazonia and land use change: from natural biogenic to biomass burning conditions

In the wet season, a large portion of the Amazon region constitutes one of the most pristine continental areas, with very low concentrations of atmospheric trace gases and aerosol particles. However, land use change modifies the biosphere-atmosphere interactions in such a way that key processes that maintain the functioning of Amazonia are substantially altered. This study presents a comparison between aerosol properties observed at a preserved forest site in Central Amazonia (TT34 North of Manaus) and at a heavily biomass burning impacted site in south-western Amazonia (PVH, close to Porto Velho). Amazonian aerosols were characterized in detail, including aerosol size distributions, aerosol light absorption and scattering, optical depth and aerosol inorganic and organic composition, among other properties. The central Amazonia site (TT34) showed low aerosol concentrations (PM2.5 of 1.3 +/- 0.7 microg m(-3) and 3.4 +/- 2.0 microg m(-3) in the wet and dry seasons, respectively), with a median particle number concentration of 220 cm(-3) in the wet season and 2200 cm(-3) in the dry season. At the impacted site (PVH), aerosol loadings were one order of magnitude higher (PM2.5 of 10.2 +/- 9.0 microg m(-3) and 33.0 +/- 36.0 microg m(-3) in the wet and dry seasons, respectively). The aerosol number concentration at the impacted site ranged from 680 cm(-3) in the wet season up to 20 000 cm(-3) in the dry season. An aerosol chemical speciation monitor (ACSM) was deployed in 2013 at both sites, and it shows that organic aerosol account to 81% to the non-refractory PM1 aerosol loading at TT34, while biomass burning aerosols at PVH shows a 93% content of organic particles. Three years of filter-based elemental composition measurements shows that sulphate at the impacted site decreases, on average, from 12% of PM2.5 mass during the wet season to 5% in the dry season. This result corroborates the ACSM finding that the biomass burning contributed overwhelmingly to the organic fine mode aerosol during the dry season in this region. Aerosol light scattering and absorption coefficients at the TT34 site were low during the wet season, increasing by a factor of 5, approximately, in the dry season due to long range transport of biomass burning aerosols reaching the forest site in the dry season. Aerosol single scattering albedo (SSA) ranged from 0.84 in the wet season up to 0.91 in the dry. At the PVH site, aerosol scattering coefficients were 3-5 times higher in comparison to the TT34 site, an indication of strong regional background pollution, even in the wet season. Aerosol absorption coefficients at PVH were about 1.4 times higher than at the forest site. Ground-based SSA at PVH was around 0.92 year round, showing the dominance of scattering aerosol particles over absorption, even for biomass burning aerosols. Remote sensing observations from six AERONET sites and from MODIS since 1999, provide a regional and temporal overview. Aerosol Optical Depth (AOD) at 550 nm of less than 0.1 is characteristic of natural conditions over Amazonia. At the perturbed PVH site, AOD550 values greater than 4 were frequently observed in the dry season. Combined analysis of MODIS and CERES showed that the mean direct radiative forcing of aerosols at the top of the atmosphere (TOA) during the biomass burning season was -5.6 +/- 1.7 W m(-2), averaged over whole Amazon Basin. For high AOD (larger than 1) the maximum daily direct aerosol radiative forcing at the TOA was as high as -20 W m(-2) locally. This change in the radiation balance caused increases in the diffuse radiation flux, with an increase of Net Ecosystem Exchange (NEE) of 18-29% for high AOD. From this analysis, it is clear that land use change in Amazonia shows alterations of many atmospheric properties, and these changes are affecting the functioning of the Amazonian ecosystem in significant ways.

Organic constituents on the surfaces of aerosol particles from Southern Finland, Amazonia, and California studied by vibrational sum frequency generation

This article summarizes and compares the analysis of the surfaces of natural aerosol particles from three different forest environments by vibrational sum frequency generation. The experiments were carried out directly on filter and impactor substrates, without the need for sample preconcentration, manipulation, or destruction. We discuss the important first steps leading to secondary organic aerosol (SOA) particle nucleation and growth from terpene oxidation by showing that, as viewed by coherent vibrational spectroscopy, the chemical composition of the surface region of aerosol particles having sizes of 1 μm and lower appears to be close to size-invariant. We also discuss the concept of molecular chirality as a chemical marker that could be useful for quantifying how chemical constituents in the SOA gas phase and the SOA particle phase are related in time. Finally, we describe how the combination of multiple disciplines, such as aerosol science, advanced vibrational spectroscopy, meteorology, and chemistry can be highly informative when studying particles collected during atmospheric chemistry field campaigns, such as those carried out during HUMPPA-COPEC-2010, AMAZE-08, or BEARPEX-2009, and when they are compared to results from synthetic model systems such as particles from the Harvard Environmental Chamber (HEC). Discussions regarding the future of SOA chemical analysis approaches are given in the context of providing a path toward detailed spectroscopic assignments of SOA particle precursors and constituents and to fast-forward, in terms of mechanistic studies, through the SOA particle formation process.

The atmospheric chemistry of trace gases and particulate matter emitted by different land uses in Borneo

We report measurements of atmospheric composition over a tropical rainforest and over a nearby oil palm plantation in Sabah, Borneo. The primary vegetation in each of the two landscapes emits very different amounts and kinds of volatile organic compounds (VOCs), resulting in distinctive VOC fingerprints in the atmospheric boundary layer for both landscapes. VOCs over the Borneo rainforest are dominated by isoprene and its oxidation products, with a significant additional contribution from monoterpenes. Rather than consuming the main atmospheric oxidant, OH, these high concentrations of VOCs appear to maintain OH, as has been observed previously over Amazonia. The boundary-layer characteristics and mixing ratios of VOCs observed over the Borneo rainforest are different to those measured previously over Amazonia. Compared with the Bornean rainforest, air over the oil palm plantation contains much more isoprene, monoterpenes are relatively less important, and the flower scent, estragole, is prominent. Concentrations of nitrogen oxides are greater above the agro-industrial oil palm landscape than over the rainforest, and this leads to changes in some secondary pollutant mixing ratios (but not, currently, differences in ozone). Secondary organic aerosol over both landscapes shows a significant contribution from isoprene. Primary biological aerosol dominates the super-micrometre aerosol over the rainforest and is likely to be sensitive to land-use change, since the fungal source of the bioaerosol is closely linked to above-ground biodiversity.

Atmospheric oxidation capacity sustained by a tropical forest

Terrestrial vegetation, especially tropical rain forest, releases vast quantities of volatile organic compounds (VOCs) to the atmosphere, which are removed by oxidation reactions and deposition of reaction products. The oxidation is mainly initiated by hydroxyl radicals (OH), primarily formed through the photodissociation of ozone. Previously it was thought that, in unpolluted air, biogenic VOCs deplete OH and reduce the atmospheric oxidation capacity. Conversely, in polluted air VOC oxidation leads to noxious oxidant build-up by the catalytic action of nitrogen oxides (NO(x) = NO + NO2). Here we report aircraft measurements of atmospheric trace gases performed over the pristine Amazon forest. Our data reveal unexpectedly high OH concentrations. We propose that natural VOC oxidation, notably of isoprene, recycles OH efficiently in low-NO(x) air through reactions of organic peroxy radicals. Computations with an atmospheric chemistry model and the results of laboratory experiments suggest that an OH recycling efficiency of 40-80 per cent in isoprene oxidation may be able to explain the high OH levels we observed in the field. Although further laboratory studies are necessary to explore the chemical mechanism responsible for OH recycling in more detail, our results demonstrate that the biosphere maintains a remarkable balance with the atmospheric environment.