Total OH Reactivity Measurements Research Articles

A Review of the Direct Measurement of Total OH Reactivity: Ambient Air and Vehicular Emission

A Review of the Direct Measurement of Total OH Reactivity: Ambient Air and Vehicular Emission

The human oxidation field.

Hydroxyl (OH) radicals are highly reactive species that can oxidize most pollutant gases. In this study, high concentrations of OH radicals were found when people were exposed to ozone in a climate-controlled chamber. OH concentrations calculated by two methods using measurements of total OH reactivity, speciated alkenes, and oxidation products were consistent with those obtained from a chemically explicit model. Key to establishing this human-induced oxidation field is 6-methyl-5-hepten-2-one (6-MHO), which forms when ozone reacts with the skin-oil squalene and subsequently generates OH efficiently through gas-phase reaction with ozone. A dynamic model was used to show the spatial extent of the human-generated OH oxidation field and its dependency on ozone influx through ventilation. This finding has implications for the oxidation, lifetime, and perception of chemicals indoors and, ultimately, human health.

Total OH Reactivity Measurements in a Suburban Site of Shanghai

AbstractTotal hydroxyl radicals (OH) reactivity, as a measure of the OH total loss rate and the significance of atmospheric oxidation, is broadly used to evaluate the OH budget in the atmosphere. Here, measurements of the total OH reactivity were performed at a suburban air quality monitoring supersite (Dianshan Lake) of Shanghai, between 1 August and 15 September, 2020, using the Comparative Reactivity Method. The measured total OH reactivity ranged from 13.9 to 107.4 s−1 with an average of 38.4 s−1. Meanwhile, the calculated total OH reactivity varied between 8.9 and 84.4 s−1 from concentrations of atmospheric trace gases measured by multiple in‐situ techniques including an online gas chromatograph equipped with mass spectrometry and flame ionization detection (GC‐MS/FID) and two proton‐transfer‐reaction time‐of‐flight mass spectrometers (PTR‐ToF‐MSs). A 12.1% missing OH reactivity was evident from the comparison between the measured and calculated total OH reactivities. Inorganic trace gases, methane, and 54 observed Photochemical Assessment Monitoring Station compounds detected by GC‐MS/FID, accounted for 25.7%, 1.3%, and 10%, respectively, of the calculated total OH reactivity. In addition, 292 species detected by PTR‐ToF‐MSs contributed the rest 63%. The top 10 VOC contributors to the measured total OH reactivity were C5H8, CH2O, C2H4O, C6H6O, C4H8O, C3H6O, C10H16, C5H10O, C5H8O2, and C8H10, respectively, whose total contribution reached 42%. Correlation analysis for the missing OH reactivity suggests that primary biogenic VOCs could be a potential source for the missing OH reactivity during the daytime, and that secondary oxidation products might also be connected to the missing OH reactivity.

Effect of Ozone, Clothing, Temperature, and Humidity on the Total OH Reactivity Emitted from Humans.

People influence indoor air chemistry through their chemical emissions via breath and skin. Previous studies showed that direct measurement of total OH reactivity of human emissions matched that calculated from parallel measurements of volatile organic compounds (VOCs) from breath, skin, and the whole body. In this study, we determined, with direct measurements from two independent groups of four adult volunteers, the effect of indoor temperature and humidity, clothing coverage (amount of exposed skin), and indoor ozone concentration on the total OH reactivity of gaseous human emissions. The results show that the measured concentrations of VOCs and ammonia adequately account for the measured total OH reactivity. The total OH reactivity of human emissions was primarily affected by ozone reactions with organic skin-oil constituents and increased with exposed skin surface, higher temperature, and higher humidity. Humans emitted a comparable total mixing ratio of VOCs and ammonia at elevated temperature-low humidity and elevated temperature-high humidity, with relatively low diversity in chemical classes. In contrast, the total OH reactivity increased with higher temperature and higher humidity, with a larger diversity in chemical classes compared to the total mixing ratio. Ozone present, carbonyl compounds were the dominant reactive compounds in all of the reported conditions.

Measurements of Total OH Reactivity During CalNex‐LA

AbstractTotal OH reactivity was measured during the California Research at the Nexus of Air Quality and Climate Change field campaign at the Pasadena ground site using a turbulent flow tube reactor with laser‐induced fluorescence detection of the OH radical. Collocated measurements of volatile organic compounds (VOCs), inorganic species, and meteorological parameters were made and used to calculate the total OH reactivity, which was then compared to the measured values. An analysis of the OH reactivity measurements finds that although the measured reactivity correlated well with the calculated reactivity, the measurements were consistently greater than the calculations for all times during the day, with an average missing OH reactivity of 8–10 s−1, accounting for approximately 40% of the measured total OH reactivity. An analysis of correlations with both anthropogenic tracers of combustion and oxygenated VOCs as well as air trajectories during the campaign suggest that the missing OH reactivity was likely due to a combination of both unmeasured local emissions and unmeasured oxidation products transported to the site. Approximately 50% of the missing OH reactivity may have been due to emissions of unmeasured volatile chemical products, such as pesticides, cleaning agents, and personal care products.

Long-term total OH reactivity measurements in a boreal forest

Abstract. Total hydroxyl radical (OH) reactivity measurements were conducted at the second Station for Measuring Ecosystem–Atmosphere Relations (SMEAR II), a boreal forest site located in Hyytiälä, Finland, from April to July 2016. The measured values were compared with OH reactivity calculated from a combination of data from the routine trace gas measurements (station mast) as well as online and offline analysis with a gas chromatographer coupled to a mass spectrometer (GC–MS) and offline liquid chromatography. Up to 104 compounds, mostly volatile organic compounds (VOCs) and oxidized VOCs, but also inorganic compounds, were included in the analysis, even though the data availability for each compound varied with time. The monthly averaged experimental total OH reactivity was found to be higher in April and May (ca. 20 s−1) than in June and July (7.6 and 15.4 s−1, respectively). The measured values varied much more in spring with high reactivity peaks in late afternoon, with values higher than in the summer, in particular when the soil was thawing. Total OH reactivity values generally followed the pattern of mixing ratios due to change of the boundary layer height. The missing reactivity fraction (defined as the difference between measured and calculated OH reactivity) was found to be high. Several reasons that can explain the missing reactivity are discussed in detail such as (1) missing measurements due to technical issues, (2) not measuring oxidation compounds of detected biogenic VOCs, and (3) missing important reactive compounds or classes of compounds with the available measurements. In order to test the second hypothesis, a one-dimensional chemical transport model (SOSAA) has been used to estimate the amount of unmeasured oxidation products and their expected contribution to the reactivity for three different short periods in April, May, and July. However, only a small fraction (<4.5 %) of the missing reactivity can be explained by modelled secondary compounds (mostly oxidized VOCs). These findings indicate that compounds measured but not included in the model as well as unmeasured primary emissions contribute the missing reactivity. In the future, non-hydrocarbon compounds from sources other than vegetation (e.g. soil) should be included in OH reactivity studies.

Shipborne measurements of total OH reactivity around the Arabian Peninsula and its role in ozone chemistry

Abstract. The Arabian Peninsula is characterized by high and increasing levels of photochemical air pollution. Strong solar irradiation, high temperatures and large anthropogenic emissions of reactive trace gases result in intense photochemical activity, especially during the summer months. However, air chemistry measurements in the region are scarce. In order to assess regional pollution sources and oxidation rates, the first ship-based direct measurements of total OH reactivity were performed in summer 2017 from a vessel traveling around the peninsula during the AQABA (Air Quality and Climate Change in the Arabian Basin) campaign. Total OH reactivity is the total loss frequency of OH radicals due to all reactive compounds present in air and defines the local lifetime of OH, the most important oxidant in the troposphere. During the AQABA campaign, the total OH reactivity ranged from below the detection limit (5.4 s−1) over the northwestern Indian Ocean (Arabian Sea) to a maximum of 32.8±9.6 s−1 over the Arabian Gulf (also known as Persian Gulf) when air originated from large petroleum extraction/processing facilities in Iraq and Kuwait. In the polluted marine regions, OH reactivity was broadly comparable to highly populated urban centers in intensity and composition. The permanent influence of heavy maritime traffic over the seaways of the Red Sea, Gulf of Aden and Gulf of Oman resulted in median OH sinks of 7.9–8.5 s−1. Due to the rapid oxidation of direct volatile organic compound (VOC) emissions, oxygenated volatile organic compounds (OVOCs) were observed to be the main contributor to OH reactivity around the Arabian Peninsula (9 %–35 % by region). Over the Arabian Gulf, alkanes and alkenes from the petroleum extraction and processing industry were an important OH sink with ∼9 % of total OH reactivity each, whereas NOx and aromatic hydrocarbons (∼10 % each) played a larger role in the Suez Canal, which is influenced more by ship traffic and urban emissions. We investigated the number and identity of chemical species necessary to explain the total OH sink. Taking into account ∼100 individually measured chemical species, the observed total OH reactivity can typically be accounted for within the measurement uncertainty (50 %), with 10 dominant trace gases accounting for 20 %–39 % of regional total OH reactivity. The chemical regimes causing the intense ozone pollution around the Arabian Peninsula were investigated using total OH reactivity measurements. Ozone vs. OH reactivity relationships were found to be a useful tool for differentiating between ozone titration in fresh emissions and photochemically aged air masses. Our results show that the ratio of NOx- and VOC-attributed OH reactivity was favorable for ozone formation almost all around the Arabian Peninsula, which is due to NOx and VOCs from ship exhausts and, often, oil/gas production. Therewith, total OH reactivity measurements help to elucidate the chemical processes underlying the extreme tropospheric ozone concentrations observed in summer over the Arabian Basin.

Total OH reactivity measurements for the OH-initiated oxidation of aromatic hydrocarbons in the presence of NOx

The total OH reactivity of the secondary products formed from the OH-initiated oxidation of toluene, p-xylene, and 1,3,5-trimethylbenzene was directly measured in the presence of NOx using a laboratory environmental chamber in order to investigate unidentified reactive species in urban air. The total OH reactivity was also calculated from the concentrations of the reactants and products, which were monitored by Fourier-transform infrared spectroscopy. The difference between the measured and calculated OH reactivity, the so-called missing OH reactivity, comprised 58–81% of the total OH reactivity of the secondary products. These results suggest that the secondary products formed from the oxidation of aromatic hydrocarbons may be important candidates in accounting for the missing OH reactivity in the analyses of urban environments. The Master Chemical Mechanism (MCM) calculations were performed to predict the temporal variation in the total OH reactivity for the oxidation of aromatic hydrocarbons. The MCM calculations successfully reproduced the observed total OH reactivity when the particle and semi-volatile product concentrations were negligibly low. The MCM calculations were used to identify the missing secondary products. The results suggest that important components of the missing OH reactivity are unsaturated multifunctional products such as unsaturated dicarbonyls, unsaturated epoxydicarbonyls, and furanones.

How the OH reactivity affects the ozone production efficiency: case studies in Beijing and Heshan, China

Abstract. Total OH reactivity measurements were conducted on the Peking University campus (Beijing) in August 2013 and in Heshan (Guangdong province) from October to November 2014. The daily median OH reactivity was 20 ± 11 s−1 in Beijing and 31 ± 20 s−1 in Heshan, respectively. The data in Beijing showed a distinct diurnal pattern with the maxima over 27 s−1 in the early morning and minima below 16 s−1 in the afternoon. The diurnal pattern in Heshan was not as evident as in Beijing. Missing reactivity, defined as the difference between measured and calculated OH reactivity, was observed at both sites, with 21 % missing reactivity in Beijing and 32 % missing reactivity in Heshan. Unmeasured primary species, such as branched alkenes, could contribute to missing reactivity in Beijing, especially during morning rush hours. An observation-based model with the RACM2 (Regional Atmospheric Chemical Mechanism version 2) was used to understand the daytime missing reactivity in Beijing by adding unmeasured oxygenated volatile organic compounds and simulated intermediates of the degradation from primary volatile organic compounds (VOCs). However, the model could not find a convincing explanation for the missing reactivity in Heshan, where the ambient air was found to be more aged, and the missing reactivity was presumably attributed to oxidized species, such as unmeasured aldehydes, acids and dicarbonyls. The ozone production efficiency was 21 % higher in Beijing and 30 % higher in Heshan when the model was constrained by the measured reactivity, compared to the calculations with measured and modeled species included, indicating the importance of quantifying the OH reactivity for better understanding ozone chemistry.

What effect does VOC sampling time have on derived OH reactivity?

Abstract. State-of-the-art techniques allow for rapid measurements of total OH reactivity. Unknown sinks of OH and oxidation processes in the atmosphere have been attributed to what has been termed “missing” OH reactivity. Often overlooked are the differences in timescales over which the diverse measurement techniques operate. Volatile organic compounds (VOCs) acting as sinks of OH are often measured by gas chromatography (GC) methods which provide low-frequency measurements on a timescale of hours, while sampling times are generally only a few minutes. Here, the effect of the sampling time and thus the contribution of unmeasured VOC variability on OH reactivity is investigated. Measurements of VOC mixing ratios by proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) conducted during two field campaigns (ClearfLo and PARADE) in an urban and a semi-rural environment were used to calculate OH reactivity. VOCs were selected to represent variability for different compound classes. Data were averaged over different time intervals to simulate lower time resolutions and were then compared to the mean hourly OH reactivity. The results show deviations in the range of 1 to 25 %. The observed impact of VOC variability is found to be greater for the semi-rural site.The selected compounds were scaled by the contribution of their compound class to the total OH reactivity from VOCs based on concurrent gas chromatography measurements conducted during the ClearfLo campaign. Prior to being scaled, the variable signal of aromatic compounds results in larger deviations in OH reactivity for short sampling intervals compared to oxygenated VOCs (OVOCs). However, once scaled with their lower share during the ClearfLo campaign, this effect was reduced. No seasonal effect on the OH reactivity distribution across different VOCs was observed at the urban site.

Intercomparison of the comparative reactivity method (CRM) and pump–probe technique for measuring total OH reactivity in an urban environment

Abstract. The investigation of hydroxyl radical (OH) chemistry during intensive field campaigns has led to the development of several techniques dedicated to ambient measurements of total OH reactivity, which is the inverse of the OH lifetime. Three techniques are currently used during field campaigns, including the total OH loss rate method, the pump–probe method, and the comparative reactivity method. However, no formal intercomparison of these techniques has been published so far, and there is a need to ensure that measurements of total OH reactivity are consistent among the different techniques. An intercomparison of two OH reactivity instruments, one based on the comparative reactivity method (CRM) and the other based on the pump–probe method, was performed in October 2012 in a NOx-rich environment, which is known to be challenging for the CRM technique. This study presents an extensive description of the two instruments, the CRM instrument from Mines Douai (MD-CRM) and the pump–probe instrument from the University of Lille (UL-FAGE), and highlights instrumental issues associated with the two techniques. It was found that the CRM instrument used in this study underestimates ambient OH reactivity by approximately 20 % due to the photolysis of volatile organic compounds (VOCs) inside the sampling reactor; this value is dependent on the position of the lamp within the reactor. However, this issue can easily be fixed, and the photolysis of VOCs was successfully reduced to a negligible level after this intercomparison campaign. The UL-FAGE instrument may also underestimate ambient OH reactivity due to the difficulty to accurately measure the instrumental zero. It was found that the measurements are likely biased by approximately 2 s-1, due to impurities in humid zero air. Two weeks of ambient sampling indicate that the measurements performed by the two OH reactivity instruments are in agreement, within the measurement uncertainties for each instrument, for NOx mixing ratios up to 100 ppbv. The CRM technique has hitherto mainly been used in low-NOx environments, i.e. environments with ambient NOx mixing ratios lower than a few ppbv, due to a measurement artifact generated by ambient NO inside the sampling reactor. However, this study shows that this technique can also be used under NOx-rich conditions if a NOx-dependent correction is carefully applied on the OH reactivity measurements. A full suite of 52 VOCs, NOx, and other inorganic species were monitored during this intercomparison. An investigation of the OH reactivity budget for this urban site suggests that this suite of trace gases can account for the measured total OH reactivity.

Detailed characterizations of the new Mines Douai comparative reactivity method instrument via laboratory experiments and modeling

Abstract. The hydroxyl (OH) radical is an important oxidant in the troposphere, which controls the lifetime of most air quality- and climate-related trace gases. However, there are still uncertainties concerning its atmospheric budget, and integrated measurements of OH sinks have been valuable to improve this aspect. Among the analytical tools used for measuring total OH reactivity in ambient air, the comparative reactivity method (CRM) is spreading rapidly in the atmospheric community. However, measurement artifacts have been highlighted for this technique, and additional work is needed to fully characterize them. In this study, we present the new Mines Douai CRM instrument, with an emphasis on the corrections that need to be applied to ambient measurements of total OH reactivity. Measurement artifacts identified in the literature have been investigated, including (1) a correction for a change in relative humidity between the measurement steps leading to different OH levels, (2) the formation of spurious OH in the sampling reactor when hydroperoxy radicals (HO2) react with nitrogen monoxide (NO), (3) not operating the CRM under pseudo-first-order kinetics, and (4) the dilution of ambient air inside the reactor. The dependences of these artifacts on various measurable parameters, such as the pyrrole-to-OH ratio and the bimolecular reaction rate constants of ambient trace gases with OH, have also been studied. Based on these observations, parameterizations are proposed to correct ambient OH reactivity measurements. On average, corrections of 5.2 ± 3.2, 9.2 ± 15.7, and 8.5 ± 5.8 s−1 were respectively observed for (1), (2) and (3) during a field campaign performed in Dunkirk, France (summer 2014). Numerical simulations have been performed using a box model to check whether experimental observations mentioned above are consistent with our understanding of the chemistry occurring in the CRM reactor. Two different chemical mechanisms have been shown to reproduce the magnitude of corrections (2) and (3). In addition, these simulations reproduce their dependences on the pyrrole-to-OH ratio and on bimolecular reaction rate constants of gases reacting with OH. The good agreement found between laboratory experiments and model simulations gives us confidence in the proposed parameterizations. However, it is worth noting that the numerical values given in this study are suitable for the Mines Douai instrument and may not be appropriate for other CRM instruments. It is recommended that each group characterize its own instrument following the recommendations given in this study. An assessment of performances for the Mines Douai instrument, including a propagation of errors from the different corrections, indicates a limit of detection of 3.0 s−1 and total uncertainties of 17–25 % for OH reactivity values higher than 15 s−1 and NOx mixing ratios lower than 30 ppbv.

VOC–OHM: A new technique for rapid measurements of ambient total OH reactivity and volatile organic compounds using a single proton transfer reaction mass spectrometer

Measurements of total hydroxyl radical (OH) reactivity and volatile organic compounds (VOC) are necessary for improving our understanding of reactive emissions and atmospheric oxidation in air pollution and atmospheric chemistry studies. Proton transfer reaction mass spectrometers (PTR-MS) can measure ambient VOCs and the total ambient OH reactivity. However, till date this has always required deployment of two PTR-MS instruments, wherein one instrument measures ambient VOCs and the other instrument measures the total OH reactivity using the comparative reactivity method (CRM). Due to material (e.g. power, space) or financial constraints, deploying two PTR-MS instruments is not always possible and yet it is desirable to quantify both VOCs and OH reactivity. Here, we present a novel hyphenated technique christened VOC-OHM (for Volatile Organic Compounds–OH reactivity Measurement) that enables rapid ambient measurements of both VOCs and total OH reactivity using a single PTR-MS. The technique can provide more specificity for identification of compounds using a PTR-QMS through an estimate of the rate coefficient of the major isobaric contributor with the hydroxyl radical as shown in the case of m/z=69 for isoprene and furan, which are nominal isobars but have rate coefficients that differ by one order of magnitude. It also demonstrates a new safer and portable substitute for pressurized zero air bottles that have been required thus far in CRM OH reactivity deployments. VOC–OHM successfully couples the typical VOC and CRM experimental set ups without undermining the PTR-MS's ability to measure either parameter. The design of the VOC–OHM system, its validation, optimization and results of field tests are described in detail. The VOC–OHM system measures the ambient VOCs and OH reactivity every hour for ∼20min durations each, with an ambient data gap of ∼13min in between. Thus rapid temporal changes in the ambient chemical composition and reactivity are easily quantified. The sampling periods and VOC speciation achieved using VOC–OHM can be customized depending on user preferences, providing more options for the majority of users possessing a single PTR-MS.

Measurements of total hydroxyl radical reactivity during CABINEX 2009 – Part 1: field measurements

Abstract. Total hydroxyl radical (OH) reactivity was measured at the PROPHET (Program for Research on Oxidants: PHotochemistry, Emissions, and Transport) forested field site in northern Michigan during the 2009 Community Atmosphere–Biosphere INteraction EXperiment (CABINEX). OH reactivity measurements were made with a turbulent-flow reactor instrument at three heights from the forest floor above (21 and 31 m) and below (6 m) the canopy at three different time periods during the CABINEX campaign. In addition to total OH reactivity measurements, collocated measurements of volatile organic compounds (VOCs), inorganic species, and ambient temperature were made at the different heights. These ancillary measurements were used to calculate the total OH reactivity, which was then compared to the measured values. Discrepancies between the measured and calculated OH reactivity, on the order of 1–24 s−1, were observed during the daytime above the canopy at the 21 and 31 m heights, as previously reported for this site. The measured OH reactivity below the canopy during the daytime was generally lower than that observed above the canopy. Closer analysis of the measurements of OH reactivity and trace gases suggests that the missing OH reactivity could come from oxidation products of VOCs. These results suggest that additional unmeasured trace gases, likely oxidation products, are needed to fully account for the OH reactivity measured during CABINEX.

Total OH reactivity measurements in ambient air in a southern Rocky mountain ponderosa pine forest during BEACHON-SRM08 summer campaign

Total OH reactivity was measured during the Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics and Nitrogen-Southern Rocky Mountain 2008 field campaign (BEACHON-SRM08) held at Manitou Experimental Forest (MEF) in Colorado USA in August, 2008. The averaged total OH reactivity was 6.7 s−1, smaller than that measured in urban (33.4 s−1, Yoshino et al., 2012) and suburban (27.7 s−1, Yoshino et al., 2006) areas in Tokyo in the same season, while sporadically high OH reactivity was also observed during some evenings. The total OH reactivity measurements were accompanied by observations of traces species such as CO, NO, NOy, O3 and SO2 and Volatile Organic Compounds (VOCs). From the calculation of OH reactivity based on the analysis of these trace species, 46.3% of OH reactivity for VOCs came from biogenic species that are dominated by 2-methyl-3-buten-2-ol (MBO), and monoterpenes. MBO was the most prominent contribution to OH reactivity of all trace species. A comparison of observed and calculated OH reactivity shows that the calculated OH reactivity is 29.5% less than the observed value, implying the existence of missing OH sinks. One of the candidates of missing OH is thought to be the oxidation products of biogenic species.

Seasonal measurements of total OH reactivity emission rates from Norway spruce in 2011

Abstract. Numerous reactive volatile organic compounds (VOCs) are emitted into the atmosphere by vegetation. Most biogenic VOCs are highly reactive towards the atmosphere's most important oxidant, the hydroxyl (OH) radical. One way to investigate the chemical interplay between biosphere and atmosphere is through the measurement of total OH reactivity, the total loss rate of OH radicals. This study presents the first determination of total OH reactivity emission rates (measurements via the comparative reactivity method) based on a branch cuvette enclosure system mounted on a Norway spruce (Picea abies) throughout spring, summer and autumn 2011. In parallel VOC emission rates were monitored by a second proton-transfer-reaction mass spectrometer (PTR-MS), and total ozone (O3) loss rates were obtained inside the cuvette. Total OH reactivity emission rates were in general temperature and light dependent, showing strong diel cycles with highest values during daytime. Monoterpene emissions contributed most, accounting for 56–69% of the measured total OH reactivity flux in spring and early summer. However, during late summer and autumn the monoterpene contribution decreased to 11–16%. At this time, a large missing fraction of the total OH reactivity emission rate (70–84%) was found when compared to the VOC budget measured by PTR-MS. Total OH reactivity and missing total OH reactivity emission rates reached maximum values in late summer corresponding to the period of highest temperature. Total O3 loss rates within the closed cuvette showed similar diel profiles and comparable seasonality to the total OH reactivity fluxes. Total OH reactivity fluxes were also compared to emissions from needle storage pools predicted by a temperature-only-dependent algorithm. Deviations of total OH reactivity fluxes from the temperature-only-dependent emission algorithm were observed for occasions of mechanical and heat stress. While for mechanical stress, induced by strong wind, measured VOCs could explain total OH reactivity emissions, during heat stress they could not. The temperature-driven algorithm matched the diel variation of total OH reactivity emission rates much better in spring than in summer, indicating a different production and emission scheme for summer and early autumn. During these times, unmeasured and possibly unknown primary biogenic emissions contributed significantly to the observed total OH reactivity flux.

Total OH reactivity measurements using a new fast Gas Chromatographic Photo-Ionization Detector (GC-PID)

Abstract. The primary and most important oxidant in the atmosphere is the hydroxyl radical (OH). Currently OH sinks, particularly gas phase reactions, are poorly constrained. One way to characterize the overall sink of OH is to measure directly the ambient loss rate of OH, the total OH reactivity. To date, direct measurements of total OH reactivity have been either performed using a Laser-Induced Fluorescence (LIF) system ("pump-and-probe" or "flow reactor") or the Comparative Reactivity Method (CRM) with a Proton-Transfer-Reaction Mass Spectrometer (PTR-MS). Both techniques require large, complex and expensive detection systems. This study presents a feasibility assessment for CRM total OH reactivity measurements using a new detector, a Gas Chromatographic Photoionization Detector (GC-PID). Such a system is smaller, more portable, less power consuming and less expensive than other total OH reactivity measurement techniques. Total OH reactivity is measured by the CRM using a competitive reaction between a reagent (here pyrrole) with OH alone and in the presence of atmospheric reactive molecules. The new CRM method for total OH reactivity has been tested with parallel measurements of the GC-PID and the previously validated PTR-MS as detector for the reagent pyrrole during laboratory experiments, plant chamber and boreal field studies. Excellent agreement of both detectors was found when the GC-PID was operated under optimum conditions. Time resolution (60–70 s), sensitivity (LOD 3–6 s−1) and overall uncertainty (25% in optimum conditions) for total OH reactivity were similar to PTR-MS based total OH reactivity measurements. One drawback of the GC-PID system was the steady loss of sensitivity and accuracy during intensive measurements lasting several weeks, and a possible toluene interference. Generally, the GC-PID system has been shown to produce closely comparable results to the PTR-MS and thus in suitable environments (e.g. forests) it presents a viably economical alternative for groups interested in total OH reactivity observations.

Total OH reactivity measurements in Paris during the 2010 MEGAPOLI winter campaign

Abstract. Hydroxyl radicals play a central role in the troposphere as they control the lifetime of many trace gases. Measurement of OH reactivity (OH loss rate) is important to better constrain the OH budget and also to evaluate the completeness of measured VOC budget. Total atmospheric OH reactivity was measured for the first time in an European Megacity: Paris and its surrounding areas with 12 million inhabitants, during the MEGAPOLI winter campaign 2010. The method deployed was the Comparative Reactivity Method (CRM). The measured dataset contains both measured and calculated OH reactivity from CO, NOx and VOCs measured via PTR-MS, GC-FID and GC-MS instruments. The reactivities observed in Paris covered a range from 10 s−1 to 130 s−1, indicating a large loading of chemical reactants. The present study showed that, when clean marine air masses influenced Paris, the purely local OH reactivity (20 s−1) is well explained by the measured species. Nevertheless, when there is a continental import of air masses, high levels of OH reactivity were obtained (120–130 s−1) and the missing OH reactivity measured in this case jumped to 75%. Using covariations of the missing OH reactivity to secondary inorganic species in fine aerosols, we suggest that the missing OH reactants were most likely highly oxidized compounds issued from photochemically processed air masses of anthropogenic origin.

Summertime total OH reactivity measurements from boreal forest during HUMPPA-COPEC 2010

Abstract. Ambient total OH reactivity was measured at the Finnish boreal forest station SMEAR II in Hyytiälä (Latitude 61°51' N; Longitude 24°17' E) in July and August 2010 using the Comparative Reactivity Method (CRM). The CRM – total OH reactivity method – is a direct, in-situ determination of the total loss rate of hydroxyl radicals (OH) caused by all reactive species in air. During the intensive field campaign HUMPPA-COPEC 2010 (Hyytiälä United Measurements of Photochemistry and Particles in Air – Comprehensive Organic Precursor Emission and Concentration study) the total OH reactivity was monitored both inside (18 m) and directly above the forest canopy (24 m) for the first time. The comparison between these two total OH reactivity measurements, absolute values and the temporal variation have been analyzed here. Stable boundary layer conditions during night and turbulent mixing in the daytime induced low and high short-term variability, respectively. The impact on total OH reactivity from biogenic emissions and associated photochemical products was measured under "normal" and "stressed" (i.e. prolonged high temperature) conditions. The advection of biomass burning emissions to the site caused a marked change in the total OH reactivity vertical profile. By comparing the OH reactivity contribution from individually measured compounds and the directly measured total OH reactivity, the size of any unaccounted for or "missing" sink can be deduced for various atmospheric influences. For "normal" boreal conditions a missing OH reactivity of 58%, whereas for "stressed" boreal conditions a missing OH reactivity of 89% was determined. Various sources of not quantified OH reactive species are proposed as possible explanation for the high missing OH reactivity.

Total OH reactivity measurements in laboratory studies of the photooxidation of isoprene

The hydroxyl radical (OH) is the principal oxidizing agent in Earth's troposphere. Measurements of the total OH reactivity, defined as the reciprocal of the lifetime of OH, have been employed in field studies to estimate the entire amount of reactive trace gases in the atmosphere. Here, the OH reactivity technique has been employed in laboratory studies of the OH initiated atmospheric oxidation of the important biogenic volatile organic compound, isoprene. A significant discrepancy was observed between measured and calculated OH reactivity, indicating that OH was reacting with some oxidation products not measured in the experiment. The “missing” OH reactivity gradually increased as the oxidation progressed and contributed up to 40% of the total OH reactivity. In the presence of NO, the “missing” OH reactivity is likely due to a number of first-generation products produced in low yields, while in the absence of NO, alternative pathways that produce less reactive products are also occurring.This work demonstrates that OH reactivity measurements can be successfully incorporated into atmospheric simulation chamber studies of VOC degradation. The OH reactivity technique has the potential to be a powerful tool for elucidating finer details of VOC oxidation.