Peroxy radicals in the summer free troposphere: seasonality and 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 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.

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Atmos. Chem. Phys. Discuss., 8, 17841–17889, 2008
www.atmos-chem-phys-discuss.net/8/17841/2008/
© Author(s) 2008. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmospheric
Chemistry
and Physics
Discussions
This discussion paper is/has been under review for the journal Atmospheric Chemistry
and Physics (ACP). Please refer to the corresponding final paper in ACP if available.
Peroxy radicals in the summer free
troposphere: seasonality and
heterogeneous loss
A. E. Parker1,*, P. S. Monks1, K. P. Wyche1, J. M. Balzani-L¨ o¨ ov2, J. Staehelin2,
S. Reimann3, G. Legreid3, M. K. Vollmer3, and M. Steinbacher3
1Department of Chemistry, University of Leicester, Leicester, UK
2Institute for Atmospheric and Climate Science, ETH, Zurich, Switzerland
3EMPA, Dubendorf, Switzerland
*now at: PC2A, Universit´ e des Sciences et Technologies de Lille, Lille, France
Received: 27 June 2008 – Accepted: 27 August 2008 – Published: 30 September 2008
Correspondence to: P. S. Monks (p.s.monks@leicester.ac.uk)
Published by Copernicus Publications on behalf of the European Geosciences Union.
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Abstract
The sum of peroxy radicals (HO2+ΣiRiO2) and supporting trace gases were measured
on the Jungfraujoch (3580ma.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 0.0063s−1can be derived for the peroxy
radical loss in the snowy conditions that could be 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±4pptv is calculated for the ozone compensation
point for the snow free days.
5
10
1Introduction
Peroxy radicals (HO2+ΣiRiO2) are key intermediates and chain carriers in the photo-
chemical cycling of ozone in the troposphere (e.g. Monks, 2005). Peroxy radicals are
formed via the oxidation of anthropogenic and biogenic species in the atmosphere such
as CO, CH4and other organic compounds. Ozone is produced via the peroxy radical
catalysed oxidation of NO to NO2and subsequent photolysis of NO2, whilst ozone can
also be destroyed through reaction with HO2(Monks, 2005). Owing to the short lifetime
of peroxy radicals (HO2has a lifetime on the order of a minute in clean air, much less
than a minute in polluted air (Monks, 2005)), they give a good indication in combina-
tion with NO of in-situ photochemical ozone production and loss. In addition, the self-
and cross-reactions of peroxy radicals to form peroxides (e.g. H2O2) are a major sink
for HO2and OH (Reeves and Penkett, 2003). The measurement of peroxy radicals
thus remains of central importance in atmospheric chemistry, and rapid progress has
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8, 17841–17889, 2008
Peroxy radicals in the
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troposphere
A. E. Parker et al.
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been made in recent years with many deployments utilising various techniques for both
ground and airborne studies (Cantrell et al., 2003; Edwards et al., 2003; Green et al.,
2003, 2006; Heard and Pilling, 2003; Mihelcic et al., 2003; Mihele and Hastie, 2003;
Salisbury et al., 2001; Sommariva et al., 2004).
The high altitude research station Jungfraujoch is situated on a saddle point located
at 7◦59?2??E, 46◦32?53??N and 3580ma.s.l. between two mountains, the Jungfrau
to the north-west (4158ma.s.l.) and the M¨ onch to the south-east (4099ma.s.l.) in
the Swiss Alps. The air pressure at the Sphinx laboratory ranges from 619mbar to
675mbar, with a mean pressure of 653.3mbar and air temperature ranges from −37◦C
to +10◦C, with a mean temperature of −8.2◦C. In winter and very often in spring and au-
tumn the Jungfraujoch is located in the free troposphere (Lugauer et al., 1998; Zanis et
al., 2007, 2000a). As the Jungfraujoch is situated on a mountain saddle point, it experi-
ences air from essentially two directions, north-west and south-east. To the north-west
air arrives at the Jungfraujoch from the Swiss Plateau and Northwestern Europe, whilst
from the south-east it arrives from the direction of Italy and the Po Valley.
Although the site at the Jungfraujoch is sometimes influenced by boundary layer air
masses (Lugauer et al., 1998; Reimann et al., 2004; Zanis et al., 2003; Zellweger et
al., 2000), it is ideal for the study of the European free troposphere. The free tropo-
sphere (FT) is the region of the atmosphere located between the planetary boundary
layer and the tropopause. The colder temperatures and lack of deposition in the free
troposphere compared to the boundary layer lead to longer chemical lifetimes for many
species (e.g. ozone) in the free troposphere compared to the boundary layer. As much
of the transport of chemical species within the atmosphere takes place in the free tro-
posphere, the combination of long-range transport and longer chemical lifetimes mean
that the chemistry of the free troposphere is of fundamental importance in determin-
ing the chemical composition of regions remote from pollutant source regions (Europe,
2008).
This paper describes a series of peroxy radical and supporting measurements made
at the Jungfraujoch during the summer of 2005. The measurements compliment a
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Peroxy radicals in the
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series of similar measurements made that have mapped the transition from winter-time
to spring-time photochemistry at the Jungfraujoch (Zanis et al., 1999, 2003, 2000a,
2000b). The measurements are explored in term of the production and loss processes
of the peroxy radicals and the concomitant production of loss of ozone in the seasonal
context. The summertime 2005 campaign was marked by periods of significant snow-
fall, the effect of which on the radical chemistry is also explored.
5
2 Measurement details
2.1 The PERCA instrument
The Chemical Amplification technique was introduced by Cantrell in the early 1980s
(Cantrell and Stedman, 1982, 1984) and has been widely deployed since then (Cantrell
et al., 1993; Fleming et al., 2006a, b; Green et al., 2003, 2006; Mihele and Hastie,
2003; Monks et al., 1998; Zanis et al., 2000a). In brief, the PERCA technique utilises
the radical catalysed conversion of NO and CO into NO2and CO2respectively via
addition of NO (3ppmv) and CO (6% v/v) to the inlet region. NO2is subsequently de-
tected via aqueous luminol (5-amino-2,3-dihydro-1,4-pthalazinedione) solution chemi-
luminescence at λ=424nm with an improved LMA-3 detector as described by Green
et al. (2006).
RO2+NO → RO+NO2
(R1)
RO+O2→ R?CHO+HO2
(R2)
HO2+NO → OH+NO2
(R3)
OH+CO → H+CO2
(R4)
H+O2+M → HO2+M(R5)
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Peroxy radicals in the
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CO+NO+O2→ CO2+NO2
The yield of NO2is equal to CL×[HO2+ΣiRiO2+ΣiRiO+OH], where CL is the chain
length, i.e. the number of HO2/OH conversion cycles that occur before termination.
A significant background NO2signal is also observed from other sources such as the
reaction of ozone with the reagent NO. Assuming a chain length of 100 (i.e. each rad-
ical molecule produces 100 molecules of NO2) and a radical concentration of 20pptv,
the radical chain cycle would produce 2ppbv of NO2. Under polluted conditions, am-
bient ozone could contribute up to 100ppbv of NO2. Consequently, it is necessary to
periodically measure only the background NO2produced by means other than peroxy
radical conversion. This is achieved by injecting CO downstream of the NO injection
point. OH produced as a result of Reaction (R3) cannot be recycled into HO2as Reac-
tion (R4) and Reaction (R5) do not take place, and instead some OH reacts with NO in
a chain termination step (Reaction R7), whilst remaining radicals are lost to the walls
of the inlet.
(R6)
OH+NO+M → HONO+M
A flow of inert gas (nitrogen) is added in place of the CO so that in amplification mode
NO and CO are injected upstream of N2, and in background mode NO and N2are
injected upstream of CO. This ensures that the properties of the sample gas flow re-
main unchanged in both operation modes. It also helps to reduce pressure pulsing in
the detected signal and allows the flows to settle again more quickly after switching.
The sensitivity of the PERCA instrument to humidity is well known (Mihele and Hastie,
1998; Mihele et al., 1999) and consequently a water correction as per Salisbury et
al. (2002) has been applied to all data in this work. This correction is relatively small
out of the boundary layer as humidity is generally low.
The uncertainty in the PERCA measurements leads to a calculated accuracy of
about 35%. The sources of error and estimated magnitude of error are as follows:
Radical calibration (j(CH3I) measurement (15%), mass flow controller calibration (zero
air and CH3I) (5%), CH3I permeation tube leak (5%), volume of photolysis cell (5%)),
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Peroxy radicals in the
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NO2detection (background variability (10%), thermal instability of luminol (20%) and
the water correction factor error (20%)). This gives an uncertainty value of 18% for
radical calibration and 22.4% for NO2calibration. Therefore, the overall radical mea-
surement uncertainty is 35%.
2.2Photolysis frequency measurements
5
A range of photolysis frequencies were measured during this work with a fast, mono-
lithic single-monochromator spectral radiometer with a temperature-stabilized diode
array detector. The spectral radiometer is a commercial instrument available from Me-
terologie Consult GmbH, Germany. A full description of the spectral radiometer de-
ployed during these measurements, including characterisation and calibration details
is given in (Edwards and Monks, 2003). A range of photolysis frequencies were derived
from the actinic flux measured with the spectral radiometer as shown in Table 1 along
with the reference used for quantum yield and absorption cross-section data.
10
2.3Other measurements
Many additional measurements were carried out during this work and are summarized
in Table 1. Ozone was measured with a commercially available Thermo Environmen-
tal Instruments 49c UV absorption instrument, whilst CO was measured with another
commercial instrument, a HORIBA APMA-360 which utilises the nondispersive infrared
(NDIR) technique. NO, NOxand NOywere all measured with a commercially avail-
able Ecophysics CraNOx instrument equipped with two temperature controlled CLD
770ALpptv chemiluminescence detectors. NOxwas measured by conversion to NO
on a PLC 760 photolytic convertor, whilst NOywas converted on a gold catalyst heated
to 300◦C with the addition of 2% CO (Messer-Griesheim GmbH, 99.997%) to act as a
reducing agent. NO2was then reported by subtraction of NO from NOx(Zellweger et
al., 2000).
Peroxy acetyl nitrate (PAN) was measured with a commercially available Meterologie
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troposphere
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Consult GmbH gas chromatograph (GC) with electron capture detection (ECD) and
calibration unit. The technique is described in Schmidt et al. (1998), and the instrument
deployed at the Jungfraujoch during 2005 is described in Balzani L¨ o¨ ov et al. (2007).
Formaldehyde was measured using an instrument based on the Hantzsch fluores-
cence technique similar to the AERO LASER CH2O analyser AL4021. The Hantzsch
technique is a liquid phase technique, and as such requires the HCHO to be analysed
to be transferred from the gas to liquid phase. Once in the liquid phase the HCHO
is reacted with 2,4-pentadione and NH3to produce 3,5-diacetyl-1,4-dihydrolutidine
(DDL). DDL fluoresces at 510nm if excited at 412nm and thus can be measured.
The Hantzsch technique is described in Kelly and Fortune (1994), and the instrument
deployed at the Jungfraujoch during 2005 is described in Balzani L¨ o¨ ov et al. (2007).
Methane was measured via GC-FID and NMHCs (n-pentane, iso-pentane, n-hexane,
benzene, toluene, trimethylbenzene, n-butane, isobutane, butadiene, ethylbenzene,
isoprene, o-xylene and m-, p-xylene) were measured with an Agilent 5793N GC-MS
with a pre-concentration system after Simmonds et al. (1995). The system is described
in detail by Reimann et al. (2004).
There were also measurements of OVOCs (methanol, ethanol, propanol, butanol,
2-methyl-3-buten-2-ol (MBO), propanal, isopropanal, butanal, pentanal, hexanal, ac-
etaldehyde, benzaldehyde, acrolein, methylacetate, ethylacetate, butylacetate, methyl
vinyl ketone (MVK), methyl ethyl ketone(MEK), methacrolein, acetone and methyl ter-
tiary butyl ether (MTBE)) made at the Jungfraujoch with a GC-MS system. The instru-
ment was based on an Agilent HP 6890-HP 5973N and constructed by EMPA (Legreid
et al., 2007, 2008). In addition relative humidity, temperature, pressure, wind speed
and wind direction were measured by the Swiss Meteorological Service.
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Peroxy radicals in the
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3Results
3.1Radicals and tracers observations
The campaign time series of peroxy radicals, O3, CO, NO and NO2are shown in Fig. 1
and Fig. 2 shows the time series of peroxy radicals with j(O1D). It can be seen that
peroxy radicals in the period 6 September 2005–12 September 2005 inclusive in ad-
dition to some days after this period are suppressed relative to the period up to and
including 5 September 2005. The suppression of peroxy radicals corresponds to days
on which there was heavy snowfall. In order to quantify the effect of snowfall, a met-
ric was devised by which days with mean relative humidity over 76% were considered
“snowy”, whilst those with a mean relative humidity under 76% were considered “snow
free”. Table 3 shows which days were assigned to which category.
The data in Fig. 3a illustrates that during snow fall peroxy radical concentrations were
much lower. The mean peak radical value is approximately 3 times greater on clear
days than when it was snowing. It would be expected that j(O1D) is also reduced on
the snowy days owing to attenuation of light by snow. However j(O1D) is not reduced to
the same extent as the peroxy radicals. The mean diurnal cycles for j(O1D) for “snow
free” and “snowy” days are shown in Fig. 3b.
The mean diurnal cycle for “snow free” days divided by the mean diurnal cycle for
“snowy” days for the sum of peroxy radicals, j(O1D), ozone and carbon monoxide are
shown in Fig. 4a. In contrast to peroxy radicals and j(O1D), the relative concentrations
of ozone and carbon monoxide are very similar on both “snow free” and “snowy” days.
This similarity is not true however for some other trace species such as NO, NO2,
HCHO and PAN. Figure 4b shows the mean diurnal cycle for “snow free” days divided
by the mean diurnal cycle for “snowy” days for NO, NO2, HCHO and PAN. It can be
seen that these species (and especially the NOx) are enhanced on the “snowy” days
compared to the “snow free” days.
Owing to the topography at the Jungfraujoch, air approaches from essentially two
directions only, the north west (towards Kleine Scheidegg and the Swiss plateau) and
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south east (down the glacier towards Italy). This channelling is born out in a polar plot
of peroxy radicals against wind direction as shown in Fig. 5. Figure 5 is filtered by
wind speed such that only those points where wind speed was greater than 3ms−1are
included and it can be seen that for the majority of points, the wind was either from the
north west or south east.
On those days classified as “snowy”, the air was mostly from the south east and on
“snow free” days the air was predominantly from the northwest. There is clearly then
a fundamental chemical difference in air masses depending on origin, with air from the
south east arriving predominantly on “snowy” days and showing suppressed peroxy
radicals and enhanced NOxcompared to “snow free” days which are characterised
by air of north west origin. The suppression of peroxy radicals is not however solely
because of the enhanced NOx, as shall be seen later.
Back trajectories have been run using a method adapted for use in the complex to-
pography of the Swiss Alps (Legreid et al., 2008), and have been used to investigate air
mass origins for the two periods of interest, “snow free” and “snowy”. The trajectories
show that for days classified as “snow free”, the air masses arriving at the Jungfraujoch
although from a wide range of starting positions, are from north of the alps and prin-
cipally the Swiss plateau. However, on the days classified as “snowy” the air masses
are from the south over northern Italy and the heavily polluted Po Valley. Examples of
these back trajectories are shown in Fig. 6.
The Po Valley is the river basin of the Po River in northern Italy that flows from
Monviso in the Alps to the Adriatic Sea south of Venice. The Po Valley is the major
industrial centre of north Italy, with major cities situated within it including Turin, Milan,
Padua, Brescia and Verona. The Po Valley is a hotspot for pollution (Martilli et al.,
2002; Neftel et al., 2002; Silibello et al., 1998), with anthropogenic pollution levels that
are amongst the highest in Europe (Dommen et al., 2002; Thielmann et al., 2002);
previous measurement campaigns found up to 185ppbV of ozone in the foothills of the
Alps north of Milan (Pr´ evˆ ot et al., 1997).
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Peroxy radicals in the
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3.2Peroxy radical production and loss
The suppression of peroxy radicals in southerly air could be due to two effects – less
production or greater loss. Reduced j(O1D) on “snowy” days owing to the reduced
visibility caused by the snow would lead to reduced production, however this may be
counteracted by the generally more humid air encountered on these days. Enhanced
NOxin the southerly air would lead to increased peroxy radical loss from the increase in
reaction of peroxy radicals with NOx. The industrialised Po Valley could be the source
of this extra NOx, and there is also the possibility of NOxproduction from lightning
as the moist air from the Mediterranean region rises rapidly as it reaches the Alps,
leading to lightning. There could also be another unknown peroxy radical sink other
than self-reaction or reaction with NOxcausing the reduction in peroxy radical.
In order to further elucidate the reasons for the suppressed peroxy radicals in the
“snowy” air, a steady state analysis of the production and loss of peroxy radicals dur-
ing this campaign has been carried out using the method described by Mihele and
Hastie (2003). In steady state it can be shown that
?
2.kself.α(1 + β)
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[HO2+ΣiRiO2] ≈
2f.j(O1D)[O3](1 + γ)
=
?
f.j(O1D)[O3]
kself
?
1
α
?
1
1 + β
?
1 + γ
(1)
Where α describes the partitioning between HO2and RO2, thus:
[HO2]
[HO2+ΣiRiO2]
β is a measure of the dominant loss process for peroxy radicals, γ is a measure of
additional peroxy radical production from sources other than ozone photolysis and
kselfis a composite rate constant for the peroxy radical loss process via self-reaction.
β=LNOx/LSRand therefore if β is less than one, then the dominant loss process is
radical self-reaction rather than loss through reaction with NOx. A γ of zero indicates
no excess production, whilst a γ of less than zero indicates that a higher concentration
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Peroxy radicals in the
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of peroxy radicals is calculated from ozone photolysis than is actually observed (Mihele
and Hastie, 2003).
A time series for β is shown in Fig. 7a. It can be seen that β is almost exclusively
greater than 1, indicating that loss of peroxy radicals is virtually always dominated by
reaction with NOxrather than peroxy radical self-reactions. As the rate of reaction of
[HO2+ΣiRiO2]+NO is of a similar order to the rate of peroxy radical self reaction, only
relatively low concentrations of NOxare required for the loss of peroxy radicals through
reaction with NOxto become important or even dominate. For a diurnally averaged
time series, the mixing ratio of NO is greater than that of peroxy radicals by a minimum
of 2–3 times midway through the day, and more at other times. The value of β reflects
the suppression of peroxy radicals and enhancement of NOxon “snowy” compared to
“snow free” days, with median β four times larger on “snowy” days.
As mentioned, γ is a measure of additional peroxy radical production from sources
other than ozone photolysis. Figure 7b is a plot of γ, from which it can be seen that on
days designated “snowy” there is a marked decrease in γ compared to days designated
“snow free”.
The median γ for “snow free” days is γ=0.19, indicating that whilst the majority of
peroxy radical production is from the photolysis of ozone to produce O(1D) and then
subsequent reaction with water vapour to produce OH, followed by OH initiated oxida-
tion of CO, CH4and other organic compounds, there is an appreciable level of peroxy
radical production from other sources.
However, for “snowy” days the median γ value is γ=−0.49, indicating that either
there is an overestimation of peroxy radical production or an underestimation of peroxy
radical losses. As production via ozone photolysis is likely to be fairly well constrained
(as j(O1D) and water vapour were both measured) and any additional production terms
would lead to γ being even more negative, it is likely that there is an underestimation
of peroxy radical losses.
The radical loss term can be written as:
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L[HO2+ΣiRiO2]=2.kself.α(1+β)[HO2+ΣiRiO2]2
(3)
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where the loss from self-reaction is from the following reactions:
HO2+HO2→ H2O2+O2
(R8)
HO2+RO2→ ROOH+O2
(R9)
RO2+RO2→ ROOR+O2
which leads to the following self-reaction loss rate,
(R10)
LSR=2k8[HO2][HO2]+2k9[HO2][RO2]+2k10[RO2][RO2](4)
Assuming that k8≈k9, and that k10is slow enough that Reaction (R10) can be dis-
carded, Eq. (4) reduces to that found in Mihele and Hastie (2003):
LSR=2.kself.α.[HO2+ΣiRiO2]2
(5)
5
However, the assumption in the derivation of the self-reaction loss rate that k8≈k9
only holds if RO2is solely in the form of CH3O2, neglecting the contribution of other
species. If the acetylperoxy radical (CH3C(O)O2) is introduced as a generic fast react-
ing peroxy radical to test the effect of additional RO2species in the form of the ratio
of δ=?CH3C(O)O2
LSR=2.
k13α (1−α)δ+k14(1−α)2δ (1−δ)+k15(1−α)2δ2?
where
?/([CH3O2]+[CH3C(O)O2]), then the self-reaction loss rate reduces
k8α2+k11α (1−α)(1−δ)+k12(1−α)2(1−δ)2+
10
thus:
?
.[HO2+ΣiRiO2]2
(6)
HO2+CH3O2→ products(R11)
CH3O2+CH3O2→ products(R12)
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HO2+CH3C(O)O2→ products
CH3O2+CH3C(O)O2→ products
CH3C(O)O2+CH3C(O)O2→ products
If δ is set to 0.5, that is half of the RO2present is in the form of acetylperoxy (the
remaining half being CH3O2), then at 298K, the combined peroxy radical self-reaction
rate is approximately double the self-reaction rate when δ is zero. Further to this, the
rate constant for the reaction of acetylperoxy with NO is approximately 2.6 times faster
than that for methylperoxy (CH3O2) with NO at 298K. Consequently, the loss rate of
peroxy radicals through reaction with NOxwill also be greater than if all the RO2were
CH3O2.
The inclusion of acetylperoxy at δ=0.5 has a dramatic affect on γ−γ for “snow free”
days is increased from γ=0.19 to γ=0.92, and for “snowy” days γ is increased from
γ=−0.49 to γ=−0.24. Thus it can be seen that even with the questionable assumption
of half of the organic peroxy radicals present being peroxyacetyl the increase in loss
rate due to peroxy radical self-reaction is not enough to account for the negative γ
observed. In fact, the atmosphere at the Jungfraujoch is relatively clean. Figure 8
shows the percentage loss of OH due to CO, CH4, HCHO and all other VOCs and
OVOCs, as defined by the sum of the median values of kOH[VOC] of interest divided by
the sum of median values of kOH[VOC] for all species, where kOHis the rate of reaction
of the VOC with OH. Isopropanal and butanol are excluded as they are both in common
use at Jungfraujoch and thus their measurements may be contaminated.
Nonetheless, even if there were a significant proportion of non-methyl fast reaction
organic peroxy radicals present, γ is still significantly negative for “snowy” days, in-
dicating a missing loss term. To investigate the missing loss term, Eq. (1) has been
rearranged and expanded for γ to include an extra loss term Lex, thus
?[HO2+?
17853
(R13)
(R14)
(R15)
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10
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γ=
iRiO2]2.α.(1+β).kself+Lex/2
f.j(O1D).[O3]
?
−1(7)

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ACPD
8, 17841–17889, 2008
Peroxy radicals in the
summer free
troposphere
A. E. Parker et al.
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where
Lex=kex[HO2+ ΣiRiO2]
In order to obtain γ=0 for “snowy” days from Eq. (7), kexwas adjusted until the median
value of γ for “snowy” days became zero. As such, kexis a pseudo first order rate
constant that incorporates all extra loss processes after peroxy radical self-reaction
losses and loss through reaction with NOx. To obtain median γ=0 for “snowy” days,
kexwas set so that kex=0.0063s−1. A question then arises as to the origin of these
extra “losses”.
One possibility is heterogeneous losses on the snow particles. Previous studies
have outlined the potential importance of HO2uptake on aerosol particles in the ma-
rine boundary layer (Haggerstone et al., 2005), on water (Hanson et al., 1992; Morita et
al., 2004), on water ice (Cooper and Abbatt, 1996), and on aqueous sulphate aerosol
(Cooper and Abbatt, 1996; Thornton and Abbatt, 2005) amongst others. Aerosol up-
take of HO2has been invoked to explain model overestimation of measured values
(Smith et al., 2006; Sommariva et al., 2006, 2004) as has cloud processing (Olson et
al., 2004). There have been suggestions of falling ice crystals scavenging aerosols
and gases absorbing onto the crystal surface or even diffusing into its bulk (Grannas
et al., 2007). However, there have been no reported studies of the loss or otherwise of
HO2on snow.
(8)
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3.3 Additional peroxy radical production sources
20
It has been seen that the median of γ is greater than zero for “snow free” days, thereby
signifying an additional production route over ozone photolysis. This additional pro-
duction is significant with γ=0.19, and could come from a variety of sources including
ozonolysis of alkenes, NO3oxidation of alkenes and the photolysis of species such as
HCHO and HONO. NO3oxidation of alkenes is only significant at night due to the rapid
photolysis of NO3by sunlight (Monks, 2005) save for in very polluted atmospheres
where daytime NO3can become important (Geyer et al., 2003). Unfortunately HONO
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Page 15

ACPD
8, 17841–17889, 2008
Peroxy radicals in the
summer free
troposphere
A. E. Parker et al.
Title Page
AbstractIntroduction
ConclusionsReferences
TablesFigures
??
??
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and NO3were not measured during this campaign, so it is not possible to determine
the possible contribution they make to peroxy radical concentrations. In addition there
were very sparse measurements of alkenes, so it is not possible to estimate the contri-
bution of alkene ozonolysis to peroxy radical concentrations.
Formaldehyde and j(HCHO) were measured during the campaign. The median pro-
duction rate from the radical production channel of HCHO photolysis on the “snow free”
days where HCHO measurements are available show that formaldehyde contributes up
to 13.7% as much as the median production from ozone photolysis on the same days.
This corresponds to 0.14 of the total γ of 0.19, and as such suggests that photoly-
sis of formaldehyde is the most significant radical production source other than ozone
photolysis.
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10
3.4 Seasonality
Previous peroxy radical measurements have been made at the Jungfraujoch in April
and May 1996 during the Free Tropospheric Experiment 1996 (FREETEX’96) (Zanis et
al., 1999, 2000b), in March and April 1998 during Free Tropospheric Experiment 1998
(FREETEX’98) (Carpenter et al., 2000; Zanis et al., 2000a) and in February and March
2001 during Free Tropospheric Experiment 2001 (FREETEX’01) (Zanis et al., 2003).
The mean daytime maximum peroxy radical concentration as reported by Zanis et
al. (2003) for FREETEX’96, ’98, and ’01 along with the mean daytime maximum per-
oxy radical concentration for all days and for “snow free” days for this campaign are
presented in Table 4 along with the average diurnal cycle for peroxy radicals from
FREETEX’96, ’98 and ’01 (Zanis et al., 2003) and all days and “snow free” days from
this work in Fig. 9.
It can be seen that the gradual seasonal increase in maximum mean daily peroxy
radicals observed by Zanis et al. (2003) continues through to the data taken during this
work if only “snow free” days are considered. However, if all days are considered then
there is a decrease back to the maximum mean seen in mid-March to mid-April.
Zanis et al. (2000a) calculated theoretical peroxy radical concentrations for the 15th
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