Peroxy Radical Concentrations Research Articles

Atmospheric Peroxy Radical Measurements Using Dual-Channel Chemical Amplification Cavity Ringdown Spectroscopy

Peroxy (HO2 and RO2) radicals are important intermediates in tropospheric oxidation of hydrocarbons, and their accurate atmospheric measurements remain challenging. In this work, the peroxy radical chemical amplification (PERCA) method was combined with cavity ringdown spectroscopy (CRDS) to develop a dual-channel instrument for measurements of atmospheric peroxy radicals. In the amplification channel, the peroxy radicals were converted in an excess amount of NO and CO into a higher level of NO2 and measured along with the background NO2, while in the reference channel, only the background NO2 (ambient NO2 and NO2 converted from O3 reaction with NO) was monitored. The NO2 levels from both channels were measured simultaneously at a high time resolution (~1 s) using two identical CRDS systems with one 408.5-nm diode laser, and their difference gave the amplified NO2 from PERCA. The peroxy radical concentration was obtained from the amplified NO2 and the calibrated amplification factor or chain length (CL). The optimized CL was 190 ± 20 (1σ) using laboratory-generated HO2 and CH3O2 radical sources. The detection sensitivity was 4 ppt/10 s (3σ). Ambient measurements in Riverside, CA were carried out. This dual-channel diode-laser PERCA-CRDS instrument was compact and capable of providing real-time, in situ, and sensitive measurements of atmospheric peroxy radicals with fast time response.

Peroxy radical detection for airborne atmospheric measurements using absorption spectroscopy of NO<sub>2</sub>

Abstract. Development of an airborne instrument for the determination of peroxy radicals (PeRCEAS – peroxy radical chemical enhancement and absorption spectroscopy) is reported. Ambient peroxy radicals (HO2 and RO2, R being an organic chain) are converted to NO2 in a reactor using a chain reaction involving NO and CO. Provided that the amplification factor, called effective chain length (eCL), is known, the concentration of NO2 can be used as a proxy for the peroxy radical concentration in the sampled air. The eCL depends on radical surface losses and must thus be determined experimentally for each individual setup. NO2 is detected by continuous-wave cavity ring-down spectroscopy (cw-CRDS) using an extended cavity diode laser (ECDL) at 408.9 nm. Optical feedback from a V-shaped resonator maximizes transmission and allows for a simple detector setup. CRDS directly yields absorption coefficients, thus providing NO2 concentrations without additional calibration. The optimum 1σ detection limit is 0.3 ppbv at an averaging time of 40 s and an inlet pressure of 300 hPa. Effective chain lengths were determined for HO2 and CH3O2 at different inlet pressures. The 1σ detection limit at an inlet pressure of 300 hPa for HO2 is 3 pptv for an averaging time of 120 s.

Suppression of new particle formation from monoterpene oxidation by NO<sub>x</sub>

Abstract. The impact of nitrogen oxides (NOx = NO + NO2) on new particle formation (NPF) and on photochemical ozone production from real plant volatile organic compound (BVOC) emissions was studied in a laboratory setup. At high NOx conditions ([BVOC] / [NOx] < 7, [NOx] > 23 ppb) new particle formation was suppressed. Instead, photochemical ozone formation was observed resulting in higher hydroxyl radical (OH) and lower nitrogen monoxide (NO) concentrations. When [NO] was reduced back to levels below 1 ppb by OH reactions, NPF was observed. Adding high amounts of NOx caused NPF to be slowed by orders of magnitude compared to analogous experiments at low NOx conditions ([NOx] ~300 ppt), although OH concentrations were higher. Varying NO2 photolysis enabled showing that NO was responsible for suppression of NPF. This suggests that peroxy radicals are involved in NPF. The rates of NPF and photochemical ozone production were related by power law dependence with an exponent approaching −2. This exponent indicated that the overall peroxy radical concentration must have been similar when NPF occurred. Thus, permutation reactions of first-generation peroxy radicals cannot be the rate limiting step in NPF from monoterpene oxidation. It was concluded that permutation reactions of higher generation peroxy-radical-like intermediates limit the rate of new particle formation. In contrast to the strong effects on the particle numbers, the formation of particle mass was substantially less sensitive to NOx concentrations. If at all, yields were reduced by about an order of magnitude only at very high NOx concentrations.

Direct Kinetic Study of OH and O3 Formation in the Reaction of CH3C(O)O2 with HO2

The reaction between HO2 and CH3C(O)O2 has three exothermic product channels, forming OH (R3a), peracetic acid (R3b), and acetic acid plus O3 (R3c). The branching ratios of the OH- and ozone-forming reaction channels were determined using a combination of laser-induced fluorescence (LIF, for time-resolved OH concentration measurement) and transient absorption spectroscopy (TAS, for time-resolved O3 concentration measurement) following pulsed laser generation of HO2 and CH3C(O)O2 from suitable precursors. TAS was also used to determine the initial concentration of the reactant peroxy radicals. The data were evaluated by numerical simulation using kinetic models of the measured concentration profiles; a Monte Carlo approach was used to estimate the uncertainties of the rate constants (k3) and branching ratios (α) thus obtained. The reaction channel forming OH (R3a) was found to be the most important with α3a = 0.61 ± 0.09 and α3c = 0.16 ± 0.08. The overall rate coefficient of the title reaction was found to be k3 = (2.1 ± 0.4) × 10(-11) cm(3) molecule(-1) s(-1) for both HO2 and DO2. Use of DO2 resulted in an increase in α3a to 0.80 ± 0.14. Comparison with former studies shows that OH formation via (R3) has been underestimated significantly to date. Possible reasons for these discrepancies and atmospheric implications are discussed.

OH and HO<sub>2</sub> radical chemistry during PROPHET 2008 and CABINEX 2009 – Part 1: Measurements and model comparison

Abstract. Hydroxyl (OH) and hydroperoxyl (HO2) radicals are key species driving the oxidation of volatile organic compounds that can lead to the production of ozone and secondary organic aerosols. Previous measurements of these radicals in forest environments with high isoprene, low NOx conditions have shown serious discrepancies with modeled concentrations, bringing into question the current understanding of isoprene oxidation chemistry in these environments. During the summers of 2008 and 2009, OH and peroxy radical concentrations were measured using a laser-induced fluorescence instrument as part of the PROPHET (Program for Research on Oxidants: PHotochemistry, Emissions, and Transport) and CABINEX (Community Atmosphere-Biosphere INteractions EXperiment) campaigns at a forested site in northern Michigan. Supporting measurements of photolysis rates, volatile organic compounds, NOx (NO + NO2 and other inorganic species were used to constrain a zero-dimensional box model based on the Regional Atmospheric Chemistry Mechanism, modified to include the Mainz Isoprene Mechanism (RACM-MIM). The CABINEX model OH predictions were in good agreement with the measured OH concentrations, with an observed-to-modeled ratio near one (0.70 ± 0.31) for isoprene mixing ratios between 1–2 ppb on average. The measured peroxy radical concentrations, reflecting the sum of HO2 and isoprene-based peroxy radicals, were generally lower than predicted by the box model in both years.

The diurnal variability of atmospheric nitrogen oxides (NO and NO<sub>2</sub>) above the Antarctic Plateau driven by atmospheric stability and snow emissions

Abstract. Atmospheric nitrogen oxides (NO and NO2) were observed at Dome C, East Antarctica (75.1° S, 123.3° E, 3233 m), for a total of 50 days, from 10 December 2009 to 28 January 2010. Average (±1σ) mixing ratios at 1.0 m of NO and NO2, the latter measured for the first time on the East Antarctic Plateau, were 111 (±89) and 98 (±89) pptv, respectively. Atmospheric mixing ratios are on average comparable to those observed previously at South Pole, but in contrast show strong diurnal variability: a minimum around local noon and a maximum in the early evening coincide with the development and collapse of a convective boundary layer. The asymmetric diurnal cycle of NOx concentrations and likely any other chemical tracer with a photolytic surface source is driven by the turbulent diffusivity and height of the atmospheric boundary layer, with the former controlling the magnitude of the vertical flux and the latter the size of the volume into which snow emissions are transported. In particular, the average (±1σ) NOx emission flux from 22 December 2009 to 28 January 2010, estimated from atmospheric concentration gradients, was 8.2 (±7.4) × 1012 molecule m−2 s−1 belongs to the largest values measured so far in the polar regions and explains the 3-fold increase in mixing ratios in the early evening when the boundary layer becomes very shallow. Dome C is likely not representative for the entire East Antarctic Plateau but illustrates the need of an accurate description of the boundary layer above snow in atmospheric chemistry models. A simple nitrate photolysis model matches the observed median diurnal NOx flux during the day but has significant low bias during the night. The difference is significant taking into account the total random error in flux observations and model uncertainties due to the variability of NO3− concentrations in snow and potential contributions from NO2− photolysis. This highlights uncertainties in the parameterization of the photolytic NOx source in natural snowpacks, such as the poorly constrained quantum yield of nitrate photolysis. A steady-state analysis of the NO2 : NO ratios indicates that peroxy (HO2 + RO2) or other radical concentrations in the boundary layer of Dome C are either higher than measured elsewhere in the polar regions or other processes leading to enhanced NO2 have to be invoked. These results confirm the existence of a strongly oxidising canopy enveloping the East Antarctic Plateau in summer.

The intrinsic thermal-oxidative stabilization effect of chemically reduced graphene oxide on polypropylene

The antioxidative effect of chemically reduced graphene oxide (rGO) on the thermal-oxidative stability of polypropylene (PP) was evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). rGO was prepared by reduction of graphene oxide (GO) and characterized by atomic force microscopy, X-ray photoelectron spectroscopy and X-ray diffraction. PP/rGO nanocomposites were then prepared without using a compatibilizer by melt blending. It was found that the thermal-oxidative degradation of PP was retarded noticeably by the rGO. The stabilization mechanism of rGO was discussed in terms of the changes in carbonyl bands and oxygen diffusion. It was proposed that the improved thermal-oxidation stability of PP/rGO nanocomposites can be attributed to the decline in both the concentration of peroxy radicals and oxygen permeability. The acceptor-like electronic property afforded by the long conjugated CC bonds and the barrier effect of rGO were suggested to be responsible for the improved thermal-oxidation stability of PP.

Measurements of OH and RO2 radicals at the coastal Antarctic site of Dumont d'Urville (East Antarctica) in summer 2010–2011

Measurements of OH and total peroxy RO2 (HO2 plus organic peroxy) radicals were conducted in December 2010/January 2011 at the coastal East Antarctic site of Dumont d'Urville (DDU, 66°40′S 140°01′E) as part of the Oxidant Production over Antarctic Land and its Export (OPALE) project. Compared to measurements carried out at the West Antarctic coast, relatively high concentrations of radicals were found with 24 h average values of 2.1 × 106 and 3.3 × 108 molecule cm−3 for OH and peroxy radicals, respectively. On the basis of the steady state calculations, the observed high concentration of peroxy radicals is in good agreement with the observed levels of O3 and HCHO representing via their photolysis the major primary radical sources. The observed OH levels at DDU could be explained only assuming some RO2 to OH conversion mechanism equivalent to the presence of NO in the range of 10 to 50 pptv. As neither NO nor halogen oxides were measured at DDU the mechanism of this recycling could not be explicitly identified. However, an examination of variability of radical levels as a function of the origin (oceanic versus continental) of sampled air masses suggests a more important OH production from RO2 recycling in continental air masses.

Modelled and measured concentrations of peroxy radicals and nitrate radical in the U.S. Gulf Coast region during TexAQS 2006

Measurements of total peroxy radicals (HO2 + RO2) and nitrate radical (NO3) were made on the NOAA research vessel R/V Brown along the U.S. Gulf Coast during the TexAQS 2006 field campaign. The measurements were modelled using a constrained box-model based upon the Master Chemical Mechanism (MCM). The agreement between modelled and measured HO2 + RO2 was typically within ∼40% and, in the unpolluted regions, within 30%. The analysis of the model results suggests that the MCM might underestimate the concentrations of some acyl peroxy radicals and other small peroxy radicals. The model underestimated the measurements of NO3 by 60–70%, possibly because of rapid heterogeneous uptake of N2O5. The MCM model results were used to estimate the composition of the peroxy radical pool and to quantify the role of DMS, isoprene and alkenes in the formation of RO2 in the different regions. The measurements of HO2 + RO2 and NO3 were also used to calculate the gas-phase budget of NO3 and quantify the importance of organic peroxy radicals as NO3 sinks. RO2 accounted, on average, for 12–28% of the total gas-phase NO3 losses in the unpolluted regions and for 1–2% of the total gas-phase NO3 losses in the polluted regions.

Effect of isoprene emissions from major forests on ozone formation in the city of Shanghai, China

Abstract. Ambient surface level concentrations of isoprene (C5H8) were measured in the major forest regions located south of Shanghai, China. Because there is a large coverage of broad-leaved trees in this region, high concentrations of isoprene were measured, ranging from 1 to 6 ppbv. A regional dynamical/chemical model (WRF-Chem) is applied for studying the effect of such high concentrations of isoprene on the ozone production in the city of Shanghai. The evaluation of the model shows that the calculated isoprene concentrations agree with the measured concentrations when the measured isoprene concentrations are lower than 3 ppb, but underestimate the measurements when the measured values are higher than 3 ppb. Isoprene was underestimated only at sampling sites near large bamboo plantations, a high isoprene source, indicating the need to include geospatially resolved bamboo distributions in the biogenic emission model. The assessment of the impact of isoprene on ozone formation suggests that the concentrations of peroxy radicals (RO2) are significantly enhanced due to the oxidation of isoprene, with a maximum of 30 ppt. However, the enhancement of RO2 is confined to the forested regions. Because the concentrations of NOx were low in the forest regions, the ozone production due to the oxidation of isoprene (C5H8 + OH → → RO2 + NO → → O3) is low (less than 2–3 ppb h−1). The calculation further suggests that the oxidation of isoprene leads to the enhancement of carbonyls (such as formaldehyde and acetaldehyde) in the regions downwind of the forests, due to continuous oxidation of isoprene in the forest air. As a result, the concentrations of HO2 radical are enhanced, resulting from the photo-disassociation of formaldehyde and acetaldehyde. Because the enhancement of HO2 radical occurs in regions downwind of the forests, the enhancement of ozone production (6–8 ppb h−1) is higher than in the forest region, causing by higher anthropogenic emissions of NOx. This study suggests that the biogenic emissions in the major forests to the south of Shanghai have important impacts on the levels of ozone in the city, mainly due to the carbonyls produced by the continuous oxidation of isoprene in the forest air.

New intrinsic oxygen related defect bands in oxygen implanted silica

Type III silica samples were implanted with O using a multi-energy process that produced a layer of constant concentration to within ± 5% beginning ~ 80 nm from the surface and extending to ~ 640 nm below the surfaces of samples. The concentrations in the layer ranged from 0.035 atomic percent to ~ 2.1 atomic percent. The optical absorption was measured from 2 to 6.5 eV. The absorption due to the implanted ions was analyzed using the difference spectra of the various samples. These difference spectra showed that optical bands at ~ 4.7 eV, 5.35 eV, and ~ 6.2 eV were a primary result of O implantation. Based on these data, we attribute these bands to O-related centers. Comparison of the difference spectra of the O samples with those of Si samples implanted to approximately the same concentrations showed that bands at the three energies in the spectra of the O samples were not observed in the spectra of the Si samples. Based on the spin concentrations of peroxy radicals, spin concentrations of E′ γ, and optical absorption coefficients at 4.8–5.0 eV and at 5.83 eV, the oscillator strengths of the peroxy band (~ 4.8 eV) was estimated, from the oscillator strength of the absorption band at 5.83 eV to be < 0.014.

A new technique for the selective measurement of atmospheric peroxy radical concentrations of HO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and RO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; using a denuding method

Abstract. A technique for the selective measurement of atmospheric HO2 and RO2 using peroxy radical chemical amplification coupled to laser-induced fluorescence NO2 detection (PERCA-LIF) is demonstrated. By pulling the air through a filled pre-inlet advantage can be taken of the higher heterogeneous loss rate of HO2 relative to CH3O2. Pre-inlet conditions have been found where ca. 90% of HO2 was removed whereas the comparable CH3O2 loss was 15%. The dependence of loss rate on humidity and peroxy radical concentration has been investigated. When using glass beads as the surface for peroxy radical removal, the influence of the relative humidity on the removal efficiency becomes negligible. It may therefore be possible to apply this technique to the measurement of absolute concentrations of solely RO2 as well as the sum of HO2 and RO2. The practical utility of the PERCA-LIF coupled to a denuder has been demonstrated with atmospheric measurements.

Reply to “Comment on ‘Primary Atmospheric Oxidation Mechanism for Toluene’”

Reply to “Comment on ‘Primary Atmospheric Oxidation Mechanism for Toluene’”

Peroxy radical observations over West Africa during AMMA 2006: photochemical activity in the outflow of convective systems

Abstract. Peroxy radical measurements made on board the DLR-Falcon research aircraft over West Africa within the African Monsoon Multidisciplinary Analysis (AMMA) campaign during the 2006 wet monsoon are presented in this study. The analysis of data focuses on the photochemical activity of air masses sampled during episodes of intense convection and biomass burning. Generally, the total sum of peroxy radical mixing ratios, measured in the outflow of convective clouds, are quite variable but occasionally are coupled with the NO variations indicating the coexistence or simultaneous emission of NOx, with a potential radical precursor (i.e. formaldehyde, acetone or peroxides), which has likely been transported to higher atmospheric altitudes. Based on the measurements, significant O3 production rates around 1 ppb/h in the MCS outflow are estimated by using a box model with simplified chemistry. Peroxy radicals having mixing ratios around 20–25 pptv and with peak values of up to 60–70 pptv are measured within biomass burning plumes, detected at the coast in Ghana. Calculations of back-trajectory densities confirm the origin of these air masses being a biomass burning region at southern latitudes and close to the Gulf of Guinea, according to satellite pictures. Measured peroxy radical concentrations agree reasonably with modelled estimations taking into account simple local chemistry. Moreover, the vertical profiles taken at the aircraft base in Ouagadougou, Burkina Faso, indicate the common feature of having maximum concentrations between 2 and 4 km, in agreement with other literature values obtained under similar conditions.

Peroxy radicals in the summer free troposphere: seasonality and potential for heterogeneous loss

Abstract. The sum of peroxy radicals (HO2+ΣiRiO2) and supporting trace gases were measured on the Jungfraujoch (3580 m a.s.l.) during the late summer of 2005. The period was marked by extended times of heavy snow which led to reduction in the observed peroxy radicals during the snowy periods that was greater than the concomitant reduction in j(O1D). In the limit a first order loss rate of 0.0063 s−1 can be derived for the peroxy radical loss in the snowy conditions that could be potentially ascribed to a heterogenous loss process. On snow free days photolysis of HCHO is shown to be a significant peroxy radical source. The seasonal trends of the peroxy radical concentrations have been mapped from the winter to summer transition in line with previous experiments. Net ozone production in late summer at the Jungfraujoch was net neutral to marginally ozone destructive. A value of 28±4 pptv is calculated for the ozone compensation point for the snow free days.

Intercomparison of peroxy radical measurements obtained at atmospheric conditions by laser-induced fluorescence and electron spin resonance spectroscopy

Abstract. Measurements of hydroperoxy radical (HO2) and organic peroxy radical (RO2) concentrations were performed by two different techniques in the atmospheric simulation chamber SAPHIR in Jülich, Germany. The first technique was the well-established Matrix Isolation Electron Spin Resonance (MIESR), which provides absolute measurements with a time resolution of 30 min and high accuracy (10%, 2 σ). The other technique, ROxLIF, has been newly developed. It is based on the selective chemical conversion of ROx radicals (HO2 and RO2) to OH, which is detected with high sensitivity by laser-induced fluorescence (LIF). ROxLIF is calibrated by quantitative photolysis of water vapor at 185 nm and provides ambient measurements at a temporal resolution of 1 min and accuracy of 20% (2 σ). The measurements of HO2 and RO2 obtained by the two techniques were compared for two types of atmospheric simulation experiments. In one experiment, HO2 and CH3O2 radicals were produced by photooxidation of methane in air at tropospheric conditions. In the second experiment, HO2 and C2H5O2 were produced by ozonolysis of 1-butene in air at dark conditions. The radical concentrations were within the range of 16 to 100 pptv for HO2 and 12 to 45 pptv for RO2. Good agreement was found in the comparison of the ROxLIF and MIESR measurements within their combined experimental uncertainties. Linear regressions to the combined data set yield slopes of 1.02±0.13 (1 σ) for RO2 and 0.98±0.08 (1 σ) for HO2 without significant offsets. The results confirm the calibration of the ROxLIF instrument and demonstrate that it can be applied with good accuracy for measurements of atmospheric peroxy radical concentrations.

Chemical ionisation mass spectrometer for measurements of OH and Peroxy radical concentrations in moderately polluted atmospheres

A new version of an atmospheric pressure chemical ionisation mass spectrometer has been developed for ground based in situ atmospheric measurements of OH and total peroxy (HO2 + organic peroxy) radicals. Based on the previously developed principle of chemical conversion of OH radicals to H2SO4 in reaction with SO2 and detection of H2SO4 using an ion molecule reaction with NO 3 ─ , the new instrument is equipped with a turbulent chemical conversion reactor allowing for measurements in moderately polluted atmosphere at NO concentrations up to several ppb. Unlike other similar devices, where the primary NO 3 ─ ions are produced using radioactive ion sources, the new instrument is equipped with a specially developed corona discharge ion source. According to laboratory measurements, the overall accuracy and detection limits are estimated to be, respectively, 25% and 2 × 105 molecule cm-3 for OH and 30% and 1 × 105 molecule cm-3 for HO2 at 10 min integration times. The detection limit for measurements of OH radicals under polluted conditions is 5 × 105 molecules cm-3 at 10 min integration times. Examples of ambient air measurements during a field campaign near Paris in July 2007 are presented demonstrating the capability of the new instrument, although with reduced performance due to the employment of non isotopic SO2.

Urban photochemistry in central Tokyo: 2. Rates and regimes of oxidant (O3 + NO2) production

Net photochemical production rates of oxidant (Ox = O3 + NO2), F‐D(Ox), were determined in Tokyo during the winter and summer of 2004 using observed and calculated HO2 radical concentrations. In both cases, calculated RO2 (organic peroxy) radical concentrations were used. The rates calculated using the two HO2 data sets are similar. In summer, morning F‐D(Ox) values on smog days (those with midday O3 concentrations exceeding 100 ppbv) were higher than those on smog‐free days (with typical midday O3 concentrations of 30 ppbv); however, the amount of ozone produced in a single day, as estimated by integrating F‐D(Ox) over the daytime, was not significantly different for the two periods. This analysis suggests that the occurrence of smog events in the city center cannot readily be explained by day‐to‐day variations in the strength of in situ photochemistry. On smog days, the coupling of photochemistry and meteorology appears to be important, as air masses in which oxidants accumulated over successive days arrive at the city center at approximately midday, transported by land‐sea breeze circulation. The average maximum daytime F‐D(Ox) values in summer, 11 and 13 ppbv h−1 using observed and calculated HO2 levels, respectively, were only 1.5 and 2.2 times higher than those in winter (8 and 6 ppbv h−1). In winter, an underestimation of HO2 levels at high NO concentrations resulted in an underestimation of F‐D(Ox) when calculated using modeled HO2. While the model predicted a volatile organic compounds (VOC)‐limited regime for Ox production in winter, F‐D(Ox) based on observed HO2 did not show features of the VOC‐limited regime and only steadily increased with increasing NO mixing ratio, even when it exceeded 20 ppbv. In summer, the dependence of F‐D(Ox) on nonmethane hydrocarbons (NMHCs) and NOx concentrations was similar in the two cases, in which observed and calculated HO2 levels were used. A VOC‐limited regime, predicted on smog‐free days, changed to a NOx‐limited regime on smog days. The F‐D(Ox) values determined for Tokyo are also compared with values for other cities.

Diurnal peroxy radical chemistry at a remote coastal site over the sea of Japan

The Rishiri Fall Experiment (RISFEX) was performed in September 2003 at a remote coastal site (45.07°N, 141.12°E, and 35 m asl) on Rishiri Island in the Sea of Japan. Peroxy radicals were measured from 15–21 September using a peroxy radical chemical amplifier (PERCA) instrument. Simultaneously measured were O3, NO, NO2, CO, nonmethane hydrocarbons (NMHCs), biogenic volatile organic compounds (BVOCs), carbonyl compounds, aromatics, volatile organoiodines, organobromines, black carbon, aerosols and photolysis frequencies of j(O1D) and j(NO2) as well as meteorological parameters. The data set covers the measurements under relative background conditions and polluted episodes over the period. The midday 30‐min averages of peroxy radicals ranged from 10 to 40 pptv with the highest observed on the event when the air masses were transported over the polluted continent in northern China before reaching the site. The significant radical signals were observed repeatedly in the early morning and late afternoon when j(O1D) was greatly reduced, resulting in the diurnal cycles of peroxy radicals much broader than those expected from the photolysis of O3 alone. A box model based on RACM (Regional Atmospheric Chemistry Mechanism) was used to calculate OH, HO2 and RO2 concentrations from measured stable species and parameters. The calculated peroxy radical concentrations agree well with measured ones during 0900–1530 Japan Standard Time (JST) (JST = UT + 9 hours) from 19 to 21 September. However, in the early morning and later afternoon when monoterpenes were the dominant VOC, the model overpredicts peroxy radicals by up to 85%. The including of uptake of radicals to aerosols into the model cannot account for the discrepancy. An underprediction by the model up to 45% was observed during noon hours on 18 September when isoprene concentration was high. It appears that unknown terpenes are present, which might produce extra radicals beyond the predicted by the model. The measured‐to‐modeled ratio of peroxy radical concentrations is near unity for [NO] &lt; 120 pptv, but rises up to 15 for [NO] &gt; 120 pptv, suggesting the presence of an unknown radical production process which is related with high NO concentration.

Seasonal dependence of peroxy radical concentrations at a Northern hemisphere marine boundary layer site during summer and winter: evidence for radical activity in winter

Abstract. Peroxy radicals (HO2+Σ RO2) were measured at the Weybourne Atmospheric Observatory (52° N, 1° E), Norfolk using a PEroxy Radical Chemical Amplifier (PERCA) during the winter and summer of 2002. The peroxy radical diurnal cycles showed a marked difference between the winter and summer campaigns with maximum concentrations of 12 pptv at midday in the summer and maximum concentrations as high as 30 pptv (10 min averages) in winter at night. The corresponding nighttime peroxy radical concentrations were not as high in summer (3 pptv). The peroxy radical concentration shows a distinct anti-correlation with increasing NOx during the daylight hours. At night, peroxy radicals increase with increasing NOx indicative of the role of NO3 chemistry. The average diurnal cycles for net ozone production, N(O3) show a large variability in ozone production, P(O3), and a large ozone loss, L(O3) in summer relative to winter. For a daylight average, net ozone production in summer was higher than winter (1.51±0.5 ppbv h−1 and 1.11±0.47 ppbv h−1, respectively). The variability in NO concentration has a much larger effect on N(O3) than the peroxy radical concentrations. Photostationary state (PSS) calculations show an NO2 lifetime of 5 min in summer and 21 minutes in the winter, implying that steady-state NO-NO2 ratios are not always attained during the winter months. The results show an active peroxy radical chemistry at night and that significant oxidant levels are sustained in winter. The net effect of this with respect to production of ozone in winter is unclear owing to the breakdown in the photostationary state.