ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-15-8179-2015Night-time measurements of HOx during the RONOCO project and analysis of the sources of HO2WalkerH. M.https://orcid.org/0000-0001-5144-1605StoneD.https://orcid.org/0000-0001-5610-0463InghamT.VaughanS.CainM.https://orcid.org/0000-0003-2062-6556JonesR. L.KennedyO. J.McLeodM.OuyangB.PyleJ.https://orcid.org/0000-0003-3629-9916BauguitteS.BandyB.ForsterG.EvansM. J.https://orcid.org/0000-0003-4775-032XHamiltonJ. F.HopkinsJ. R.https://orcid.org/0000-0002-0447-2633LeeJ. D.https://orcid.org/0000-0001-5397-2872LewisA. C.LidsterR. T.PunjabiS.MorganW. T.HeardD. E.d.e.heard@leeds.ac.ukhttps://orcid.org/0000-0002-0357-6238School of Chemistry, University of Leeds, Leeds,
UKNational Centre for Atmospheric Science, University of
Leeds, Leeds, UKDepartment of Chemistry, University of Cambridge,
Cambridge, UKNational Centre for Atmospheric Science, University of
Cambridge, Cambridge, UKFacility for Airborne Atmospheric Measurements (FAAM),
Cranfield University, Cranfield, UKNational Centre for Atmospheric Science, FAAM, Cranfield
University, Cranfield, UKCentre for Ocean and Atmospheric Sciences, School of
Environmental Sciences, University of East Anglia, Norwich,
UKNational Centre for Atmospheric Science, University of
East Anglia, Norwich, UKWolfson Atmospheric Chemistry Laboratories, Department
of Chemistry, University of York, York, UKNational Centre for Atmospheric Science, University of
York, York, UKSchool of Earth, Atmospheric and Environmental Sciences,
University of Manchester, Manchester, UKNational Centre for Atmospheric Science, University of
Manchester, Manchester, UKD. E. Heard (d.e.heard@leeds.ac.uk)23July20151514817982006November201430January201512May201524May2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/15/8179/2015/acp-15-8179-2015.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/15/8179/2015/acp-15-8179-2015.pdf
Measurements of the radical species OH and HO2 were
made using the fluorescence assay by gas expansion (FAGE) technique during a
series of night-time and daytime flights over the UK in summer 2010 and
winter 2011. OH was not detected above the instrument's 1σ limit of
detection during any of the night-time flights or during the winter daytime
flights, placing upper limits on [OH] of
1.8 × 106 molecule cm-3 and
6.4 × 105 molecule cm-3 for the summer and winter
flights, respectively. HO2 reached a maximum concentration of
3.2 × 108 molecule cm-3 (13.6 pptv) during a night-time
flight on 20 July 2010, when the highest concentrations of NO3 and
O3 were also recorded. An analysis of the rates of reaction of OH,
O3, and the NO3 radical with measured alkenes indicates that the
summer night-time troposphere can be as important for the processing of
volatile organic compounds (VOCs) as the winter daytime troposphere. An
analysis of the instantaneous rate of production of HO2 from the
reactions of O3 and NO3 with alkenes has shown that, on average,
reactions of NO3 dominated the night-time production of HO2 during
summer and reactions of O3 dominated the night-time HO2 production
during winter.
Introduction
Trace gases emitted into the atmosphere, including pollutants
and greenhouse gases, are removed primarily by oxidation. The hydroxyl
radical, OH, is the most important oxidising species in the daytime
troposphere, reacting with numerous species, including volatile organic
compounds (VOCs), CO, SO2, and long-lived anthropogenic halogenated
compounds. During the day, primary production of OH (i.e. initialisation of
the radical chain) occurs predominantly via photolysis of ozone at λ≤340 nm, followed by the reaction of the resulting electronically
excited oxygen atom, O(1D), with water vapour. The OH-initiated
oxidation of VOCs leads to the production of the hydroperoxy radical,
HO2, and together the two radicals form the HOx family. A key
reaction in the conversion of OH to HO2 is the reaction with CO:
OH+CO→H+CO2H+O2+M→HO2+M.
The reaction of OH with VOCs results in the production of organic peroxy
radicals, RO2:
OH+RH→H2O+RR+O2+M→RO2+M
Reactions of HO2 and RO2 with NO propagate the HOx radical
chain, regenerating OH:
RO2+NO→RO+NO2RO+O2→R′O+HO2HO2+NO→OH+NO2
The production of OH through the photolysis of ozone (and other species at
longer wavelengths) is limited to daylight hours, and the oxidation of trace
gases at night proceeds through alternative mechanisms. Two mechanisms are
known to initiate HOx radical chemistry and oxidation chemistry at
night: ozonolysis of alkenes and reactions of the nitrate radical, NO3,
with alkenes.
Reactions of ozone with alkenes occur via the addition of ozone to the double
bond to form a five-membered ring called a primary ozonide. The primary
ozonide decomposes to form one of two possible pairs of products, each pair
consisting of a carbonyl compound and a vibrationally and rotationally
excited carbonyl oxide termed a Criegee intermediate (CI). The simplest
gas-phase CI, CH2OO, and the alkyl-substituted CH3CHOO have been
observed directly by photoionisation mass spectrometry (Taatjes et al., 2008,
2012, 2013; Beames et al., 2012, 2013; Welz et al., 2012;
Stone et al., 2014a), by infrared
absorption spectroscopy (Su et al., 2013), and by microwave spectroscopy
(Nakajima and Endo, 2013, 2014). Excited CIs may be stabilised by collision
with surrounding molecules (Donahue et al., 2011; Drozd and Donahue, 2011) or
may undergo isomerisation or decomposition to yield products including OH, H,
and subsequently HO2 (Paulson and Orlando, 1996; Kroll et al., 2001a, b,
2002; Johnson and Marston, 2008). Stabilised CIs (SCIs) are known to react
with a variety of compounds, including H2O, NO2, SO2, and a
variety of organic compounds (e.g. Mauldin III et al., 2012; Taatjes et al.,
2012, 2013, 2014; Ouyang et al., 2013; Stone et al., 2014a). There is
experimental evidence for the formation of OH from the thermal decomposition
of SCIs, on a much longer timescale than the decomposition or isomerisation
of excited CIs (Kroll et al., 2001a, b). The OH produced through these
ozonolysis mechanisms will proceed to oxidise other VOC species. Criegee
intermediates formed in the ozonolysis of alkenes are known to be an
important source of HOx during the day and at night (Paulson and
Orlando, 1996; Donahue et al., 1998; Kanaya et al., 1999; Salisbury et al.,
2001; Geyer et al., 2003; Ren et al., 2003a, 2006; Heard et al., 2004;
Harrison et al., 2006; Sommariva et al., 2007). The gas-phase ozonolysis of
unsaturated VOCs, and in particular the role and subsequent chemistry of the
Criegee intermediate, have been reviewed in detail by Johnson and
Marston (2008), Donahue et al. (2011), Vereecken and Francisco (2012), and
Taatjes et al. (2014).
Another key night-time oxidant, NO3, is formed primarily by the reaction
of NO2 with ozone. NO3 reacts with a range of species in the
troposphere, and its reaction with alkenes is known to be an important
night-time oxidation mechanism (Salisbury et al., 2001; Geyer et al., 2003;
Sommariva et al., 2007; Emmerson and Carslaw, 2009; Brown et al., 2011). The
reaction between NO3 and an alkene proceeds primarily via addition to a
double bond to form a nitrooxyalkyl radical, R–ONO2. At atmospheric
pressure, the main fate of the nitrooxyalkyl radical is reaction with O2
(Berndt and Böge, 1994) to produce a nitrooxyalkyl peroxy radical,
O2–R–ONO2. The nitrooxyalkyl peroxy radical can react with
NO2, HO2, RO2, NO, and NO3, of which the latter two
reactions lead to the formation of the nitrooxyalkoxy radical,
O–R–ONO2. The nitrooxyalkoxy radical can undergo isomerisation,
decomposition, or reaction with O2. Reaction with O2, analogous to
the reaction of organic alkoxy radicals, yields HO2:
O–R–ONO2+O2→O–R′–ONO2+HO2.
Thus, the night-time oxidation of hydrocarbons by NO3 leads to the
production of HO2. The reaction of HO2 with NO (Reaction R7),
O3, and NO3 can generate OH:
HO2+O3→OH+2O2HO2+NO3→OH+NO2+O2.
Atkinson and Arey (2003) published a detailed review of the tropospheric
degradation of VOCs, including reaction with O3 and NO3. A
comprehensive review of night-time radical chemistry is given by Brown and
Stutz (2012).
Examples of modelling studies and observations of HOx radicals
and VOC oxidation at night. PERCA: peroxy radical chemical amplification;
LIF: laser-induced fluorescence; DOAS: differential optical absorption
spectroscopy; MCM: Master Chemical Mechanism; MIESR: matrix isolation
electron spin resonance; RACM: Regional Atmospheric Chemistry Mechanism;
CRDS: cavity ring-down spectroscopy; CIMS: chemical ionisation mass
spectrometry; GC: gas chromatography; PTRMS: proton transfer reaction mass
spectrometry; FTIR: Fourier transform infrared spectroscopy; DUALER: DUAl
channel peroxy radical chemical amplifier; OA-CRD: off-axis cavity ring-down
spectroscopy; CRM-PTR-MS: comparative reactivity method proton transfer mass
spectrometry.
Location, campaign, dateMethodsResultsReferenceMace Head, Ireland, Eastern Atlantic Spring Experiment (EASE97), 1997Measurements: [HO2+RO2] measured by PERCA; HOx measured by LIF; NO3 measured by DOAS. Modelling: campaign-tailored box model constrained to measurements, based on MCM.Two nights of HOx measurements: HO2= 1–2 and 0.5–0.7 pptv; OH not detected above limit of detection (∼ 2.5 × 105 cm-3). NO3 dominated radical production in westerly (clean) air masses; O3 dominated in NE, SE, and SW air masses and dominated radical production overall during the campaign.Salisbury et al. (2001); Creasey et al. (2002)Pabstthum, Germany, Berlin Ozone Experiment (BERLIOZ), 1998Measurements: HOx measured by LIF; NO3 measured by DOAS and MIESR. Modelling: zero-dimensional model using lumped VOC reactivity, constrained to measured species.Night-time OH = 1.85 × 105 cm-3, compared to modelled value of 4.1 × 105 cm-3. Night-time HO2= 3 × 107 cm-3, model results in agreement. NO3 chemistry responsible for 53 % of HO2 and 36 % of OH during the night. O3+ alkene responsible for 47 % of HO2 and 64 % of OH during the night.Geyer et al. (2003);Holland et al. (2003)Birmingham, Pollution of the Urban Midlands Atmosphere (PUMA), 1999 and 2000Measurements: HOx measured by LIF. Modelling: photochemical box model constrained to measurements, based on MCM.Daytime OH initiation dominated by O3+ alkenes, HONO photolysis, and O(1D) + H2O during summer. O3+ alkenes dominated in winter. O3+ alkenes main radical source at night.Emmerson et al. (2005); Harrison et al. (2006)New York, PM2.5 Technology Assessment and Characteristics Study-New York (PMTACS-NY), 2001Measurements: HOx measured by LIF.Night-time OH ∼ 7 × 105 cm-3 and night-time HO2∼ 8 × 106 cm-3. Increase in HOx after midnight attributed to increase in O3 due to transport. O3+ alkenes main source of night-time HOx.Ren et al. (2003a, b)Mace Head, North Atlantic Marine Boundary Layer Experiment (NAMBLEX), 2002Measurements: HOx measured by LIF; NO3 measured by DOAS. Modelling: zero-dimensional box models constrained to measured species, based on MCM.Night-time HO2= 2–3 × 107 cm-3; OH below detection limit (6 × 104 cm-3). Model overestimated HO2. On average, O3+ alkene reactions contributed 59 % and NO3+ alkene reactions contributed 41 % to RO2 production at night, but NO3 and RO2 concentrations were always higher in semi-polluted air masses than in clean marine air masses and NO3 reactions dominated in these conditions.Fleming et al. (2006); Smith et al. (2006); Sommariva et al. (2007)Writtle, London, Tropospheric ORganic CHemistry experiment (TORCH), 2003Measurements: HOx measured by LIF, RO2 measured by PERCA, during a heatwave or pollution episode. Modelling: zero-dimensional box model constrained to measured species.OH and HO2 observed above the limit of detection on several nights. OH peaked at 8.5 × 105 cm-3; HO2 peaked at 1 × 108 cm-3. Model overpredicted night-time OH and HO2 on average by 24 % and 7 % and underpredicted [HO2+ΣRO2] by 22 %.Lee et al. (2006); Emmerson et al. (2007); Emmerson and Carslaw (2009)Mexico City Metropolitan Area (MCMA 2003)Measurements: HOx measured by LIF, NO3 measured by DOAS. Modelling: zero-dimensional model based on MCM v3.1, constrained to measured species.Polluted city location characterised by high levels of NO, NO2, and O3. Maximum night-time OH ∼ 1 × 106 cm-3; maximum night-time HO2∼ 6 pptv. Night-time production of radicals dominated by O3+ alkene reactions (76–92 %); NO3+ alkene plays a minor role. Daytime radical production ∼ 25 times higher than night.Shirley et al. (2006); Sheehy et al. (2010); Volkamer et al. (2010)New York City, PMTACS-NY winter 2004Measurements: HOx measured by LIF. Modelling: zero-dimensional model based on RACM and constrained by measurements.Mean maximum OH = 0.05 pptv; mean maximum HO2= 0.7 pptv. Model underprediction of HO2 was pronounced when NO was high. O3+ alkene reactions were dominant night-time source.Ren et al. (2006)
Continued.
Gulf of Maine, Northeast United States, New England Air Quality Study (NEAQS), 2004Measurements: NO3 and N2O5 measured by CRDS. Modelling: zero-dimensional model based on MCM v3.1, constrained to measured species. No measurements of OH, HO2, or RO2.Ship-based measurements onboard RV Ronald H. Brown in the Gulf of Maine, influenced by unpolluted marine air masses and polluted air masses from the USA and Canada. Maximum modelled night-time HO2= 7.0 × 108 cm-3. Base model overestimated NO3 and NO2 observations by 30–50 %. In anthropogenic air masses, reaction with VOCs and RO2 each accounted for 40 % of modelled NO3 loss.Sommariva et al. (2009)Houston, Texas, Texas Air Quality Study (TexAQS), 2006Measurements: NO3 and N2O5 measured by CRDS; VOCs measured by CIMS, GC, and PTRMS. No direct measurements of OH, HO2, or RO2.Loss rates and budgets of NO3 and highly reactive VOCs calculated. NO3 primarily lost through reaction with VOCs. VOC oxidation dominated by NO3, which was 3–5 times more important than O3.Brown et al. (2011)Pearl River Delta, China, Program of Regional Integrated Experiments of Pearl River Delta Region (PRIDE-PRD), 2006Measurements: HOx measured by LIF; OH reactivity measured by laser-flash photolysis and LIF; VOCs measured by FTIR and GC. Modelling: box model based on RACM and the Mainz Isoprene Mechanism and constrained by measurements.Rural site 60 km downwind of large urban region (Guangzhou), with low local wind speeds favouring accumulation of air pollutants. Maximum night-time OH (hourly average) = 5 × 106 cm-3; maximum night-time HO2 (hourly average) = 1 × 109 cm-3. Unknown recycling mechanism required for the model to reproduce measured night-time values. OH reactivity peaked at night. Missing night-time reactivity attributed to unmeasured secondary organic compounds.Lou et al. (2010); Lu et al. (2012, 2013)Beijing, Campaigns of Air Quality Research in Beijing and Surrounding Regions (CAREBEIJING2006), 2006Measurements: HOx measured by LIF; OH lifetime measured by laser-flash photolysis and LIF; VOCs measured by GC. Modelling: box model based on RACM and the Mainz Isoprene Mechanism and constrained by measurements.Suburban rural site south of Beijing, under the influence of slowly moving, aged polluted air from the south. OH reactivity peaked at night. Model generally underestimated observed night-time OH concentrations.Lu et al. (2013, 2014)Cape Verde, Reactive Halogens in the Marine Boundary Layer (RHaMBLe), 2007Measurements: HOx measured by LIF. Modelling: box model based on MCM with added halogen chemistry scheme, constrained to measurements of long-lived species.Clean tropical Atlantic measurement site with occasional continental influence. OH was not measured at night. HO2 was detected on two nights, up to 2.5 × 107 cm-3. Model underprediction of HO2 was significantly reduced by constraining the model to 100 pptv of peroxy acetyl nitrate (PAN) at night.Whalley et al. (2010)Huelva, Spain, Diel Oxidant Mechanisms in relation to Nitrogen Oxides (DOMINO), 2008Measurements: [HO2+RO2] measured by DUALER; HOx measured by LIF; NO3 and N2O5 measured by OA-CRD; OH reactivity measured by CRM-PTR-MS. No measurements of anthropogenic VOCs.Coastal forested site with strong urban-industrial and weak biogenic influences. Maxima in [HO2+RO2] and [HO2] were observed around noon and midnight. Enhanced night-time [HO2+RO2] (up to 80 pptv) was observed in air masses from the urban-industrial region. Maximum night-time HO2=8 pptv. Measured NO3 was generally below LOD; calculated NO3 up to 20 pptv. Calculated production of RO2 from NO3+ alkenes accounts for 47–54 % of observed [HO2+RO2]. Ozonolysis of unmeasured alkenes could account for remaining [HO2+RO2].Andrés-Hernández et al. (2013)
The oxidising capacity of the nocturnal troposphere is thought to be
controlled by the reactions described above, with a limited role for OH and
HO2 due to the absence of their photolytic sources. The oxidation of
VOCs at night can have significant effects on daytime air quality and
tropospheric ozone production (Brown et al., 2004, 2006, 2011; Wong and
Stutz, 2010). Several field measurement campaigns have involved night-time
measurements of OH, HO2, RO2, and NO3 (see Table 1) and have
highlighted the importance of the vertical profile of night-time radical
concentrations and chemistry (Geyer and Stutz, 2004a, b; Stutz et al., 2004;
Volkamer et al., 2010), but prior to the current work, there had been no
aircraft-based studies of night-time chemistry involving measurements of both
NO3 and HO2 to enable the vertical profiling of the lower
atmosphere and a full evaluation of the nocturnal radical budget. Table 1
gives details of some previous measurements and modelling of night-time
HOx concentrations in polluted or semi-polluted environments. Highlights
from these studies are discussed here, with particular attention paid to
those involving measurements of HOx, NO3, and O3 and to those
in which the contributions made by O3 and NO3 to night-time radical
chemistry have been considered.
Geyer et al. (2003) report radical measurements and modelling from the 1998
Berlin Ozone Experiment
(BERLIOZ). Measurements of NO3, RO2, HO2, and OH were made by
matrix isolation electron spin resonance (MIESR), chemical amplification
(CA), and laser-induced fluorescence (LIF) spectroscopy at a site
approximately 50 km from Berlin. HO2 was detected at night, with
concentrations frequently as high as
5 × 107 molecule cm-3 (approximately 2 pptv) and an
average concentration of 1 × 108 molecule cm-3 over 1 h
(02:00 to 03:00) of nocturnal measurements during an intensive period of the
study (Holland et al., 2003). OH was usually below the limit of detection of
the LIF instrument (3.5 × 105 molecule cm-3). Modelling
revealed that nitrate radical reactions with terpenes were responsible for
producing 53 % of HO2 and 36 % of OH radicals in the night, with
ozonolysis accounting for the production of the remaining 47 % of
HO2 and 64 % of OH radicals. A positive linear correlation between
RO2 and NO3 was observed and was reproduced by the model.
Reactions of O3 with alkenes were found to be responsible for the
majority of the formation of OH during the winter PUMA (Pollution of the
Urban Midlands Atmosphere) campaign (a low-photolysis urban environment)
(Heard et al., 2004; Emmerson et al., 2005; Harrison et al., 2006).
Measurements of OH, HO2, and RO2 were unavailable at night, but
model-predicted values of these radicals were used to calculate that 90 %
of night-time initiation via HO2 was from O3 reactions. Without
measurements of NO3 during the campaign, there was no estimate of its
contribution to radical initiation.
Modelling results from the MCMA-2003 (Mexico City) field campaign (Volkamer
et al., 2010) indicate that night-time radical production at roof-top level
(approximately 16 m above the ground) was dominated by ozonolysis of
alkenes, and that reactions of NO3 with alkenes played only a minor
role. The measurement site was located in a polluted urban environment, with
high levels of NO, NO2, and O3. NO3 was observed at a maximum
concentration of 50 pptv during the night at a mean height above the ground
of 70 m. Roof-top level concentrations of NO3 were estimated using a
linear scaling factor, calculated from the observed O3 vertical
gradient, and were found to be, on average, 3 times lower than the
concentrations measured at 70 m. This predicted vertical gradient accounts
for the relative unimportance of NO3 reactions in radical initiation at
roof-top level. The propagation of RO2 radicals to HO2 and OH, by
reaction with NO3, was found to be negligible.
The 2006 Texas Air Quality Study (TexAQS) involved a series of night-time
flights onboard the NOAA P-3 aircraft over Houston, Texas, and along the Gulf
Coast (Brown et al., 2011). Loss rates and budgets of NO3 and highly
reactive VOCs were calculated, but there were no measurements of OH,
HO2, and RO2 during the flights. Budgets for NO3 show that it
was lost primarily through reactions with unsaturated VOCs, but the
contribution to NO3 loss through reaction with peroxy radicals was
uncertain because of the lack of direct measurements of RO2 during the
flights. NO3 dominated VOC oxidation, being 3 to 5 times more important
than O3.
In summary, NO3 and O3 have both been found to dominate radical
initiation in the night-time troposphere, and in some situations the two
mechanisms were found to be equally important. The relative importance of
O3- and NO3-initiated oxidation depends on the availability of
NO3, which is determined by the amount of NOx present in the
atmosphere and the ratio of NO to NO2, and on the concentration and
species distribution of VOCs (Bey et al., 2001; Geyer et al., 2003). A
modelling study by Bey et al. (2001) suggests that nocturnal radical
initiation is driven by alkene ozonolysis in urban environments or in
environments with low NOx concentrations, while both O3 and
NO3 contribute to radical initiation in rural environments with moderate
NOx levels. It is expected that NO3 dominates nocturnal radical
initiation in air masses containing sufficient NO2 and O3 for
NO3 production while being deprived of NO (e.g. air masses downwind of
urban areas). Geyer and Stutz (2004b) have found that the effects of
suppressed mixing in the nocturnal boundary layer can also control whether
NO3 or O3 dominates night-time radical chemistry.
In this paper we report airborne measurements of OH and HO2 made during
the RONOCO (ROle of Nighttime chemistry in controlling the Oxidising Capacity
of the atmOsphere) and SeptEx (September Experiment) projects in 2010 and
2011. The rates of reaction between O3, NO3, and OH with the
alkenes measured during the flights are investigated. The analysis of radical
production from the night-time reactions of O3 and NO3 with alkenes
is also given. Comparisons are made between the daytime and night-time
chemistry studied and between the summer and winter measurement periods.
Details and results of a box modelling study, and a comparison to the
observations, are given by Stone et al. (2014b).
Details of the RONOCO and SeptEx fieldwork
RONOCO is a Natural Environment Research Council (NERC)-funded consortium project aimed at
improving our understanding of the mechanisms and impact of nocturnal
oxidation chemistry over the UK. The RONOCO fieldwork consisted of two
measurement campaigns, in July 2010 and January 2011. Additional fieldwork,
SeptEx, was conducted in September 2010. The RONOCO and SeptEx flights were
conducted onboard the BAe-146 research aircraft operated by the Facility for
Airborne Atmospheric Measurements (FAAM). Both field measurement campaigns
were based at East Midlands Airport (52.8∘ N, 1.3∘ W) in the
UK. During RONOCO the majority of the flying took place at night, with
occasional flights beginning or ending in daylight hours to study chemical
behaviour at dusk and dawn. Flights during SeptEx were mainly during the day,
providing a useful comparison to the nocturnal chemistry.
Flights were conducted between altitudes of 50 and 6400 m, above the UK
and the North Sea. Figure 1 shows the flight tracks during the summer,
SeptEx, and winter measurements coloured by altitude. Measurements of OH and
HO2 were made using the University of Leeds aircraft-based fluorescence
assay by gas expansion (FAGE) instrument. A suite of supporting
measurements, including CO, O3, NO, H2O, VOCs, NO3, and HCHO,
were made during the flights and have been used in the current work.
Table 2 summarises the techniques used to measure
these species.
Flight tracks for (a) summer RONOCO, (b) SeptEx,
and (c) winter RONOCO measurement campaigns, coloured by altitude.
Air mass histories for each flight have been calculated using the UK Met
Office Numerical Atmospheric-dispersion Modelling Environment (NAME). NAME is
a three-dimensional Lagrangian particle dispersion model (Jones et al.,
2007), which is run here using the UK Meteorological Office's Unified Model
meteorological fields. Model “particles”, restricted to a 300 m deep layer
from the surface, were released along the flight path and were tracked
backwards through the modelled atmosphere. Model particle densities were
integrated over 24 h periods, beginning at 24, 48, 72, and 96 h before each
flight. The resulting “footprint” maps show the regions where the measured
air has been in contact with the surface over the 4 days preceding a flight.
An example is shown in Fig. 2, which shows model particle densities
integrated over the 24 h period beginning 48 h prior to flight B535. The
majority of the summer flights were characterised by air masses originating
from the west and south-west of the UK, having Atlantic or continental
European influences. The SeptEx flights were predominantly influenced by air
masses from the north-east, east, and south-east of the UK, with northern
European influences. The winter flights were mainly characterised by air
masses arriving from the west of the UK, bringing Atlantic
influences.
Table 3 gives mean and maximum mixing ratios of CO, O3, NO, and NO2
measured during RONOCO and SeptEx. The mean mixing ratios of NO measured
during the summer RONOCO flights are much lower than ground-based night-time
measurements (e.g. 1.0 ppbv during the Tropospheric ORganic CHemistry experiment (TORCH) (Emmerson and Carslaw, 2009),
0–20 ppbv during the PM2.5 Technology Assessment and Characteristics Study-New York (PMTACS-NY); Ren et al., 2006) but are comparable with
previous airborne night-time measurements (e.g. < 30 pptv during
the Texas Air Quality Study (TexAQS; Brown et al., 2011). Mean values of NO up to 14 pptv were reported
by Salisbury et al. (2001) for semi-polluted air masses sampled at Mace Head.
These comparisons indicate that the RONOCO and SeptEx flights enabled the
sampling of air masses generally removed from the influence of NO in fresh
surface emissions. Table 3 also highlights the unusual chemical conditions
encountered during flight B537 on 20 July 2010, discussed further in
Sect. 4.1. Night-time altitude profiles of NO3, O3,
trans-2-butene, and propene (the latter two being illustrative of
the alkenes measured) are given in Fig. 3.
Details of supporting measurements.
SpeciesInstrument, techniqueTime resolution; limit of detection (LOD)ReferencesCOAero Laser AL5002 Fast Carbon Monoxide Monitor. Excitation and fast-response fluorescence at λ= 150 nm.1 s; 3.5 ppbvGerbig et al. (1999)O3Thermo Scientific TEi49C ozone analyser. Absorption spectroscopy at λ= 254 nm.1 s; 0.6 ppbvHewitt et al. (2010)NO, NO2, NOx (NO + NO2)Air Quality Design dual-channel fast-response NOx instrument. Chemiluminescence from NO + O3 reaction. Conversion of NO2 to NO by photolysis.10 s; 3 pptv for NO, 15 pptv for NO2Stewart et al. (2008)NO2, ΣANs, ΣPNsTD-LIF (thermal dissociation laser-induced fluorescence). Detection of NO2 by laser-induced fluorescence. Thermal decomposition of ΣANs (total alkyl nitrate) and ΣPNs (total peroxy nitrate) to NO2.1 s; 9.8 pptv for NO2, 28.1 pptv for ΣANs, 18.4 pptv for ΣPNsDari-Salisburgo et al. (2009); Di Carlo et al. (2013)AlkenesWhole air samples (WAS) analysed by laboratory-based gas chromatography with flame ionisation detection (GC-FID).Typically 30 s; variable limits of detectionHopkins et al. (2003)NO3, N2O5BBCEAS (broadband cavity-enhanced absorption spectroscopy) of NO3 at λ= 642–672 nm. N2O5 measured following thermal dissociation to NO3+ NO2.1 s; 1.1 pptv for NO3, 2.4 pptv for NO3+ N2O5Kennedy et al. (2011)HCHOHantzsch technique: liquid-phase reaction of formaldehyde followed by excitation and fluorescence of resulting adduct at λ= 510 nm.60 s; 81 pptvStill et al. (2006)
Mean mixing ratios of selected gas-phase species, and air
temperature, measured during RONOCO and SeptEx. The flight and season during
which the maximum values were measured are given in parentheses. NO2
data are from the TD-LIF instrument. Zero values indicate measurements below
the limit of detection.
Footprint map for flight B535 on 17 July 2010, showing
model particle densities (g s m-3) in a 300 m deep layer from the
surface, integrated over a 24 h period beginning 48 h prior to the
flight.
Night-time altitude profiles of (a) NO3;
(b) O3; (c)trans-2-butene; and
(d) propene, showing 60 s data (grey points) and mean values in
500 m altitude bins (black lines).
ExperimentalThe Leeds FAGE aircraft instrument
The University of Leeds aircraft FAGE instrument has been described in detail
by Commane et al. (2010). A brief description is given here. The instrument,
which was designed specifically for use onboard the FAAM BAe-146 research
aircraft (Floquet, 2006), is housed in two double-width 19 in. aircraft
racks, with the inlet, detection cells, and pump set being separate from the
two racks. Ambient air is sampled through a 0.7 mm diameter “pinhole” into
a cylindrical inlet (length: 50 cm; diameter: 5 cm) which extends through a
window blank on the starboard side of the aircraft.
Downstream of the inlet are two low-pressure fluorescence cells positioned in
series, the first for the detection of OH and the second for the detection of
HO2. During the RONOCO and SeptEx flights, the pressure inside the cells
ranged from 1.9 Torr at ground level to 1.2 Torr at 6 km.
Laser light at λ∼ 308 nm is generated by a diode-pumped
Nd:YAG-pumped
tunable Ti:sapphire laser (Photonics Industries DS-532-10 and TU-UV-308 nm)
and delivered to the fluorescence cells via optical fibres, on an axis
perpendicular to the gas flow. A small fraction of the Ti:sapphire second
harmonic (λ= 462 nm) is directed to the probe of a wavemeter to
enable measurement of the laser wavelength to within 0.001 nm. A UV
photodiode is positioned opposite the laser input arm on each fluorescence
cell to measure laser power.
The sampled air forms a supersonic gas expansion beam in which the rate of
collision between OH radicals and ambient air molecules is reduced. The OH
fluorescence lifetime is therefore extended to several hundred nanoseconds,
significantly longer than the laser pulse, so that the measured signal can
be temporally discriminated from laser scattered light. OH is excited from
its ground state, X2Πiv′′=0, to
its first electronically excited state, A2Σ+v′=0, at λ∼ 308 nm. The
resulting on-resonance fluorescence is detected by a UV-sensitive channel
photomultiplier tube on an axis perpendicular to both the gas flow and the
laser light. HO2 is detected by titration with an excess of NO
(Reaction R7), the resulting OH being detected as described.
The FAGE instrument was calibrated prior to and following each field
measurement period, using a well-established method (Edwards et al., 2003;
Faloona et al., 2004; Commane et al., 2010). Light at λ=184.9 nm
from a mercury pen-ray lamp photolyses water vapour in a flow of synthetic
air inside an aluminium flow tube, generating OH and HO2 at known
concentrations. The aircraft FAGE instrument's limit of detection (LOD) for
OH and HO2 is determined by the instrument's sensitivity and the
standard deviation of the background signal. During the RONOCO and SeptEx
fieldwork, the 1σ LOD for a 5 min averaging period ranged between
0.64 and 1.8 × 106 molecule cm-3 for OH and between 5.9
and 6.9 × 105 molecule cm-3 for HO2.
RO2-based interference in FAGE measurements of HO2
It has recently been shown that the reaction of alkene-derived
β-hydroxyalkyl peroxy radicals, RO2, with NO inside the HO2
detection cell can lead to interference in FAGE HO2 measurements (Fuchs
et al., 2011; Whalley et al., 2013). The magnitude of the interference
depends on the parent alkene, the residence time and mean temperature inside
the cell, and the amount of NO injected. The interference therefore depends
on the chemical environment and differs between FAGE instruments. In view of
this, the University of Leeds ground-based and aircraft FAGE instruments have
been tested for RO2 interference. Thorough descriptions of the
ground-based experimental method and results, and the results of a modelling
study, are given by Whalley et al. (2013). The strongest interference in the
aircraft instrument measurements was observed for ethene-derived RO2,
amounting to an increase of 39.7 ± 4.8 % in the observed HO2
signal, with a cell pressure of 1.8 Torr, an estimated detection cell
temperature of 255 K (obtained from rotational excitation spectra performed
previously), and [NO]cell= 1014 molecule cm-3.
Whalley et al. (2013) show that the chemistry responsible for the observed
interferences is well known and that a model using the Master Chemical
Mechanism (MCM, version 3.2: Jenkin et al., 1997; Saunders et al., 2003;
Bloss et al., 2005, via http://mcm.leeds.ac.uk/MCM) can reproduce the
interferences once tuned to the conversion efficiency of HO2 to OH in
the FAGE detection cell. Accordingly, Stone et al. (2014b) have applied the
results of the ethene-derived RO2 interference testing in a modelling
study to assess the effect of the interference on the HO2 measurements
made during the RONOCO and SeptEx campaigns. A box model using a detailed MCM
scheme was used to calculate a total potential interference in the RONOCO
HO2 measurements. The model was constrained to the conditions in the
detection cell (1.8 Torr, 255 K, [NO]
∼ 1014 molecule cm-3). Equal concentrations of HO2 and
∑RO2 (sum of all peroxy radicals in the MCM generated from
the parent hydrocarbon) were used to initialise the model. The model run time
was varied until the model-predicted interference from ethene-derived
RO2 radicals was equal to the experimentally determined interference,
thereby tuning the model to the conversion efficiency of HO2 to OH. An
interference factor, f, was calculated for each RO2 in the MCM as
follows:
f=OHHO2+RO2-OHHO2OHHO2,
where OHHO2+RO2 and OHHO2 are the modelled
concentrations of OH produced from the reactions of RO2 and HO2 and
the concentration from HO2 alone, respectively. The greatest
interference was calculated to come from isoprene-derived peroxy radicals,
followed by aromatic compounds and C2 to C5 alkenes. The smallest
modelled interference is from the C1 to C3 alkanes. The
interference factors were applied to model-predicted RO2 speciation and
concentrations for the RONOCO flights. Model-predicted RO2 species were
dominated by CH3O2 (33 %; f= 1.1 %) and HO2
(24 %; f= 0.0 %), with smaller contributions from RO2
derived from iso-butene (12 %; f=0.5 %),
cis-2-butene and trans-2-butene (10 %; f= 0.05 %), and isoprene (2 %; f=7.6 %). RO2 species
with high interference factors were a minor component of the total RO2.
A modelled value of HO2 including the total potential interference,
HO2∗, was
calculated using
HO2∗=HO2mod+∑ifiRO2,imod.
Direct comparison between modelled values of [HO2∗] and the
FAGE-measured values of [HO2] was therefore made possible. The
model-predicted interference during the RONOCO campaign is described by
HO2∗=1.15HO2+2×105 molecule cm-3.
The average model-predicted interference in the HO2 measurements is 14 %. The HO2 measurements made during RONOCO and SeptEx were not
adjusted since speciated RO2 measurements were not available. The
measurements are hereafter referred to as HO2∗.
The magnitude of the RO2 interference can be reduced by lessening the
concentration of NO in the detection cell. This also reduces the instrument
sensitivity to HO2. Since the conversion of RO2 to OH requires at
least two NO molecules, while the conversion of HO2 requires only one
molecule, the ratio of HO2 signal to RO2 signal can be made
favourable by reducing [NO] (Whalley et al., 2013). This effect has been
investigated for the ground-based instrument and will be investigated for the
aircraft instrument prior to future HOx measurement campaigns. An
overview of the laboratory and computational studies of the interference in
different FAGE instruments is given in a recent review by Stone et
al. (2012).
BBCEAS measurements of NO3 and N2O5
NO3 and N2O5 were measured by the University of Cambridge
broadband cavity-enhanced absorption spectroscopy (BBCEAS) instrument. The
instrument was designed and built specifically for the RONOCO project and is
described in detail in Kennedy et al. (2011). A brief description is given
here.
The instrument consists of three 94 cm long high-finesse optical cavities
formed by pairs of highly reflecting mirrors. The cavities are irradiated by
incoherent broadband continuous wave light sources. Two of the cavities, for
the detection of N2O5 and NO3, are irradiated by red
light-emitting diodes (LEDs) centred at 660 nm. The third cavity, for the
detection of NO2, is irradiated by a blue LED centred at 460 nm. The
light from the LEDs is collimated using optical fibres and a focussing lens
at the input of each cavity. A spectrometer, consisting of a spectrograph and
charge couple device (CCD), is positioned at the end of each cavity to
measure the wavelength-dependent intensity of transmitted light.
Ambient air is sampled through a rear-facing inlet on the aircraft fuselage,
positioned approximately 4 m from the aircraft nose and 10 cm from the
aircraft body. The air from the inlet is divided into two flows. The flow
directed to the N2O5 cavity is heated to 120 ∘C to ensure
near complete (> 99.6 %) thermal dissociation of
N2O5 to NO2 and NO3. The cavity itself is heated to
80 ∘C and is used to measure the sum of the concentrations of
ambient NO3 plus NO3 from thermal decomposition of N2O5.
The second flow is unheated and is directed first through the NO3 cavity
and then through the NO2 cavity. Background spectra are recorded at
half-hour intervals during flights by halting the flow of ambient air and
purging the cavities with nitrogen.
NO3 is detected by its strong
B2E′-X2A′2
electronic transition centred at 662 nm. The concentration of NO3 is
determined by separating the finely structured NO3 absorption features
from the broad features caused by Rayleigh and Mie scattering using a fitting
technique analogous to that employed in differential optical absorption
spectroscopy (DOAS). A strong water vapour absorption feature that spectrally
overlaps with NO3 absorption around 662 nm is simulated for the
pressure and temperature measured in the cavity and is removed from the
measured absorption spectrum. The concentration of N2O5 is
determined by subtracting the concentration of ambient NO3 measured in
the unheated cavity from the sum of the concentrations of ambient and
dissociated NO3 measured in the heated cavity.
Contributions to uncertainties in ambient measurements of NO3 and
N2O5, including wall losses of NO3 and N2O5,
temperature- and pressure-dependent absorption cross sections of NO3 and
H2O, and the length of the cavity occupied by the sample, have been
thoroughly investigated in laboratory experiments or addressed in the data
analysis routine. In addition, wall losses of NO3 and N2O5
were determined before and after each flight to account for changes in the
surface properties of the inlet and detection cell walls, which were found to
be negligible. The total uncertainty in the measured concentration of ambient
NO3 was 11 %. The uncertainty in the measured concentration of
ambient N2O5 is determined for each individual ambient measurement,
being dependent on the NO3/ N2O5 ratio, and was on the
order of 15 %. During RONOCO flights, the 1σ limits of detection
for NO3 and the sum of NO3+ N2O5 were 1.1 and 2.4 pptv,
respectively, for a 1 s integration time.
Overview of OH and HO2∗ measurements
FAGE measurements were made on 16 flights during RONOCO and 9 flights during
SeptEx. There was insufficient laser power during flights B534 to B536 in the
summer campaign to measure both OH and HO2∗ by dividing the laser
light between the two cells. OH was therefore not measured during these
flights. Low laser power throughout the summer fieldwork caused relatively
high fluctuations in laser power overall and therefore higher background
variability. This resulted in higher limits of detection for OH
(1.8 × 106 molecule cm-3) and HO2∗
(6.9 × 105 molecule cm-3).
Table 4 summarises the OH and HO2∗ measurements during RONOCO and
SeptEx and gives the instrument's average 1σ limit of detection for a
5 min averaging period. OH was not detected above the limit of detection
during the summer or winter RONOCO flights, resulting in upper limits of
1.8 × 106 and 6.4 × 105 molecule cm-3 for
mean summer and winter concentrations, respectively. These upper limit values
are similar to previously reported night-time OH measurements (Geyer et al.,
2003; Holland et al., 2003; Ren et al., 2003b; Emmerson and Carslaw, 2009).
The mean daytime OH concentration during SeptEx was
1.8 × 106 molecule cm-3, which was above the limit of
detection. The mean HO2∗ mixing ratio was highest during SeptEx
(2.9 pptv) and was higher during summer (1.6 pptv) than during winter
(0.7 pptv). The OH and HO2∗ data sets for RONOCO and SeptEx are
shown as altitude profiles in Figs. 4 and 5, respectively.
Table 5 gives the mean and maximum HO2∗ mixing ratios at different
times of day during summer, SeptEx, and winter. Dawn, day, dusk, and night
are defined by the solar zenith angle as follows: dawn and dusk are between
90 and 102∘ and are distinguished by the time of day; day is between
0 and 90∘; night is between 102 and 180∘.
The mean dusk HO2∗ mixing ratio in summer was higher than the mean
night-time mixing ratio, suggesting that photochemical production was still
active at dusk in summer. The reverse was true for the winter data, with the
highest mean HO2∗ mixing ratio being at night. This suggests that when
photochemical production was suppressed in the winter daytime due to low
photolysis rates, production via reactions of NO3 and O3 with
alkenes was an important route to radical initiation. The RONOCO HO2∗
measurements are similar to night-time, ground-based, urban measurements. For
example, during the TORCH campaign, [HO2] peaked at 1 × 108 molecule cm-3 at night (Emmerson et al., 2007), and during
the PMTACS-NY 2001 field campaign, night-time HO2 concentrations of
8 × 106 molecule cm-3 were measured (Ren et al.,
2003b).
Combined daytime and night-time mean concentrations of OH and mean
mixing ratios of HO2∗ with the FAGE instrument's average 1σ
limits of detection for a 5 min averaging period during the RONOCO and
SeptEx fieldwork.
Altitude profile of OH measured during SeptEx showing 60 s data
(grey points) and mean values in 500 m altitude bins (black lines). Error
bars are 1 SD.
Mean and, in parentheses, maximum HO2∗ mixing ratios measured
during RONOCO and SeptEx.
Altitude profiles of HO2∗ measured in RONOCO and SeptEx
during (a) dawn; (b) day; (c) dusk; and
(d) night, showing 60 s data (grey points) and mean values in
500 m altitude bins (black lines). Error bars are 1 SD.
Flight track of flight B537 on 20 July 2010, coloured by
altitude.
Footprint maps for flight B537 on 20 July 2010, showing
model particle densities (g s m-3) in a 300 m deep layer from the
surface, integrated over 24 h periods beginning (a) 24 h, (b) 48 h, (c) 72 h, and (d) 96 h prior to the flight.
Time series of altitude (top panel, blue), HO2∗ (middle
panel, black, with grey shading representing the uncertainty in the
measurements), O3 (bottom panel, red), and NO3 (bottom panel,
green) during night-time flight B537 on 20 July 2010.
Case study flight B537: high night-time HO2∗ concentrations
The highest HO2∗ concentration (3.2 × 107 molecule cm;-3 13.7 pptv) was measured during night-time flight B537 on
20 July 2010. Take-off from East Midlands Airport was at 22:00 local
time (21:00 UTC, sunset at 20:18 UTC). The flight track, coloured by
altitude, is shown in Fig. 6. The flight involved a profile descent from
3350 to 460 m down the Norfolk coast and a missed approach at Southend
Airport (51.6∘ N, 0.70∘ E). Plumes from European
continental outflow (see Fig. 7) were intersected by a series of runs at
altitudes between 460 m and the upper boundary of the polluted layer.
Flight B537 is an unusual flight within the RONOCO data set, with high
concentrations of CO, O3, NO3, and high temperatures compared to
the values measured during other night-time flights (see Table 3). The
ambient aerosol surface area was significantly higher during B537 (nearly
800 µ m2 cm-3) than during other flights (between 100
and 400 µ m2 cm-3), and the organic aerosol
concentration was significantly enhanced (Morgan et al., 2015). Footprint
maps for flight B537, indicating regions where the sampled air was in contact
with the surface prior to the flight, are shown in Fig. 7. The air sampled
during the flight originated primarily over northern France, Belgium, and
Germany.
A region of high surface pressure was positioned over the UK on the 20 July,
with a mean air pressure of 1012.6 hPa over the 24 h prior to the flight.
The mean air temperature 24 h prior to the flight (22:00, 19 July 2010, to
22:00, 20 July 2010), measured at a number of Met Office weather stations in
Greater London, was 22.6 ∘C, and reached a maximum value of
28.6 ∘C. Wind speeds prior to the flight were low, with an average
value of 4.7 knots (2.4 m s-1). No rainfall was recorded at any of
the Greater London weather stations during the 24 h prior to the flight. At
Heathrow Airport (51.5∘ N, 0.45∘ W), 12.4 h of sunshine
were recorded on the 20 July. High temperatures, combined with low wind
speed, exposure to solar radiation, and little precipitation promote the
formation of ozone as a result of photochemical processing of VOCs emitted at
the surface (e.g. Lee et al., 2006) and offer an explanation for the high
ozone mixing ratios measured during flight B537. Peak surface daytime ozone
concentrations measured in Teddington, London, on 20 July were on the order
of 2.0 × 1012 molecule cm-3 (∼ 78 ppbv) (data
available at http://www.airquality.co.uk). Similar levels were recorded
at a number of locations within Greater London.
Figure 8 shows a time series of altitude, HO2∗, O3, and
NO3 mixing ratios during the flight, demonstrating very similar
behaviour between the two radical species. During the missed approach at
Southend Airport the mixing ratios of HO2∗ and NO3 increased
with decreasing altitude, to reach values of 4.5 and 35 pptv, respectively,
at 50 m above the ground. The maximum HO2∗ and NO3 mixing
ratios were measured over the North Sea east of Ipswich (52.16∘ N,
2.34∘ E) at an altitude of 509 m, in the outflow of the London
plume. Figure 9 shows scatter plots of HO2∗ against NO3 and
O3 during flight B537 and during the other night-time flights during
RONOCO. Strong positive correlation is evident between HO2∗ and
NO3 during B537 (r=0.97), while during the remaining night flights
there is still a significant, though weaker, correlation (r=0.58).
Moderate negative correlation is evident between HO2∗ and O3
during B537 (r=-0.46), with weak positive correlation existing for the
other night-time flights (r= 0.19). The data suggest that NO3 was
an important initiator of HOx radicals during flight B537 and that
O3 played a limited role overall during the night-time flights. Further
investigation of the roles of NO3 and O3 in alkene oxidation and
radical initiation at night is described in Sect. 5.
Oxidation of alkenes and production of HO2: method of analysis
Following the work of Salisbury et al. (2001), the total rates of reaction,
Φ, of O3 and NO3 with the alkenes measured during RONOCO and
SeptEx have been calculated:
ΦO3=∑ialkenekO3+alkiO3alkeneiΦNO3=∑ialkenekNO3+alkiNO3alkenei.
The reactions of O3 and NO3 with alkenes yield OH, HO2, and
RO2 radicals. A consideration of the reaction mechanisms of NO3 and
O3 enables the calculation of the rate of instantaneous production of
HO2 (PHO2) from the reactions of NO3 and
O3 with the alkenes measured during RONOCO, using the chemistry scheme,
rate constants, and branching ratios in the MCM (Jenkin et al., 1997;
Saunders et al., 2003).
HO2∗ vs. (a) NO3 and (b) O3
during flight B537 (blue, filled circles) and during all other night-time
flights (black, open circles). The lines are lines of best fit to the data.
Figure 10 shows a generalised reaction scheme for the reaction of NO3
with an alkene. The reaction between NO3 and an alkene proceeds via the
addition of NO3 to the double bond to form a nitrooxyalkyl radical,
followed by rapid reaction with oxygen to yield a nitrooxyalkyl peroxy
radical, RO2 (shown as a single step in Fig. 10). The RO2 radical
can react with a number of species, of which NO, NO3, and RO2 lead
to the production of an alkoxy radical (RO). Radical termination occurs via
the reaction of RO2 with HO2 to yield a peroxide (ROOH) or with
RO2 to yield carbonyl (RC(O)CH3) and alcohol (RCH2OH)
products. Reaction of RO with oxygen proceeds via the abstraction of a
hydrogen atom to yield HO2 or an aldehyde (RCHO). This generalised
scheme can be applied to the reactions of NO3 with all the alkenes
measured. The rate of instantaneous production of HO2 is found by first
calculating the fraction of RO2 that reacts to produce RO
(FRO) and the fraction of RO that reacts to produce HO2
(FHO2):
FRO=k3NO+k4NO3+0.6k5RO2k2HO2+k3NO+k4NO3+k5RO2FHO2=k6O2k7+k6O2,
where RO2 represents all peroxy radicals. Average values of FRO for
the NO3+ alkene reactions range between 0.50 for trans-2-pentene- and
1-pentene-derived RO2 species and 0.61 for ethene-derived RO2
species. FHO2 varies between 0 and 1 for the alkenes
studied. Overall, the rate of production of HO2
(PHO2) from reactions of NO3 with alkenes is
then given by
PHO2=kiNO3alkenei×FRO×FHO2.
The reaction scheme for the reaction of O3 with alkenes is more
complicated because the number and type of radicals produced in the
O3+ alkene reaction depends on the structure of the alkene. The
simplest case is the reaction of ozone with ethene. Ozone adds to the double
bond to form a five-membered ring called a primary ozonide. Decomposition of
the ozonide yields an excited Criegee intermediate (CH2OO∗) and
a carbonyl compound (in this case formaldehyde, HCHO). The energy-rich
Criegee intermediate can be stabilised by collision with a third body or
undergo decomposition to yield products including OH, CO, and HO2. The
primary ozonide produced in the O3+ propene reaction (see Fig. 11) can
decompose via two channels, yielding carbonyls and Criegee intermediates with
different structures and different products, including RO2. Reaction of
RO2 with NO, NO3, and RO2 (all peroxy radicals) yields RO,
which in turn yields HO2.
Generalised reaction scheme for production of RO2 and
HO2 following reaction of NO3 with an alkene.
Reaction scheme for O3+ propene, showing production of
HO2 and the methyl peroxy radical, CH3O2.
The rates of production of HO2 from reactions of O3 with alkenes
(PHO2) have been calculated as follows:
PHO2,Direct=kiO3alkenei×αHO2PHO2,RO2=kiO3alkenei×αRO2×FRO×FHO2PHO2=PHO2,Direct+PHO2,RO2,
where PHO2,Direct is the rate of direct
HO2 production from Criegee intermediate decomposition, αHO2 is the branching ratio to HO2-producing
channels from the Criegee intermediate, PHO2,RO2 is the rate of HO2 production from RO2
radicals produced in the O3+ alkene reaction, αRO2 is the branching ratio to RO2-producing
channels from the Criegee intermediate, FRO is the fraction of
RO2 radicals that react to produce RO radicals, and
FHO2 is the fraction of RO radicals that react to
produce HO2 radicals, which is equal to 1 for all the alkenes studied.
Average values of FRO for the O3+ alkene reactions range between
0.54 for 1-pentene-derived RO2 species and 0.64 for 1-butene- and
trans-2-pentene-derived RO2 species.
The reactions of RO2 with NO to form RONO2 have been omitted from
the calculations because the branching ratio is small (0.001 to 0.02) for
the radicals studied (Carter and Atkinson, 1989; Lightfoot et al., 1992).
The reaction of CH3O2 with NO2 to form
CH3O2NO2 has been omitted from the calculations, since the
reverse reaction is much faster than the forward direction (kf=6.4× 10-12 s-1; krev=1.08 s-1 at a mean
temperature of 286.5 K during RONOCO).
The primary aims of the analysis presented here are threefold: (1) to
calculate the total rate of initiation through reactions of NO3 and
O3 with alkenes; (2) to determine the relative importance of NO3
and O3 in night-time HO2 production; (3) to investigate differences
in radical production between different seasons and different times of day.
The correlation between [HO2]∗ and [NO3], especially during
flight B537, will be investigated.
PHO2 has been calculated for each alkene measured for
every 60 s data point where all the requisite data were available and
where HO2∗ was above the limit of detection of the FAGE instrument.
Concentrations of RO2 were calculated by scaling the observed HO2∗
concentrations with the RO2/ HO2∗ ratio calculated using a
box model constrained to the concentrations of long-lived species measured
during the flights (Stone et al., 2014b), i.e. RO2,obs= HO2∗,obs× RO2,mod/ HO2∗,mod. The rates of
reaction and rates of production of HO2 presented hereafter are average
values for individual flights, seasons, or times of day.
Average night-time rates of reaction between O3, and NO3
with alkenes during (a) summer and (b) winter RONOCO
flights. Error bars represent the combined uncertainty in the measurements.
ResultsNight-time oxidation of alkenes
Figure 12 shows histograms of the rate of reaction between O3 and
NO3 with individual alkenes during summer and winter, for the night-time
data only. The reactivity of measured alkenes (ΦO3+ΦNO3) was greater by a factor of 2.2 during summer flights
than during winter flights. The reactions of NO3 are largely responsible
for this seasonal difference, since the contribution from
O3+ alkene reactions varies little between summer
(4.1 × 104 molecule cm-3 s-1) and winter
(3.9 × 104 molecule cm-3 s-1). The factor of 4.1
difference between the rate of NO3 reactions in summer
(9.8 × 104 molecule cm-3 s-1) and winter
(2.4 × 104 molecule cm-3 s-1) can be attributed to
the higher mean concentration of NO3 in summer
(5.8 × 108 molecule cm-3) compared to winter
(2.0 × 108 molecule cm-3). This seasonal difference in
NO3 concentrations is attributable to the lower mean night-time
temperature in winter (277.7 K) compared to summer (286.7 K), which
disfavours NO3 in the thermal equilibrium N2O5
NO3+ NO2. Keq[NO2], which determines
[N2O5] / [NO3], is calculated to be 4.8 in summer and 29.6
in winter. At night in summer, ΦNO3 was greater than
ΦO3 by a factor of 2.4, but in winter ΦO3 was
a factor of 1.6 greater than ΦNO3. Figure 12 illustrates the
importance of the butene isomers (within the VOCs measured) in the reactions
of O3 and NO3 and therefore radical initiation and propagation.
Reactions with iso-butene dominated NO3 reactivity in summer
(42 %) and winter (53 %), with trans-2-butene also
contributing significantly (28 % in summer and 32 % in winter).
Reactions of O3 were dominated by trans-2-butene (42 % in
summer and 34 % in winter) and propene (26 % in summer and 38 %
in winter). The importance of these alkenes is attributed to their relatively
high abundances compared to the other alkenes measured, during both summer
and winter, combined with their fast rates of reaction with O3 and
NO3.
For comparison with the reactions of O3 and NO3, the total rate of
reaction of measured alkenes with OH has been calculated using upper limits
on OH concentrations of 1.8 × 106 molecule cm-3 and
6.4 × 105 molecule cm-3 for the summer and winter
flights, respectively, based on the FAGE instrument's limit of detection. The
high upper limits make the total rate of reaction of OH with alkenes,
ΦOH, unrealistically high for both summer
(1.6 × 105 molecule cm-3 s-1) and winter
(7.8 × 104 molecule cm-3 s-1). However, the OH
reactivity will likely be considerably lower than the values calculated using
the OH upper limits. A box model constrained to concentrations of long-lived
species measured during the flights (Stone et al., 2014b) predicts a mean OH
concentration of 2.4 × 104 molecule cm-3, significantly
lower than the upper limits given by the instrument's limit of detection.
Using the mean modelled value for OH gives ΦOH=2.1× 103 molecule cm-3 s-1 for summer and
ΦOH= 2.9 × 103 molecule cm-3 s-1
for winter, indicating a diminished role for OH in alkene oxidation at night,
in agreement with previous studies (e.g. Geyer et al., 2003; Emmerson et al.,
2005).
Average rates of instantaneous production of HO2 from
reactions of O3 and NO3 with alkenes.
HO2 production rate (ΣPHO2)/ 104 molecule cm-3 s-1DawnDayDuskNightSummerNO302.83.8O30.52.21.7Total0.55.05.5WinterNO30.40.40.40.5O31.41.51.21.2Total1.81.91.61.7
Average daytime rates of reaction of (a) O3 and OH
with alkenes during SeptEx and (b) O3 and NO3 with alkenes
during winter. Note the different scales. NO3 was not detected during
daytime SeptEx flights (LOD = 1.1pptv); OH was not detected during
daytime winter flights
(LOD = 6.4 × 105 molecule cm-3). Error bars
represent the combined uncertainties in the measurements.
Daytime oxidation of alkenes
Figure 13 shows histograms of rates of reaction of O3 and OH with
alkenes during SeptEx and of O3 and NO3 with alkenes during winter
RONOCO flights, for daytime data only. OH was detected above the limit of
detection (1.2 × 106 molecule cm-3) during the SeptEx
flights, so the FAGE OH data were included in the calculations, using a
reaction scheme analogous to the one shown in Fig. 10. NO3 was not
detected during the day in SeptEx. NO3 is not expected to be present at
measurable concentrations during daylight hours due to photolysis, but a mean
concentration of 8.3 × 107 molecule cm-3 (3.3 pptv) was
measured during the day in the winter RONOCO flights. These measurements of
low mixing ratios of NO3 may be partly caused by interference from other
daytime species as observed by Brown et al. (2005) or by the variability in
the instrument baseline, which can be on the order of 1–2 pptv during
vertical profiles on the aircraft (Kennedy et al., 2011). This variability is
small compared to the range of NO3 values typically observed during
RONOCO flights (0–50 pptv during summer; 0–10 pptv during winter). During
SeptEx, ΦOH exceeded ΦO3 by a factor of 8.
Ethene and propene were the two most abundant alkenes measured during SeptEx
and contributed significantly to OH reactivity. O3 reactivity with
alkenes was dominated by propene and trans-2-butene (the six most
abundant alkenes measured during SeptEx). NO3 reactivity with alkenes
was dominated by trans-2-butene and isobutene (the three most
abundant alkene measured during winter daytime flights). The total rate of
reaction of O3 and OH with alkenes during daytime SeptEx flights
(3.7 × 105 molecule cm-3 s-1) exceeded the total
rate of reaction of O3 and NO3 during daytime winter RONOCO flights
(6.6 × 104 molecule cm-3 s-1) by a factor of 6 and
was more than double the total rate of reaction of O3 and NO3 with
alkenes during night-time summer flights
(1.4 × 105 molecule cm-3 s-1). In winter daytime
flights, ΦO3 was greater than ΦNO3 by a
factor of 2.4.
Average rates of instantaneous production of HO2 from reactions
of O3 and NO3 with alkenes during (a) summer and
(b) winter RONOCO flights. Error bars represent the combined
uncertainty in the measurements.
Figures 12b and 13b reveal that reactions of O3 dominated alkene
reactivity during both daytime and night-time winter RONOCO flights. The
concentrations of alkenes were generally higher at night, with the total
alkene concentration (sum of concentrations of alkenes measured) being
2.1 × 109 molecule cm-3 in the day and
3.4 × 109 molecule cm-3 at night. The total measured
alkene reactivity (ΦO3+ΦNO3) was
marginally higher during the day, by a factor of 1.04. This difference is
attributable mainly to the change in ΦO3.
Comparison of Figs. 12a and 13b reveals that the total measured alkene
reactivity (ΦO3+ΦNO3) was higher during
the summer night-time flights
(1.4 × 105 molecule cm-3 s-1) than during the
winter daytime flights
(6.6 × 104 molecule cm-3 s-1), indicating a low
oxidising environment during winter daytime. The additional contribution to
measured alkene reactivity from reactions with OH has been calculated using
the OH upper limits as described in Sect. 6.1. Even with this additional,
upper-limit OH reactivity (1.6 × 105 and
1.1 × 105 molecule cm-3 s-1 for summer night-time
and winter daytime, respectively), the total summer night-time alkene
reactivity remains higher than that during winter daytime, confirming the
importance of the summer nocturnal troposphere for the oxidation of the
measured alkenes.
Night-time production of HO2 from reactions of O3 and NO3 with alkenes
Table 6 gives the total rates (ΣPHO2) of instantaneous
production of HO2 from the reactions of O3 and NO3 with
alkenes. NO3 was not detected during the dawn summer RONOCO flights, and
there were no daytime RONOCO flights during summer. NO3 dominated
HO2 production during dusk and night (68 %), in agreement with Geyer
et al. (2003), who found that NO3 was responsible for 53 % of
HO2 production at night in the BERLIOZ campaign. During winter, O3
dominated HO2 production at all times, with a night-time contribution of
70 %. This is in agreement with the results from the winter PMTACS-NY
2004 field campaign (Ren et al., 2006).
The total rate of instantaneous production of HO2 at night was 3.3 times
greater in summer than in winter, with production from O3 decreasing by
a factor of 1.5 and production from NO3 decreasing by a factor of 7.8
between summer and winter. The mean temperature difference of 9 K between
summer and winter is thought to be responsible for the lower NO3
concentrations in winter (2.0 × 108 molecule cm-3,
8.2 pptv, compared to 5.8 × 108 molecule cm-3,
24.5 pptv, in summer), owing to the increased thermal stability of
N2O5, and for the reduced rate of temperature-dependent reactions
between NO3 and alkenes and subsequent reactions. There was very little
difference between summer and winter mean O3 mixing concentrations
(9.6 × 1011 molecule cm-3, 39.6 ppbv, and
9.4 × 1011 molecule cm-3, 38.6 ppbv, respectively).
The production of HO2 via reactions of NO3 and O3 with alkenes
is now examined in more detail. The rate of production from individual
alkenes was calculated and plotted in a histogram, as shown in Fig. 14 for
the summer and winter night-time data. During both summer and winter,
reactions of O3 and NO3 with trans-2-butene were important
sources of HO2, contributing on average 62 % to O3-initiated
HO2 production and 36 % to NO3-initiated production during the
summer and winter flights. Reactions of NO3 with isoprene were important
during summer, contributing 28 % to NO3-initiated production. The
importance of trans-2-butene, despite its relatively low abundance
during summer and winter night-time RONOCO flights (1.8 and 1.7 pptv,
respectively, compared to ethene mixing ratios of 55.0 and 104.5 pptv), is
attributed to its fast rates of reaction with both O3 and NO3
compared to the other alkenes measured. The importance of the
isoprene + NO3 reactions during the summer RONOCO flights is
similarly attributed to its fast rate of reaction with NO3 compared to
the other alkenes measured. In addition there is no aldehyde-forming channel
from the isoprene-derived RO radical (k7 in Fig. 10), so that the yield
of HO2 from RO is equal to 1. The reaction of isobutene with NO3
can proceed via one of two channels to produce two different RO2
radicals but only one channel, with a branching ratio of 0.2, produces
HO2. Isobutene is therefore not a dominant contributor to HO2
production, despite being the single largest contributor to NO3
reactivity during daytime and night-time RONOCO flights (Figs. 12 and 13).
Figure 14 highlights the small change in total production from O3
between summer and winter and the dramatic change in total production from
NO3 between summer and winter.
Reactions of formaldehyde with NO3 were included in the analysis where
formaldehyde data were available (mean HCHO = 955 pptv). The NO3+ HCHO reaction contributed a further 5.5 × 103 molecule cm-3 s-1 (15 %) to HO2 production from NO3 reactions,
so that production from NO3 contributed 79 % of the total
production.
Average rates of instantaneous production of HO2 from
reactions of O3 and NO3 with alkenes during flight B537. Error
bars represent the combined uncertainty in the measurements.
Production of HO2 during flight B537
Flight B537, on 20 July 2010, has been identified as an interesting flight,
with high concentrations of HO2∗
(3.2 × 108 molecule cm-3; 13.6 pptv), ozone (peaking at
1.8 × 1012 molecule cm-3; 89.9 ppbv), and NO3
(peaking at 4.1 × 109 molecule cm-3; 176.9 pptv), and a
strong positive correlation between HO2∗ and NO3 (r=0.97;
see Fig. 9). NO, NO2, and aerosol surface area were also elevated
in-flight during flight B537 compared to their mean summer values. The
highest concentration of ethene
(1.43 × 1010 molecule cm-3; 0.61 ppbv) during the
summer RONOCO flights was measured during B537. ΣPHO2 from
O3+ alkene reactions
(2.6 × 104 molecule cm-3 s-1) was higher in flight
B537 than in all the other summer flights, contributing 42 % of HO2
production, with NO3+ alkene reactions contributing
3.6 × 104 molecule cm-3 (58 %). The total rate of
HO2 production from O3 and NO3 reactions during flight B537
was 6.2 × 104 molecule cm-3 s-1. While this is
higher than the average value of ΣPHO2 for the summer
flights (5.4 × 104 molecule cm-3 s-1), it is not
the highest rate of production during the summer flights. During B534
unusually high concentrations of isoprene, cis-2-butene, and
1,3-butadiene contributed to a total rate of HO2 production of
7.9 × 104 molecule cm-3, which is the highest calculated
value.
Figure 15 shows that the reactions of O3 and NO3 with
trans-2-butene are once again important, contributing 74 % of
ΣPHO2,O3 and 45 % of ΣPHO2,NO3.
The correlation between HO2∗ and NO3 is attributed to the
production of HO2 by reactions of NO3 with alkenes, especially
trans-2-butene. Figure 16 shows HO2∗ vs. the total
instantaneous rate of production from the reactions of O3 and NO3
with alkenes during flight B537 at each 60 s data point during the flight
for which the requisite data were available. Note that the rates plotted in
Fig. 16 are higher than those shown in Fig. 15, where the rates of production
of HO2 from each alkene have been averaged across the whole flight. A
strong positive correlation exists between HO2∗ and both ΣPHO2,O3 (r=0.6) and ΣPHO2,NO3 (r=0.8), indicating the importance of these reactions for the production of
HO2 during this flight.
[HO2∗] vs. total rate of instantaneous production of
HO2 from reactions of (a) O3 and (b) NO3
during flight B537. Correlation coefficients (r) are given in each plot.
The lines are lines of best fit to the data.
Comparison with model results
The observations of OH, HO2∗, NO3, and N2O5 have
been interpreted in the context of night-time oxidation chemistry using a box
model constrained to observations of VOCs, NOx, O3, CO, and other
long-lived species measured during the RONOCO flights (Stone et al., 2014b).
The Dynamically Simple Model of Atmospheric Chemical Complexity (DSMACC)
(Emmerson and Evans, 2009; Stone et al., 2010, 2014b) was initiated with
concentrations of measured species, using a chemistry scheme based on the
Master Chemical Mechanism (MCM, version 3.2: Jenkin et al., 1997, 2003;
Saunders et al., 2003; Bloss et al., 2005, via
http://mcm.leeds.ac.uk/MCM), and was allowed to run to diurnal steady
state. The model output includes concentrations of OH, HO2, NO3,
RO2, and other species. Data from daytime flights, or during dawn or
dusk periods, were not included in the model analysis. Data from flight B537
were also excluded, owing to the atypical observations of HO2∗,
NO3, O3, and other chemical species made during this flight. The
modelling study and results are described in more detail by Stone et
al. (2014b).
The model predicts a mean OH concentration of
2.4 × 104 molecule cm-3 for the summer flights, which is
consistent with the measured OH concentrations for which the instrument's
limit of detection is an upper limit only. The base model underpredicts
HO2∗ by around 200 % and overpredicts NO3 and
N2O5 by 80 and 50 %, respectively. These discrepancies were
investigated by determining the processes controlling radical production and
loss in the model and using those results to improve model performance. Model
production of HO2 is dominated by reactions of RO + O2
(42 %), with a significant contribution from OH + CO (31 %)
despite low OH concentrations at night. ROx (= RO + RO2+ OH + HO2) radical initiation in the model is dominated by
reactions of NO3 with unsaturated VOCs (80 %), with a much smaller
contribution (18 %) from alkene ozonolysis. Modelled HO2 loss is
dominated by its reactions with NO3 (45 %) and O3 (27 %),
both of which are radical propagating routes and which are the dominant
routes to OH production in the model. In fact NO3 was found to control
both radical initiation and propagation in the model.
These results are in general agreement with the results of the analysis
presented in Sect. 6.1, though the model predicts a more important role for
NO3 (80 % of ROx radical production, which is 7.2 times the
contribution from O3+ alkenes) than is predicted by the analysis based
on the observations alone (69 % of HO2 radical production during
summer, which is 2.1 times the contribution from O3+ alkenes). The
model predicts a relatively small role for O3 in both summer and winter.
The model is constrained to measured values of O3 but overpredicts
NO3. The mean measured NO3 night-time mixing ratio was 24.5 pptv
in the summer and 8.2 pptv in the winter. The mean modelled summer and
winter values are 37.4 and 20.7 pptv, respectively. This discrepancy between
modelled and measured NO3 helps to explain the model overprediction of
the role of NO3 in HOx radical initiation during the RONOCO
flights. Modelled NO3 reactivity was dominated by iso-butene
(36 %) and trans-2-butene (27 %), and modelled O3
reactivity was dominated by trans-2-butene (51 %), in agreement
with the night-time alkene reactivities presented in Sect. 6.1.
An improvement to the model predictions of NO3, N2O5, and
HO2∗ was made by increasing the concentration of unsaturated VOCs
in the model. Increasing the total observed alkene concentration by 4 times
resulted in a modelled-to-observed ratio of 1.0 for HO2∗ and of
∼ 1.2 for NO3 and N2O5. Two-dimensional gas
chromatography (GC × GC) analysis of the whole-air samples taken
during RONOCO has revealed a large number of VOCs extra to those routinely
measured (Lidster et al., 2014). Calibration standards for the majority of
these species are not yet available, and so the quantification of their
concentrations is not possible, but their detection confirms that the model
overprediction of NO3 and underprediction of HO2∗ are
attributable to reactions of NO3 with unquantified unsaturated
hydrocarbons.
The presence of unquantified unsaturated VOCs during the RONOCO campaign,
suggested by the model and confirmed by the two-dimensional GC analysis, has
implications for the conclusions drawn from the analysis based on the
observations. The relative contributions of NO3 and O3 to
night-time radical initiation will change with the composition of unsaturated
VOCs in the sampled air, due to the different rates of reaction of NO3
and O3 with different VOC species and the rates of production of
HO2 following these reactions. The model results indicate that the
reaction of NO3 with the unquantified VOCs leads to increased production
of HO2. The role of NO3 in night-time radical production would
therefore be enhanced by the inclusion of the unquantified VOCs in the
observational analysis.
Conclusions
Night-time radical chemistry has
been studied as part of the RONOCO and SeptEx campaigns onboard the BAe-146
research aircraft during summer 2010 and winter 2011. NO3,
N2O5, OH, and HO2∗ were measured simultaneously for the
first time from an aircraft, with OH and HO2∗ being measured by
the University of Leeds aircraft FAGE instrument. OH was detected above the
limit of detection during the daytime SeptEx flights only, with a mean
concentration of 1.8 × 106 molecule cm-3. Upper limits
of 1.8 × 106 and 6.4 × 105 molecule cm-3
are placed on mean OH concentrations for the summer and winter RONOCO (night,
dawn, and dusk) measurement campaigns, respectively. HO2∗ was
detected above the limit of detection during the summer and winter RONOCO
flights and during SeptEx, with a maximum mixing ratio of 13.6 pptv measured
during the night-time flight B537 on 20 July 2010. Mean night-time
HO2∗ mixing ratios were significantly higher in summer than in
winter. Significant concentrations (up to 176.9 pptv) of NO3 were
measured during night-time flights, since the air masses sampled were
sufficiently removed from the surface that the loss of NO3 by reaction
with NO was minimised. The RONOCO flights were therefore an excellent
opportunity to study the role of NO3 in nocturnal oxidation and radical
initiation.
The rates of reaction of O3 and NO3 with the alkenes measured have
been calculated. At night during summer, NO3 dominated alkene
reactivity. Several previous night-time studies have also found NO3 to
be the dominant nocturnal oxidant (e.g. Geyer et al., 2003; Brown et al.,
2011). During night-time winter RONOCO flights the total rate of reaction of
NO3 with alkenes was much reduced, but the rate of reaction of O3
with alkenes was similar to that in summer. During day and night in winter,
O3+ alkene reactions were faster than NO3+ alkene reactions.
Overall, during RONOCO, the combined rate of alkene oxidation by O3 and
NO3 was highest at night during summer.
The calculation of the rates of the instantaneous production of HO2 from
reactions of O3 and NO3 with alkenes, using measurements made
during the flights, has revealed that night-time production was dominated by
NO3 in summer and by O3 in winter. The rate of instantaneous
production of HO2 from reactions of NO3 with alkenes decreased
significantly from summer to winter (87 %), whereas production from
O3+ alkene reactions was similar in summer and winter, decreasing by
just 31 %. Strong positive correlation between HO2∗ and
NO3, especially during flight B537, is attributed to the production of
HO2 from reactions of NO3 with alkenes, particularly
trans-2-butene and other isomers of butene.
Significant concentrations of HO2∗ were detected at night, with
the highest HO2∗ concentration (13.6 pptv) being measured during a
summer night-time flight, indicating that HOx radical chemistry remains
active at night under the right conditions. The role of HOx is
diminished in the low photolysis winter daytime atmosphere, with alkene
ozonolysis being primarily responsible for oxidation and radical initiation,
in agreement with previous studies (e.g. Heard et al., 2004; Emmerson et
al., 2005). Both the analysis presented here and the results of the box
modelling study by Stone et al. (2014b) indicate that in air masses removed
from sources of NO, NO3 plays an important role in the oxidation of
alkenes and radical initiation at night, in agreement with previous studies
(e.g. Brown et al., 2011). Alkene ozonolysis also plays a significant role
in nocturnal oxidation in agreement with Salisbury et al. (2001), Geyer et
al. (2003), Ren et al. (2003a, 2006), Emmerson et al. (2005), and
Volkamer et al. (2010). The balance between the roles of NO3 and
O3 was controlled in part by [NO3], with colder winter
temperatures forcing the NO3–N2O5 equilibrium towards
N2O5.
The total rate of reaction of O3 and NO3 with alkenes during
night-time summer flights
(1.4 × 105 molecule cm-3 s-1) was higher by a
factor of 2.1 than during daytime winter flights
(6.6 × 104 molecule cm-3 s-1). Whilst it should be
remembered that measurements at different times of day and in different
seasons reflect composition changes in air masses (such as the abundance of
reactive alkenes), this result supports the hypothesis that oxidation of
certain VOCs, in particular the reactive alkenes, in the nocturnal summer
atmosphere can be as rapid as in the winter daytime atmosphere.
A box model of night-time chemistry constrained to measurements of long-lived
species has been used to investigate the night-time chemistry sampled during
RONOCO (Stone et al., 2014b). The base model underpredicts HO2∗
and overpredicts NO3. These discrepancies were minimised by increasing
the concentration of alkenes in the model, thereby increasing the reaction of
NO3 with alkenes and the production of HO2. The presence of
unquantified unsaturated VOCs has been confirmed by 2D-GC analysis, though
the exact nature and concentrations of the `missing' species are unclear. The
inclusion of these species in the analysis presented in this paper would
likely increase the role of NO3 in the oxidation of alkenes and
production of HO2 at night.
Acknowledgements
This work was funded by the UK Natural Environment Research Council
(NE/F004664/1). The authors would like to thank ground staff, engineers,
scientists, and pilots involved in RONOCO for making this project a success.
Airborne data were obtained using the BAe 146-301 Atmospheric Research
Aircraft (ARA) flown by Directflight Ltd. and managed by the Facility for
Airborne Atmospheric Measurements (FAAM), which is a joint entity of the
Natural Environment Research Council (NERC) and the Met
Office. Edited by: S. Brown
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