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Despite declines in transportation emissions, urban North America and Europe still face unhealthy air pollution levels. This has challenged conventional understanding of the sources of their volatile organic compound (VOC) precursors. Using airborne flux measurements to map emissions of a wide range of VOCs, we demonstrate that biogenic terpenoid emissions contribute ~60% of emitted VOC OH reactivity, ozone, and secondary organic aerosol formation potential in summertime Los Angeles and that this contribution strongly increases with temperature. This implies that control of nitrogen oxides is key to reducing ozone formation in Los Angeles. We also show some anthropogenic VOC emissions increase with temperature, which is an effect not represented in current inventories. Air pollution mitigation efforts must consider that climate warming will strongly change emission amounts and composition. [ABSTRACT FROM AUTHOR]

A new instrument to measure total OH reactivity in ambient air based on the Comparative Reactivity Method (CRM) has been built and characterized at the Finnish Meteorological Institute in Helsinki, Finland. The system is based on the detection of pyrrole by a gas chromatograph with a photoionization detector and designed for long term studies. It was tested in a container close to the SMEAR III semi-urban station in Helsinki during the winter in February 2016. The sampling location next to the delivery area of the institute was influenced by local vehicle emissions and cannot be considered representative of background conditions in Helsinki. However, effects of nitrogen oxides on the measurements could be investigated there. During this campaign, 56 compounds were measured individually by 1) an in-situ gas chromatograph coupled to a mass spectrometer (GC/MS) and by 2) off-line sampling in canisters and on adsorbent filled cartridges taken at the container and subsequently analysed by GC-FID and liquid chromatography, respectively. In addition, nitrogen oxides were measured at the same location, while ozone, carbon monoxide and sulfur dioxide concentrations have been retrieved from the SMEAR III mast data. The comparison between the total OH reactivity measured and the OH reactivity derived from individual compound measurements are in better agreement for lower reactivity levels. Possible explanations for the differences are discussed in detail. [ABSTRACT FROM AUTHOR]

In the Amazon basin, high temperatures and drought unprecedented in records since 1900 occurred during the 2015/16 El Niño event. How tropical forests react to such extreme conditions in terms of volatile organic compound (VOC) emissions is of interest as the frequency of these events is predicted to increase through climate change. The diverse VOCs emitted in response to drought stress can be significant for both the plant-internal and global carbon budgets, they can influence ozone and particle production, and impact OH concentrations through their reactivity. Total OH reactivity is a directly measureable quantity that gives the reaction frequency of OH radicals with all reactive species in the atmosphere in s-1. Here we present a comparison of the OH reactivity diel cycle from November 2015, i.e. extreme drought and elevated temperatures associated with strong El Niño conditions, with the OH reactivity diel cycle from November 2012, a "normal", El Niño Southern Oscillation (ENSO)-neutral period. The diel maximum of OH reactivity during the El Niño event occurred at sunset, opposed to early afternoon under ENSO-neutral conditions. The absolute total diel OH reactivity, however, did not change significantly. Daytime OH reactivity averages were 24.3 ± 14.5 s-1 in 2012 and 24.6 ± 11.9 s-1 in 2015, respectively. Our findings suggest that a combination of stronger turbulent transport above the canopy with stress-related monoterpene and, possibly, other drought specific biogenic volatile organic compound (BVOC) emissions were responsible for the increased reactivity at sunset. [ABSTRACT FROM AUTHOR]

Proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS) is a technique commonly used to measure ambient volatile organic compounds (VOCs) in urban, rural, and remote environments. PTR-ToF-MS is known to produce artifacts from ion fragmentation, which complicates the interpretation and quantification of key atmospheric VOCs. This study evaluates the extent to which fragmentation and other ionization processes impact urban measurements of the PTR-ToF-MS ions typically assigned to isoprene (m/z 69, C 5 H 8 H +), acetaldehyde (m/z 45, CH 3 CHO +), and benzene (m/z 79, C 6 H 6 H +). Interferences from fragmentation are identified using gas chromatography (GC) pre-separation, and the impact of these interferences is quantified using ground-based and airborne measurements in a number of US cities, including Las Vegas, Los Angeles, New York City, and Detroit. In urban regions with low biogenic isoprene emissions (e.g., Las Vegas), fragmentation from higher-carbon aldehydes and cycloalkanes emitted from anthropogenic sources may contribute to m/z 69 by as much as 50 % during the day, while the majority of the signal at m/z 69 is attributed to fragmentation during the night. Interferences are a higher fraction of m/z 69 during airborne studies, which likely results from differences in the reactivity between isoprene and the interfering species along with the subsequent changes to the VOC mixture at higher altitudes. For other PTR masses, including m/z 45 and m/z 79, interferences are observed due to fragmentation and O 2+ ionization of VOCs typically used in solvents, which are becoming a more important source of anthropogenic VOCs in urban areas. We present methods to correct these interferences, which provide better agreement with GC measurements of isomer-specific molecules. These observations show the utility of deploying GC pre-separation for the interpretation PTR-ToF-MS spectra. [ABSTRACT FROM AUTHOR]

Proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS) is a technique commonly used to measure ambient volatile organic compounds (VOCs) in urban, rural, and remote environments. PTR-ToF-MS is known to produce artifacts from ion fragmentation, which complicates the interpretation and quantification of key atmospheric VOCs. This study evaluates the extent to which fragmentation and other ionization processes impacts urban measurements of the PTR-ToF-MS ions typically assigned to isoprene (m/z 69, C5H8H+), acetaldehyde (m/z 45, CH3CHO+), and benzene (m/z 79, C6H6H+). Interferences from fragmentation are identified using gas-chromatography (GC) pre-separation and the impact of these interferences are quantified using ground-based and airborne measurements in a number of US cities, including Las Vegas, Los Angeles, New York City, and Detroit. In urban regions with low biogenic isoprene emissions (e.g., Las Vegas), fragmentation from higher carbon aldehydes and cycloalkanes emitted from anthropogenic sources may contribute to m/z 69 by as much as 50% during the day, while the majority of the signal at m/z 69 is attributed to fragmentation during the night. Interferences are a higher fraction of m/z 69 during airborne studies, which likely results from differences in the reactivity between isoprene and the interfering species along with the subsequent changes to the VOC mixture at higher altitudes. For other PTR masses, including m/z 45 and m/z 79, interferences are observed due to the fragmentation and secondary ionization of VOCs typically used in solvents, which are becoming a more important source of anthropogenic VOCs in urban areas. We present methods to correct these interferences, which provide better agreement with GC measurements of isomer specific molecules. These observations show the utility of deploying GC pre-separation for the interpretation PTR-ToF-MS spectra. [ABSTRACT FROM AUTHOR]

Hydroxyl (OH) radical reactivity (kOH) has been measured for 18 years with different measurement techniques. In order to compare the performances of instruments deployed in the field, two campaigns were conducted performing experiments in the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich in October 2015 and April 2016. Chemical conditions were chosen either to be representative of the atmosphere or to test potential limitations of instruments. All types of instruments that are currently used for atmospheric measurements were used in one of the two campaigns. The results of these campaigns demonstrate that OH reactivity can be accurately measured for a wide range of atmospherically relevant chemical conditions (e.g. water vapour, nitrogen oxides, various organic compounds) by all instruments. The precision of the measurements (limit of detection<1 s-1 at a time resolution of 30 s to a few minutes) is higher for instruments directly detecting hydroxyl radicals, whereas the indirect comparative reactivity method (CRM) has a higher limit of detection of 2 s-1 at a time resolution of 10 to 15 min. The performances of the instruments were systematically tested by stepwise increasing, for example, the concentrations of carbon monoxide (CO), water vapour or nitric oxide (NO). In further experiments, mixtures of organic reactants were injected into the chamber to simulate urban and forested environments. Overall, the results show that the instruments are capable of measuring OH reactivity in the presence of CO, alkanes, alkenes and aromatic compounds. The transmission efficiency in Teflon inlet lines could have introduced systematic errors in measurements for low-volatile organic compounds in some instruments. CRM instruments exhibited a larger scatter in the data compared to the other instruments. The largest differences to reference measurements or to calculated reactivity were observed by CRM instruments in the presence of terpenes and oxygenated organic compounds (mixing ratio of OH reactants were up to 10 ppbv). In some of these experiments, only a small fraction of the reactivity is detected. The accuracy of CRM measurements is most likely limited by the corrections that need to be applied to account for known effects of, for example, deviations from pseudo first-order conditions, nitrogen oxides or water vapour on the measurement. Methods used to derive these corrections vary among the different CRM instruments. Measurements taken with a flowtube instrument combined with the direct detection of OH by chemical ionisation mass spectrometry (CIMS) show limitations in cases of high reactivity and high NO concentrations but were accurate for low reactivity (<15 s-1) and low NO (<5 ppbv) conditions. [ABSTRACT FROM AUTHOR]

Hydroxyl (OH) radical reactivity (kOH) has been measured for 18 years with different measurement techniques. In order to compare the performances of instruments deployed in the field, two campaigns were conducted performing experiments in the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich in October 2015 and April 2016. Chemical conditions were chosen either to be representative of the atmosphere or to test potential limitations of instruments. All types of instruments that are currently used for atmospheric measurements took part in one of the two campaigns. The results of these campaigns demonstrate that OH reactivity can be accurately measured for a wide range of atmospherically relevant chemical conditions (e.g. water vapor, nitrogen oxides, various organic compounds) by all instruments. The precision of the measurements (limit of detection < 1 s-1 at a time resolution of 30 seconds to a few minutes) is higher for instruments directly detecting hydroxyl radicals, whereas the indirect Comparative Reactivity Method (CRM) has a higher limit of detection of 2 s-1 at a time resolution of 10 to 15 min. The performances of the instruments were systematically tested by stepwise increasing, for example, the concentrations of carbon monoxide (CO), water vapor or nitric oxide (NO). In further experiments, mixtures of organic reactants were injected in the chamber to simulate urban and forested environments. Overall, the results show that instruments are capable of measuring OH reactivity in the presence of CO, alkanes, alkenes and aromatic compounds. The transmission efficiency in Teflon inlet lines could have introduced systematic errors in measurements for low-volatile organic compounds in some instruments. CRM instruments exhibited a larger scatter in the data compared to the other instruments. The largest differences to reference measurements or to calculated reactivity were observed by CRM instruments in the presence of terpenes and oxygenated organic compounds (mixing ratio of OH reactants were up to 10 ppbv). In some of these experiments, only a small fraction of the reactivity is detected. The accuracy of CRM measurements is most likely limited by the corrections that need to be applied in order to account for known effects of, for example, deviations from pseudo-first order conditions, nitrogen oxides or water vapor on the measurement. Methods to derive these corrections vary among the different CRM instruments. Measurements by a flow-tube instrument combined with the direct detection of OH by chemical ionization mass spectrometry (CIMS) show limitations in cases of high reactivity and high NO concentrations, but were accurate for low reactivity (< 15 s-1) and low NO (< 5 ppbv) conditions. [ABSTRACT FROM AUTHOR]

The pristine Amazon acts as a sink for ozone (O3), mainly due to deposition processes, which occur inside the canopy. Loss pathways are stomatal and surface deposition, reactions with terpenes and NOx. Sources of O3 are biomass burning especially during the drier season due to the emission of O3 precursors (VOC and NOx). O3 levels are expected to increase with ongoing deforestation: first, through release of O3 precursors from biomass burning and secondly by reduced deposition as forest canopies (esp. tropical forest), which efficiently remove O3, are replaced by grassland or crops. Apart from that, the entrainment of free tropospheric O3 contributes to boundary layer concentrations especially in the wet season.The ATTO (Amazon Tall Tower Observatory) site in the Central Amazon (02°08'38.8"S, 58°59'59.5"W), comprises a 325 meter and two 80 meter towers and acts as an ideal location to perform comprehensive long-term studies regarding forest-atmosphere interactions. During the wet months (350 mm precipitation in March), the air quality shows almost pristine conditions, whereas very strong pollution from biomass prevails in the drier season (ca. 80 mm precipitation in September). Since 2012 vertical mixing ratio profiles of H2O, CO2 and O3 have been continuously measured at multiple heights between 0.05 and 80 m, and since 2016 until 325 meters. Ozone fluxes have been determined by means of flux-gradient methods and eddy covariance.These unique long-term measurements allow detailed comparisons with previous campaigns in the Amazon basin. Monthly median O3 values show very good agreement for all measurements north of Manaus. In the south, where the influence of biomass burning is much stronger, values are much higher in the dryer season. Therefore, we use trajectory weighted fire counts together with proxy data (CO and BC) for studying the effects of biomass burning on the local O3 concentrations. For quantifying the different contributions, the boundary layer budget of O3 will be analyzed. [ABSTRACT FROM AUTHOR]

The Middle East has exceptional environmental qualities, with intense solar radiation, extensive deserts and water scarcity that promotes dust mobilisation. Furthermore, the region is subject to high levels of air pollution due to emissions from traffic and petrochemical industry. The AQABA (Air Quality and climate in the Arabian BAsin) field campaign took place in summer 2017 covering a ship track from Toulon to Kuwait through the Mediterranean, around the Arabian Peninsula and back. As one of the most reactive oxidants in the atmosphere, the hydroxyl radical (OH) is a driving force in air chemistry. It is produced photochemically by UV radiation and can react with most VOC emitted by biogenic or anthropogenic sources. Its close chemical relative, the hydroperoxyl radical (HO2) is a major source of tropospheric ozone. HOx (OH and HO2) play a role in the production of aerosols via the formation of low volatility compounds like organic acids. Production and loss of HOx are impacted by dust through its influence on photolysis frequencies and uptake of HOx precursors. Additionally, uptake of HOx oxidation products on dust particles increases their hydrophilicity and therefore may change chemical and physical surface properties.During the AQABA campaign, HOx was measured using laser-induced fluorescence (LIF) to investigate the regional influences on HOx concentrations and budgets. This study contrasts HOx concentrations and OH recycling between different environmental conditions along the ship track. This includes the Arabian Gulf at very high temperatures, with dust events, petrochemical industries and ship traffic, compared to the pristine environment over the Arabian Sea, and European dominated air masses over the Mediterranean Sea. The measurements will be compared to calculations obtained using the CAABA/MECCA box model. [ABSTRACT FROM AUTHOR]