Anthropogenic Isoprene Research Articles

Isoprene in urban Atlantic forests: Variability, origin, and implications on the air quality of a subtropical megacity

Biosphere-atmosphere interactions play a key role in urban chemistry because of biogenic volatile organic compound (BVOC) emissions. Of the BVOC, isoprene is the most emitted compound; however, it also has anthropogenic origins in urban areas. In this study, we aimed to investigate the spatio-temporal variability and atmospheric impacts of biogenic and anthropogenic isoprene in the subtropical megacity of São Paulo (MASP), Brazil. Several measurement campaigns were conducted in three different urban Atlantic forests (Matão, PEFI, and RMG), and an urban background site (IAG); this equated to a total of 268 samples for the 2018–2019 period. For all sampling points, daytime average concentrations of isoprene were two to three times higher during the rainy season (IAG: 1.75 ± 0.93 ppb; Matão: 0.87 ± 0.35 ppb; PEFI: 0.50 ± 0.30 ppb; RMG: 0.37 ± 0.18 ppb), than those observed during the dry season (IAG: 0.46 ± 0.24 ppb; Matão: 0.31 ± 0.17 ppb; PEFI: 0.17 ± 0.11 ppb; RMG: 0.11 ± 0.07 ppb). Average isoprene concentrations were similar to those observed in other places worldwide, with the exception of the Amazon forest. Our results indicate differences in isoprene concentrations between sites, suggesting that environmental conditions such as the urban heat island and vegetation types, may play a role in spatial variability. Estimates of the isoprene fraction indicated that the biogenic fraction (85%) surpassed the anthropogenic fraction during the rainy season. By contrast, the anthropogenic fraction (52%) exceeded the biogenic fraction during dry periods. These fractions have an impact on potentially forming secondary pollutants gaseous (ozone formation potential: 7.19–33.32 μg m−3), and aerosols (secondary organic aerosols formation potential: 0.41–1.88 μg m−3). These results highlight the role of biogenic isoprene and its potential impact on urban air quality in subtropical megacities; this requires further investigation under future climate change scenarios.

Tracer-based characterization of source variations of ambient isoprene mixing ratios in a hillocky megacity, India, influenced by the local meteorology

The ambient biogenic volatile organic compounds (BVOCs), mainly isoprene, are potentially involved in the formation of secondary pollutants, hence, they are significant in terms of air quality and climate. Although the largest sources of BVOCs are tropical regions, the measurements of isoprene in the Indian subcontinent are limited. We conducted the measurements of isoprene, benzene, and toluene at an urban site in a hillocky megacity of India using a high-sensitivity proton transfer reaction quadrupole mass spectrometer (PTR-QMS). The mixing ratios of isoprene were compared with those of aromatic compounds like benzene and toluene, which represent typical anthropogenic VOCs. Isoprene and isoprene/benzene (>5 ppbv ppbv−1) showed higher levels in the pre-monsoon months, most likely due to large emissions by urban vegetation during physiological activities in plants which was enhanced by the high ambient temperatures and solar radiation. While Benzene and toluene showed higher mixing ratios during winter, which were due to shallower boundary layer depths and transport of air masses from polluted Indo-Gangetic Plain during this season. The mixing ratios of VOCs show significant diurnal variation as a result of their different origins and the role of different meteorological parameters. The robust emission ratios of isoprene/benzene obtained from nighttime data were used to separate the non-anthropogenic and anthropogenic isoprene emissions. ∼30% enhancement observed in non-anthropogenic emissions to isoprene from winter to pre-monsoon season when temperatures and solar radiation were stronger, although traffic in the city. Isoprene/benzene ratio at lower temperatures (<25 °C) and solar radiation (<100 W m−2) was predominantly controlled by anthropogenic sources. Overall, toluene and isoprene are the most frequent species in terms of having the highest ozone-forming potential (OFP) values but biogenic isoprene became more important to ozone formation during the afternoon hours in the pre-monsoon months with high air temperatures (>25 °C).

Atmospheric formaldehyde at El Teide and Pic du Midi remote high-altitude sites

Formaldehyde (CH2O) is a tracer of the photochemical activity of the atmosphere. Linked to air quality, CH2O is an ozone (O3) precursor and serves as a proxy for natural and anthropogenic reactive organic emissions. As a product of the photooxidation of methane (CH4) and other hydrocarbons (e.g., isoprene), CH2O represents an important source of radicals in the remote free troposphere. This work aims at improving the characterization of this part of the troposphere where data are scarce. In particular, this study assesses the presence of CH2O at two high-altitude remote sites: El Teide (TEI, 3570 m a.s.l., Tenerife, Canary Islands, Spain) and Pic du Midi (PDM, 2877 m a.s.l., French Pyrenees). Through ground-based remote sensing measurements performed during two field campaigns in July (TEI) and September (PDM) 2013, this study presents the vertical distribution of CH2O at both locations. Results at PDM show that CH2O mixing ratios follow a decreasing vertical profile with a mean maximum of 0.5 ± 0.2 nmol mol−1 (i.e., ppbv) at the instruments' altitude. At TEI, observations indicate an uplifted layer of CH2O with a mean maximum of 1.3 ± 0.3 nmol mol−1 at 3.8 km a.s.l. (i.e., 300 m above the instrument's altitude). At both remote sites, the observed CH2O levels are higher than expected for background methane oxidation (a threefold increase in the case of TEI). Air mass back trajectory analysis links CH2O observations with abundant natural (e.g. forests) and/or anthropogenic isoprene emissions from the region nearby PDM, while the high CH2O levels detected at TEI indicate in-plume formation of CH2O resulting from its precursors emitted from west-African and Canadian fires. Finally, as a key trace gas for O3 and HOx chemistries, we estimate the upper limit of bromine monoxide (BrO) in the free troposphere at TEI and PDM to be 0.8 and 1.5 pmol mol−1 (i.e., pptv) respectively.

Biogenic and anthropogenic isoprene emissions in the subtropical urban atmosphere of Delhi

This study examines the seasonal and intraday variations in isoprene, benzene and toluene at different sites in Delhi with a view to estimate the anthropogenic and biogenic fractions of isoprene and their ozone formation potential. For this purpose, measurements of ambient isoprene, benzene and toluene were made at selected vegetative and traffic sites of Delhi using Gas Chromatography-Mass Spectrometry. The study reveals that the average isoprene concentrations were higher at vegetative sites (4.40 ± 3.35 μg/m3) as compared to traffic sites (0.56 ± 0.22 μg/m3). The benzene and toluene levels, on the other hand, were found to be higher at the traffic sites (23.22 ± 10.94 and 60.58 ± 20.27 μg/m3) as compared to vegetative sites (8.01 ± 4.56 and 24.75 ± 10.86 μg/m3) respectively. The isoprene concentrations at vegetative sites were found to be the highest in summer (6.51 ± 3.56 μg/m3) followed by post-monsoon (4.80 ± 3.18 μg/m3) and winter (1.82 ± 0.57 μg/m3). In the context of intraday variation, isoprene concentrations attained maximum values during the afternoon period. However, no significant intraday/seasonal variation was observed in isoprene concentration at traffic sites. Estimation of anthropogenic and biogenic fractions of isoprene revealed that anthropogenic contribution varied between 60 and 70% at traffic sites and less than 10% at the vegetative sites. Upon computation of ozone formation potential of isoprene, benzene and toluene it is seen that isoprene's contribution towards ozone formation is higher at vegetative sites as compared to traffic sites. But, the anthropogenic isoprene's contribution towards ozone formation is found to be minimal in the present study. In view of the existing vegetation cover (~20%) of Delhi, however, the role of isoprene in surface ozone formation cannot be understated.

Isoprene and monoterpenes over Russia and their impacts in tropospheric ozone formation

Ground-based levels of important biogenic volatile organic compounds (BVOCs), isoprene and monoterpenes, as well as NO x and O 3 measured simultaneously along the Trans-Siberian railway on a mobile railway laboratory in TROICA-12 campaign in summer 2008 are analyzed. It was shown that the highest isoprene (≥ 2.5 ppb) concentration was observed in the daytime in the Far East region where several favorable factors for its emissions occurred: a large amount of deciduous forests, high temperatures (>28°C) and light conditions. Maximum levels of monoterpenes (up to 3-9 ppb) along the Trans-Siberian railway were observed during the nighttime in the Ural region and in Central Siberia where coniferous vegetation is located. To evaluate the relative importance of isoprene and monoterpenes in ground-level ozone formation in Russian cities along the Trans-Siberian railway, where high NO x concentration leads to tropospheric ozone generation, daytime ozone-forming potential (OFP) was calculated. The chemical losses of the studied BVOCs during their transport from sources to the measurement point were taken into account. Calculated OFPs due to isoprene (OFP iso ) and monoterpenes (OFP mono ) along the Trans-Siberian railway are in average 15±13 and 18±25 ppbv of ozone, respectively. The highest OFP iso (up to 40 ppbv) were estimated in Central Siberia and in the Far East. OFP mono was the highest in the regions of coniferous vegetation, Ural and Central Siberia, and reached 80 ppbv. In the most cities along the Trans-Siberian railway, where high NO x concentration (10-20 ppbv) along with high daytime temperatures (>25°C) were observed, monoterpenes made a main contribution to tropospheric ozone formation. Only in the Far East cities, where the largest deciduous vegetation area of the Trans-Siberian railway is located, isoprene played the main role in tropospheric ozone generation. It was also noted that OFP iso increases with the population-size of the cities. It can be either due to the greater proportion of deciduous vegetation in the large cities along the Trans-Siberian railway or due to the impact of anthropogenic isoprene source. OFP mono the lowest in the medium cities and the highest in the small ones.

A Two-Decade Anthropogenic and Biogenic Isoprene Emissions Study in a London Urban Background and a London Urban Traffic Site

A relationship between isoprene and 1,3-butadiene mixing ratios was established to separate the anthropogenic and biogenic fractions of the measured isoprene in London air in both urban background (Eltham) and urban traffic (Marylebone Road) areas over two decades (1997–2017). The average daytime biogenic isoprene mixing ratios over this period reached 0.09 ± 0.04 ppb (Marylebone Road) and 0.11 ± 0.06 ppb (Eltham) between the period of 6:00 to 20:00 local standard time, contributing 40 and 75% of the total daytime isoprene mixing ratios. The average summertime biogenic isoprene mixing ratios for 1997–2017 are found to be 0.13 ± 0.02 and 0.15 ± 0.04 ppb which contribute 50 and 90% of the total summertime isoprene mixing ratios for Marylebone Road and Eltham, respectively. Significant anthropogenic isoprene mixing ratios are found during night-time (0.11 ± 0.04 ppb) and winter months (0.14 ± 0.01 ppb) at Marylebone Road. During high-temperature and high-pollution events (high ozone) there is a suggestion that ozone itself may be directly responsible for some of the isoprene emission. By observing the positive correlation between biogenic isoprene levels with temperature, photosynthetically active radiation and ozone mixing ratios during heatwave periods, the Cobb-Douglas production function was used to obtain a better understanding of the abiotic factors that stimulate isoprene emission from plants. Other reasons for a correlation between ozone and isoprene are discussed. The long-term effects of urban stressors on vegetation were also observed, with biogenic isoprene mixing ratios on Marylebone Road dropping over a 20-year period regardless of the sustained biomass levels.

Revelation of Coupling Biogenic with Anthropogenic Isoprene by Highly Time-Resolved Observations

ABSTRACTInter-annual variations of atmospheric isoprene in a major metropolis, Taipei, were reported based on a two-year continuous measurements of non-methane hydrocarbons (NMHCs) with an hourly resolution. It is by far the largest dataset ever collected in the urban subtropical environment with thriving vegetation and traffic flows. The dataset revealed the detailed interplay between the two major sources of biogenic and vehicular throughout the year. To separate the vehicular contribution from the biogenic one for the ambient isoprene, ethylene was used as an indicator of traffic emissions. While dramatic surge of isoprene was observed at noontime in hot months with the highest average peak mixing ratio of 1.6 ppbv in August, its abundance decreased to 0.2 ppbv on average in cold months. The vehicular contribution to ambient isoprene was largely masked over by the noontime surge of isoprene in hot seasons, but was still able to be vaguely observed on the slopes of the isoprene peaks mimicking the rush-hour features of ethylene. In winter, the diurnal variations of isoprene were very similar to those of ethylene, which suggests that ambient isoprene in cold months was almost of traffic origin. This study based on the continuous dataset could enhance the key findings in previous flask studies in the same metropolis. While canister sampling had a major advantage of area coverage, the highly time-resolved fixed-site monitoring could better reveal the evolution process from a biogenically overwhelmed condition in hot months to the condition where the biogenic source weakened to reveal the traffic source in cold months.

Discrepancies between modeled and observed nocturnal isoprene in an urban environment and the possible causes: A case study in Houston

Air quality simulations were conducted using the Community Multiscale Air Quality (CMAQ) model for nocturnal isoprene in September 2013 using the United States Environmental Protection Agency's (EPA's) National Emissions Inventory of 2011 (NEI, 2011). The results were evaluated against measurements collected at eight Texas Commission on Environmental Quality (TCEQ) Automated Gas Chromatographs (AutoGCs) monitoring stations. The comparisons demonstrated two distinctive behaviors: overestimation before midnight (20:00–23:00p.m. local time) versus underestimation after midnight (00:00–06:00a.m.). Analyses identify the uncertainties in nitrate radical (NO3) concentration and vertical mixing as the possible minor factors contributing to the underestimation, and the underestimated wind speed as the major factor contributing to the overestimation. Further analysis links isoprene underestimation to the uncertainties in the nocturnal isoprene anthropogenic emissions in the NEI (2011) over industrial areas in Houston. This can be substantiated by the fact that the observed nighttime isoprene concentrations increased when the wind direction veered back from southeast to northeast, placing the stations downwind of industrial facilities. A sensitivity run with adjusted anthropogenic isoprene emissions in the later part of the night (i.e., the emissions were multiplied by the hourly underestimation factors ranging from 3.81 to 14.82) yielded closer isoprene predictions after midnight with slightly improved model mean (0.15 to 0.20ppb), mean error (−0.10 to −0.04ppb), mean absolute error (0.18 to 0.15ppb), root mean squared error (RMSE, 0.27 to 0.25ppb), and index of agreement (IOA, 0.66 to 0.68). The insignificant improvement was likely due to the uncertainties in the location of the high-peaked anthropogenic emissions. The impacts of the nighttime-adjusted isoprene emissions on the isoprene oxidation products, organic nitrate and ozone, were found to be minimal. This study, however, shows that more in-situ surface nighttime measurement data is critical to constrain the underestimated nocturnal isoprene emissions in Houston.

Isoprene emissions and impacts over an ecological transition region in the U.S. Upper Midwest inferred from tall tower measurements

AbstractWe present 1 year of in situ proton transfer reaction mass spectrometer (PTR‐MS) measurements of isoprene and its oxidation products methyl vinyl ketone (MVK) and methacrolein (MACR) from a 244 m tall tower in the U.S. Upper Midwest, located at an ecological transition between isoprene‐emitting deciduous forest and predominantly non‐isoprene‐emitting agricultural landscapes. We find that anthropogenic interferences (or anthropogenic isoprene) contribute on average 22% of the PTR‐MS m/z 69 signal during summer daytime, whereas MVK + MACR interferences (m/z 71) are minor (7%). After removing these interferences, the observed isoprene and MVK + MACR abundances show pronounced seasonal cycles, reaching summertime maxima of &gt;2500 pptv (1 h mean). The tall tower is impacted both by nearby and more distant regional isoprene sources, with daytime enhancements of isoprene (but little MVK + MACR) under southwest winds and enhancements of MVK + MACR (but little isoprene) at other times. We find that the GEOS‐Chem atmospheric model with the MEGANv2.1 (Model of Emissions of Gases and Aerosols from Nature version 2.1) biogenic inventory can reproduce the isoprene observations to within model uncertainty given improved land cover and temperature estimates. However, a 60% low model bias in MVK + MACR cannot be resolved, even across diverse model assumptions for NOx emissions, chemistry, atmospheric mixing, dry deposition, land cover, and potential measurement interferences. This implies that, while isoprene emissions in the immediate vicinity of the tall tower are adequately captured, they are underestimated across the broader region. We show that this region experiences a strong seasonal shift between VOC‐limited chemistry during the spring and fall and NOx‐limited or transitional chemistry during the summer, driven by the spatiotemporal distribution of isoprene emissions. Isoprene's role in causing these chemical shifts is likely underestimated due to the underprediction of its regional emissions.

Seasonal characteristics of biogenic and anthropogenic isoprene in tropical–subtropical urban environments

Measurements of atmospheric isoprene and other selected volatile organic compounds (VOCs) were conducted in Taipei, a tropical–subtropical metropolis, to investigate diurnal variations, seasonal diversity and spatial differences in terms of the levels and sources of isoprene. Our study also investigated the responses of biogenic isoprene to both light flux and temperature in real urban settings that have a variety of plant species and traffic volumes. The robust ratio of isoprene/1,3-butadiene obtained from pure traffic emissions was used as a gauge to separate biogenic isoprene from traffic emissions. The four seasonal measurements at a typical urban site in the city showed that biogenic contributions overwhelmed their anthropogenic counterparts in summer and dominated the daytime isoprene levels in spring and autumn. Even in winter, biogenic sources still contributed a non-negligible fraction of approximately 44% to daytime isoprene. Furthermore, the concentration contour of isoprene extrapolated from the data at 38 sites throughout the city revealed that high levels of biogenic isoprene were generally a widespread phenomenon in summer in the tropical–subtropical city. A three-dimensional plot of the isoprene/1,3-butadiene ratio, ambient temperatures and radiation flux showed a temperature threshold of biogenic isoprene emissions, beyond which the biogenic contribution began to increase exponentially with enhanced ambient temperature. However, when the ambient temperature was below the threshold, there was no or negligible biogenic contribution to the ambient isoprene, regardless of the strength of the radiation flux. The temperature threshold (approximately 17–21 °C) of isoprene emissions in the tropical–subtropical city was much higher than the thresholds in cities and areas at temperate latitudes, indicating that the adaptation of vegetation to different temperature zones via isoprene emission may be characteristic.

Biogenic and anthropogenic isoprene in the near-surface urban atmosphere — A case study in Essen, Germany

Isoprene is emitted in large quantities by vegetation, exhaled by human beings and released in small quantities by road traffic. As a result of its high reactivity, isoprene is an important ozone precursor in the troposphere and can play a key role in atmospheric chemistry. Measurements of isoprene in urban areas in Central Europe are scarce. Thus, in Essen, Germany, the isoprene concentration was measured at various sites during different seasons using two compact online GC-PID systems. Isoprene concentrations were compared with those of benzene and toluene, which represent typical anthropogenic VOCs. In the summer, the diurnal variation in isoprene concentration was dependent on the biogenic emissions in the city. It was found that its maximum concentration occurred during the day, in contrast to the benzene and toluene concentrations. During the measurement period in the summer of 2012, the average hourly isoprene concentrations reached 0.13 to 0.17ppb between 10 and 20 LST. At high air temperatures, the isoprene concentration exceeded the benzene and toluene concentrations at many of the sites. Isoprene became more important than toluene with regard to ozone formation in the city area during the afternoon hours of summer days with high air temperatures. This finding was demonstrated by the contributions to OH reactivity and ozone-forming potential. It contradicts the results of other studies, which were based on daily or seasonal average values. With an isoprene/benzene ratio of 0.02, the contribution of anthropogenic isoprene decreased substantially to a very low level during the last 20years in Central Europe due to a strong reduction in road traffic emissions. In the vicinity of many people, isoprene concentrations of up to 0.54ppb and isoprene/benzene ratios of up to 1.34 were found in the atmosphere due to isoprene exhaled by humans.

Biogenic isoprene in subtropical urban settings and implications for air quality

Isoprene has potentially a large impact on secondary oxidant formation, particularly in the polluted urban atmospheres. The environmental conditions in tropical and subtropical cities with high temperatures and light flux are conducive to the production of large amounts of biogenic isoprene. Measurements of speciated volatile organic compounds (VOCs) were conducted in Taipei, a subtropical metropolis, to investigate the characteristics of biogenic and anthropogenic isoprene during the hot seasons (summer and autumn) and to assess their significance in secondary pollutant formation. The daily and daytime average concentrations of isoprene at the subtropical urban site in summer were 0.72 and 1.26 ppbv, respectively, which were considerably higher than the concentrations of isoprene in most temperate cities. Furthermore, summertime isoprene ranks highest in OH reactivity and second highest in terms of ozone formation potential (OFP) among 66 measured VOCs.The ratios of isoprene to 1,3-butadiene, an exhaust tracer, were used to estimate the fractions of biogenic and anthropogenic isoprene in the urban area. The results reveal that the biogenic contribution apparently overwhelmed the anthropogenic contribution in summertime, although traffic in the city is heavy. Furthermore, the residual isoprene (mostly biogenic) after daytime photochemical loss persisted into the nighttime and contributed a large fraction to nighttime isoprene. In autumn, daytime isoprene was also predominantly from biogenic sources because the hot and sunny conditions persist into the autumn months. The high biogenic isoprene levels in subtropical urban settings and its coherence with OH diurnal cycles accentuate the significance of biogenic isoprene and its potentially great impact on atmospheric oxidant capacity, urban air quality, and even regional climate.

Emission ratios of anthropogenic volatile organic compounds in northern mid‐latitude megacities: Observations versus emission inventories in Los Angeles and Paris

Ground‐based and airborne volatile organic compound (VOC) measurements in Los Angeles, California, and Paris, France, during the Research at the Nexus of Air Quality and Climate Change (CalNex) and Megacities: Emissions, Urban, Regional and Global Atmospheric Pollution and Climate Effects, and Integrated Tools for Assessment and Mitigation (MEGAPOLI) campaigns, respectively, are used to examine the spatial variability of the composition of anthropogenic VOC urban emissions and to evaluate regional emission inventories. Two independent methods that take into account the effect of chemistry were used to determine the emission ratios of anthropogenic VOCs (including anthropogenic isoprene and oxygenated VOCs) over carbon monoxide (CO) and acetylene. Emission ratios from both methods agree within ±20%, showing the reliability of our approach. Emission ratios for alkenes, alkanes, and benzene are fairly similar between Los Angeles and Paris, whereas the emission ratios for C7–C9 aromatics in Paris are higher than in Los Angeles and other French and European Union urban areas by a factor of 2–3. The results suggest that the emissions of gasoline‐powered vehicles still dominate the hydrocarbon distribution in northern mid‐latitude urban areas, which disagrees with emission inventories. However, regional characteristics like the gasoline composition could affect the composition of hydrocarbon emissions. The observed emission ratios show large discrepancies by a factor of 2–4 (alkanes and oxygenated VOC) with the ones derived from four reference emission databases. A bias in CO emissions was also evident for both megacities. Nevertheless, the difference between measurements and inventory in terms of the overall OH reactivity is, in general, lower than 40%, and the potential to form secondary organic aerosols (SOA) agrees within 30% when considering volatile organic emissions as the main SOA precursors.

BIOGENIC VOLATILE ORGANIC COMPOUND (VOC) EMISSION ESTIMATES FROM AN URBAN TREE CANOPY

Biogenic emissions of the volatile organic compounds isoprene and monoterpenes (BVOCs) can contribute to tropospheric ozone and secondary particle formation and have indirect effects on climate change. While there are few studies of BVOC emissions from European towns and cities, several studies in North America indicate that the urban tree canopy may be a significant source of BVOC compounds, contributing to ozone and particle formation in the urban air-shed. Here, BVOC emissions from the U.K. West Midlands (UKWM) metropolitan area were estimated and compared with anthropogenic VOC emission estimates, and with BVOC emission estimates for other urban and U.K. regions. Monoterpene and isoprene emission potential estimates for the UKWM urban land-use classes spanned as much as two orders of magnitude, from 17–104 g·km–2·h–1 and from 42–1570 g·km–2·h–1, respectively. Isoprene emission potential estimates for the UKWM urban land classes (42–530 g·km–2·h–1) were of the same order of magnitude as isoprene emission measurements for U.K. gorse heathland in early summer (43 g·km–2·h–1), and up to approximately one order of magnitude lower than those measured from U.K. Sitka spruce forests in summer (658–6760 g·km–2·h–1). In the UKWM, the land class average of ΣBVOC emission estimates (∼75–165 kg·km–2·yr–1) were approximately two orders of magnitude lower than anthropogenic VOC emission estimates for the same area (70 000 kg·km–2·yr–1). Biogenic isoprene emission estimates from at least two of the UKWM urban land-use classes were equivalent to, or greater than, estimates of anthropogenic isoprene emissions. The extrapolation methodologies are critically discussed in the context of their uncertainties, and in the context of their generic potential.