ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-14475-2016Daytime formation of nitrous acid at a coastal remote site in Cyprus
indicating a common ground source of atmospheric HONO and NOMeuselHannahhttps://orcid.org/0000-0002-0062-7976KuhnUweReiffsAndreasMallikChinmayhttps://orcid.org/0000-0002-1428-9453HarderHartwighttps://orcid.org/0000-0002-6868-714XMartinezMonicaSchuladenJanBohnBirgerhttps://orcid.org/0000-0003-4177-3934ParchatkaUweCrowleyJohn N.https://orcid.org/0000-0001-8669-0230FischerHorstTomscheLauraNovelliAnnahttps://orcid.org/0000-0003-2077-7573HoffmannThorstenJanssenRuud H. H.https://orcid.org/0000-0001-5129-7535HartogensisOscarPikridasMichaelhttps://orcid.org/0000-0002-8131-2369VrekoussisMihalishttps://orcid.org/0000-0001-8292-8352BourtsoukidisEfstratioshttps://orcid.org/0000-0001-5578-9414WeberBettinahttps://orcid.org/0000-0002-5453-3967LelieveldJoshttps://orcid.org/0000-0001-6307-3846WilliamsJonathanPöschlUlrichhttps://orcid.org/0000-0003-1412-3557ChengYafanghttps://orcid.org/0000-0003-4912-9879SuHangh.su@mpic.dehttps://orcid.org/0000-0003-4889-1669Max Planck Institute for Chemistry, Multiphase Chemistry Department,
Mainz, GermanyMax Planck Institute for Chemistry, Atmospheric Chemistry Department,
Mainz, GermanyInstitute for Energy and Climate Research (IEK-8), Research Center
Jülich, Jülich, GermanyJohannes Gutenberg University, Inorganic and Analytical Chemistry,
Mainz, GermanyWageningen University and Research Center, Meteorology and Air
Quality, Wageningen, the NetherlandsCyprus Institute, Energy, Environment and Water Research Center,
Nicosia, CyprusInstitute of Environmental Physics and Remote Sensing – IUP,
University of Bremen, Bremen, GermanyCenter of Marine Environmental Sciences – MARUM, University of
Bremen, Bremen, GermanyHang Su (h.su@mpic.de)22November20161622144751449324June201625July201627October20165November2016This 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/16/14475/2016/acp-16-14475-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/14475/2016/acp-16-14475-2016.pdf
Characterization of daytime sources of nitrous acid (HONO) is crucial to
understand atmospheric oxidation and radical cycling in the planetary
boundary layer. HONO and numerous other atmospheric trace constituents were
measured on the Mediterranean island of Cyprus during the CYPHEX
(CYprus PHotochemical EXperiment) campaign in summer 2014. Average volume
mixing ratios of HONO were 35 pptv (±25 pptv) with a
HONO / NOx ratio of 0.33, which was considerably higher than
reported for most other rural and urban regions. Diel profiles of HONO showed
peak values in the late morning (60 ± 28 pptv around 09:00 local
time) and persistently high mixing ratios during daytime
(45 ± 18 pptv), indicating that the photolytic loss of HONO is
compensated by a strong daytime source. Budget analyses revealed unidentified
sources producing up to
3.4 × 106 molecules cm-3 s-1 of HONO and up to
2.0 × 107 molecules cm-3 s-1 NO. Under humid
conditions (relative humidity > 70 %), the source strengths of HONO and NO exhibited
a close linear correlation (R2=0.72), suggesting a common source that may
be attributable to emissions from microbial communities on soil surfaces.
Introduction
Nitrous acid (HONO) is an important component of the nitrogen cycle, being
widespread in the environment. Either in its protonated form (HONO or
HNO2) or as nitrite ions (NO2-) it can be found not only in the gas
phase, on aerosol particles, in clouds and in dew droplets but also in soil,
seawater and sediments (Foster et al., 1990; Rubio et al., 2002; Acker et
al., 2005, 2008; Bianchi et al., 1997). It plays a key role in the oxidizing
capacity of the atmosphere, as it is an important precursor of the OH
radical, which initiates most atmospheric oxidations. OH radicals react with
pollutants in the atmosphere to form mostly less toxic compounds (e.g.
CO + OH → CO2+ H2O; Levy, 1971). Volatile organic
compounds (VOCs) react with OH, contributing to formation of secondary
aerosols (SOAs), which can serve as cloud condensation nuclei (CCN; Arey et
al., 1990; Duplissy et al., 2008). Furthermore OH oxidizes SO2 to
H2SO4, which condense subsequently to form aerosol particles (Zhou
et al., 2013). In this way HONO has an indirect effect on the radiative budget
and climate. In the first 2–3 h following sunrise, when OH production from
other sources (photolysis of O3 and formaldehyde) is relatively low,
photolysis of HONO can be the major source of OH radicals as HONO
concentrations may be high after accumulation during nighttime (Lammel and
Cape, 1996; Czader et al., 2012; Mao et al., 2010). On average up to 30 %
of the daily OH budget in the boundary layer is provided by HONO photolysis
(Alicke et al., 2002; Kleffmann et al., 2005; Ren et al., 2006), but it has been
reported as high as 56 % (Ren et al., 2003), with ambient HONO mixing
ratios ranging from several parts per trillion by volume (pptv)
in rural areas up to a few parts per billion by volume (ppbv) in highly
polluted regions (Acker et al., 2006a, b; Costabile et al., 2010; Li et al.,
2012; Michoud et al., 2014; Spataro et al., 2013; Su et al., 2008a; Zhou et
al., 2002a).
In early studies, atmospheric HONO was assumed to be in a photostationary
state (PSS) during daytime controlled by the gas-phase reaction of NO and OH
(Reaction R1) and two loss reactions, which are the photolysis (Reaction R2)
and the reaction with OH (Reaction R3).
OH+NO→HONOHONO⟶hv(300-405nm)OH+NOHONO+OH→NO2+H2O
However, field measurements in remote and rural locations as well as urban
and polluted regions found several-times-higher daytime HONO concentrations
than model predictions, suggesting a large unknown source (Kleffmann et al.,
2003, 2005; Su et al., 2008a, 2011; Sörgel et al., 2011a; Michoud et al.,
2014; Czader et al., 2012; Wong et al., 2013; Tang et al., 2015; Oswald et
al., 2015) even after considering direct emission of HONO from combustion
sources (Kessler and Platt, 1984; Kurtenbach et al., 2001). Heterogeneous
reactions on aerosols have been proposed as an explanation for the missing
source. The hydrolysis (Reaction R4; Finlayson-Pitts et al., 2003) and redox
reactions of NO2 have been intensively investigated on different kinds
of surfaces such as fresh soot, aged particles or organic-coated particles (Ammann et
al., 1998; Arens et al., 2001; Aubin and Abbatt, 2007; Bröske et al., 2003;
Han et al., 2013; Kalberer et al., 1999; Kleffmann et al., 1999; Kleffmann
and Wiesen, 2005; Lelievre et al., 2004). Minerals like SiO2,
CaCO3, CaO, Al2O3 and Fe2O3 showed a catalytic
effect on the hydrolysis of NO2 (Kinugawa et al., 2011; Liu et al.,
2015; Wang et al., 2003; Yabushita et al., 2009). Different kinds of surfaces
(humic acid and other organic compounds, titanium dioxide, soot) can be
photochemically activated, which leads to enhanced NO2 uptake and HONO
production (Reaction R5, George et al., 2005; Langridge et al., 2009; Monge
et al., 2010; Ndour et al., 2008; Ramazan et al., 2004; Stemmler et al.,
2007; Kebede et al., 2013). The photolysis of particulate nitric acid
(HNO3), nitrate (NO3-) and nitro-phenols (R-NO2) leads to
HONO formation as well (Baergen and Donaldson, 2013; Bejan et al., 2006;
Ramazan et al., 2004; Scharko et al., 2014; Zhou et al., 2003, 2011). But
these reactions cannot account for the HONO levels observed during daytime
(Elshorbany et al., 2012).
2NO2+H2O→HONO+HNO3surface⟶hve-⟶NO2NO2-⟶H2OHONO+OH-
On the other hand, soil nitrite, either biogenic or non-biogenic, has been
suggested as an effective source of HONO (Su et al., 2011; Oswald et al.,
2013; Mamtimin et al., 2016). Depending on soil properties such as pH and
water content and according to Henry's law, HONO can be released (Donaldson et
al., 2014b; Su et al., 2011). This is consistent with field flux measurements
showing HONO emission from the ground rather than deposition as is the case
for HNO3 (Harrison and Kitto, 1994; Kleffmann et al., 2003; Ren et al.,
2011; Stutz et al., 2002; VandenBoer et al., 2013, 2014; Villena et al., 2011; Zhou
et al., 2011). In a recent study, Weber et al. (2015) measured large HONO and
NO emissions from dryland soils with microbial surface communities (so-called
biological soil crusts). Many studies have shown decreasing HONO mixing
ratios with altitude in the lowest few hundred meters of the troposphere, due
to respective short atmospheric lifetime compared to vertical transport time
(Wong et al., 2012, 2013; Vogel et al., 2003; VandenBoer et al., 2013, 2014; Zhang
et al., 2009; Young et al., 2012). According to the modeling results of Wong
et al. (2013), we estimate that the ground HONO source could be important for
up to 200–300 m a.g.l. This indicates that HONO is more relevant for the
OH budget close to the surface than in high-altitude air masses.
Several field studies also show a correlation of the unknown HONO source with
solar radiation or the photolysis frequency of NO2JNO2 (Su
et al., 2008a; Soergel et al., 2011a; Wong et al., 2012; Costabile et al.,
2010; Michoud et al., 2014; Oswald et al., 2015; Lee et al., 2016). This
correlation can be explained either by the aforementioned photosensitized
reactions or by temperature-dependent soil–atmosphere exchange (Su et al.,
2011). According to Su et al. (2011), the release of HONO from soil surfaces
is controlled by both the soil (biogenic and chemical) production of nitrite
and the gas–liquid-phase equilibrium. The solubility is strongly
temperature-dependent, resulting in higher HONO emissions during noontime
and high-radiation JNO2 periods, and lower HONO emissions or
even HONO deposition during the nighttime as further confirmed by VandenBoer
et al. (2015). This temperature dependence exists not only for equilibrium
over soil solution but also for adsorption–desorption equilibrium over
dry and humid soil surfaces (Li et al., 2016).
In this study we measured HONO and a suite of other atmospherically relevant
trace gases in a coastal area on the Mediterranean island of Cyprus in summer
2014. Due to low local anthropogenic impact and low NOx levels in aged air
masses, but high solar radiation, this is an ideal site at which to investigate
possible HONO sources and to gain a better understanding of HONO chemistry.
Instrumentation
HONO was measured with a commercial long-path absorption photometry
instrument (effective light path 1.5 m, LOPAP, Quma, Wuppertal, Germany).
LOPAP has a collecting efficiency of > 99 % for HONO and a detection
limit of 4 pptv at a time resolution of 30 s. To avoid potential
interferences induced by long inlet lines and heterogeneous formation or loss
of HONO on the inlet walls (Kleffmann et al., 1998; Zhou et al., 2002b; Su et
al., 2008b), HONO was collected by a sampling unit installed directly in the
outdoor atmosphere, i.e., placed on a mast at a height of 5.8 m above ground
installed at the edge of a laboratory container. Furthermore, the LOPAP has
two stripping coils placed in series to reduce known interfering signals
(Heland et al., 2001). In the first stripping coil HONO is quantitatively
collected. Due to the acidic stripping solution, interfering species are
collected less efficiently but in both channels. The true concentration of
HONO is obtained by subtracting the inferences quantified in the second
channel (in this study the average is 1 pptv, at most 5 pptv) from the
total signal obtained from the first channel. For a more detailed description
of LOPAP, see Heland et al. (2001). This correction of chemical interferences
ascertained excellent agreement with the (absolute) differential optical
absorption spectroscopy (DOAS) measurements, both in a smog chamber and under
urban atmospheric conditions (Kleffmann et al., 2006). A possible
interference from peroxynitric acid (HNO4) has been proposed (Liao et
al., 2006; Kerbrat et al., 2012; Legrand et al., 2014), but this will be
insignificant at the high temperatures during the CYPHEX (CYprus
PHotochemical EXperiment) campaign, at which HNO4 is unstable. The
stripping coils are temperature-controlled by a water-based thermostat, and
the whole external sampling unit is shielded from sunlight by a small plastic
housing. The reagents were all high-purity-grade chemicals, i.e.,
hydrochloric acid (37 %, for analysis; Merck), sulfanilamide (for
analysis, > 99 %; AppliChem) and N-(1-naphthyl)-ethylenediamine
dihydrochloride (for analysis, > 98 %; AppliChem). For calibration
Titrisol® 1000 mg NO2- (NaNO2
in H2O; Merck) was diluted to 0.0015 and 0.005 mg L-1
NO2-. For preparation all solutions were used, and for cleaning of
the absorption tubes 18 MΩ H2O was used. The accuracy of the
HONO measurements was 10 %, based on the uncertainties of liquid and gas
flow, concentration of calibration standard and regression of calibration.
NO and NO2 measurements were made with a modified commercial
chemiluminescence detector (CLD 790 SR), originally manufactured by ECO
Physics (Duernten, Switzerland). The two-channel CLD based on the
chemiluminescence of the reaction between NO and O3 was used for
measurements of NO and NO2. NO2 was measured as NO using a
photolytic converter from Droplet Measurement Technologies (Boulder, USA). In
the current study, data were obtained at a time resolution of 5 s. The CLD
detection limits (determined by continuously measuring zero air at the measuring
site) for NO and NO2 measurements were 5 and 20 pptv, respectively for
an integration period of 5 s. O3 was measured with a standard UV
photometric detector (Model 49, Thermo Environmental Instruments Inc.) with a
detection limit of 1 ppb. Data are reported for an integration period of
60 s. The total uncertainties (2σ) for the measurements of NO,
NO2 and O3 were determined to be 20, 30 and 5 %, respectively,
based on the reproducibility of in-field background measurements,
calibrations, the uncertainties of the standards and the conversion
efficiency of the photolytic converter (Li et al., 2015).
Map of location: the red star shows the location of Ineia
and the measuring site. The four red points mark the main cities of Cyprus:
Nicosia, Larnaca, Limassol and Paphos (clockwise ordering). Map produced by
the Cartographic Research Lab University of Alabama; map of Cyprus: Google
Maps.
OH and HO2 radicals were measured using the HydrOxyl Radical measurement
Unit based on fluorescence Spectroscopy (HORUS) setup developed at the Max
Planck Institute for Chemistry (Mainz, Germany). HORUS is based on laser-induced
fluorescence–fluorescence assay by gas expansion (LIF-FAGE)
technique, wherein OH radicals are selectively excited at low pressure by
pulsed UV light at around 308 nm, and the resulting fluorescence of OH is
detected using gated microchannel plate (MCP) detectors (Martinez et al.,
2010; Hens et al., 2014). The HORUS instrument had an inlet pre-injector
(IPI) (Novelli et al., 2014) which allows the periodic addition of propane to
scavenge the atmospheric OH radicals. This procedure allows the removal of
potential interference species. HO2 is estimated by converting
atmospheric HO2 into OH using NO and detecting the additional OH
formed. The instrument is calibrated by measuring signals from known amounts
of OH and HO2 generated by photolysis of water vapor in humidified zero
air. The accuracy (2σ) of the OH measurements was 29 %, and the
precision (1σ) was 4.8×105 molecules cm-3.
Photolysis frequencies were determined using a spectroradiometer (Metcon
GmbH) with a single monochromator and 512 pixel CCD array as a detector
(275–640 nm). The thermostatted monochromator–detector unit was attached
via a 10 m optical fiber to a 2-Π integrating hemispheric quartz dome.
The spectroradiometer was calibrated prior to the campaign using a 1000 W
National Institute of Standards and Technology (NIST)
traceable irradiance standard. J values were calculated using molecular
parameters recommended by the IUPAC and NASA evaluation panels (Sander et
al., 2011; IUPAC, 2015). The J value for HONO was not corrected for
upwelling UV radiation and is estimated to have an uncertainty of
∼ 10 % (Bohn et al., 2008).
Aerosol measurements were also performed during the campaign. In this study
particulate nitrate and aerosol surface data were used. These were detected
by high-resolution time-of-flight aerosol mass spectrometer
(HR-ToF-AMS, Aerodyne Research Inc., Billerica, MA USA), and scanning mobility
particle sizer (SMPS 3936, TSI, Shoreview, MN USA) and aerodynamic particle
sizer (APS 3321, TSI), respectively. The mobility- and aerodynamics-based size
distributions were combined based on the algorithm proposed by Khlystov et
al. (2004).
The VOCs including α-pinene, β-pinene, isoprene, Δ3-carene, limonene and DMS (dimethyl sulfide) were detected by a commercial
gas chromatography–mass spectrometry (GC-MS) system (MSD 5973; Agilent
Technologies GmbH) coupled with an air sampler and a thermal desorber unit
(Markes International GmbH). The VOCs were trapped at 30 ∘C on a
low-dead-volume quartz cold trap (U-T15ATA; Markes International GmbH) filled
with two-bed sorbent (Tenax TA and Carbograph I). The cold trap was heated to
320 ∘C, and the sample was transferred to a 30 m GC column (DB-624,
0.25 mm I.D., 1.4 µm film; J&W Scientific). The temperature of
the GC oven was programmed to be stable at 40 ∘C for 5 min and then
rise at a rate of 5 ∘C min-1 up to 140 ∘C.
Thereafter, the rate was increased to 40 ∘C min-1 up to
230 ∘C, where it was stabilized for 3 min. Each sample was taken
every 45 min; calibrations, using a commercial gas standard mixture
(National Physical Laboratory, UK), were performed every 8–12 samples.
Carbon monoxide was measured by infrared absorption spectroscopy using a room
temperature quantum cascade laser at a time resolution of 1 s. Data are
reported as 60 s averages with a total uncertainty of ∼ 10 %,
mainly determined by the uncertainty of the NIST standard used (Li et al.,
2015).
Meteorological parameters (temperature, relative humidity (RH), wind speed and
direction, pressure, solar radiation, precipitation) were detected by
the weather station Vantage Pro2 from Davis Instruments.
Airflow conditions during the CYPHEX campaign: (a) measured
local wind direction, (b) back trajectories calculated with NOAA
Hysplit model showing examples for the two main air mass origins (48 h,
UTC = LT - 3 h).
Besides GC-MS all other operating instruments had time resolutions between
20 s and 5 min. For most analyses in this study the data were averaged to
10 min. When GC-MS data were included in the evaluation, 1 h averaged data
were used.
Site description
Cyprus is a 9251 km2 island in the southeast Mediterranean Sea (Fig. 1).
The measuring site was located on a military compound in Ineia, Cyprus
(34.9638∘ N, 32.3778∘ E), about 600 m above sea level and
approximately 5.5–8 km from the coastline (main wind direction: W–SW). The
field site is characterized by light vegetation cover, mainly comprising
small shrubs like Pistacia lentiscus, Sarcopoterium spinosum and
Nerium oleander; herbs like Inula viscosa and Foeniculum vulgare; and few typical Mediterranean trees
like Olea europaea, Pinus sp. and Ceratonia siliqua. The
area within a radius of about 15 km around the station is only weakly
populated. Paphos (88 266 citizens) is located 20 km south of the field
site; Limassol (235 000), Nicosia (325 756) and Larnaca (143 367) are 70,
90 and 110 km to the E–SE, respectively (population data according to
statistical service of the Republic of Cyprus,
http://www.cystat.gov.cy, census of population October 2011). During
the campaign (7 July–3 August 2014), clear-sky conditions prevailed and
occasionally clouds skimmed the site. No rain was observed, but the elevated
field site was impacted by fog during nighttime and early morning due to
adiabatic cooling of ascending marine humid air masses. Temperature ranged
from 18 to 28 ∘C. Within the main local wind direction of SW
(Fig. 2a) there was no direct anthropogenic influence, resulting in clean
humid air from the sea. Analysis of 48 h back trajectories showed mainly two
source regions of air mass origin (Fig. 2b). For approximately half
(46 %) of the campaign the air masses came from west of Cyprus, spending
most of their time over the Mediterranean Sea prior to arriving at the site.
During the remaining half of the campaign air masses originated from north of
Cyprus, from eastern European countries (Turkey, Bulgaria, Rumania, Ukraine
and Russia). Westerly air masses have been shown to exhibit lower
concentration of gaseous and aerosol pollutants than the predominant
northerly air masses that typically reach the site (Kleanthous et al., 2014).
They spent more time over continental terrestrial surface and were likely to
be additionally affected by biomass burning events detected in eastern Europe
within the measurement periods (FIRMS, MODIS, web fire mapper, Fig. S1 in the
Supplement). Previous back-trajectory studies in the eastern Mediterranean
support this assumption (Kleanthous et al., 2014; Pikridas et al., 2010).
Most of the time the advected air mass was loaded with high humidity as a
result of sea breeze circulation. Two periods of about 4 days with lower
relative humidity occurred. These two situations will be contrasted below.
Results
The concentrations of HONO and other atmospheric trace gases as well as
meteorological conditions observed on Cyprus from 7 July to 3 August 2014 are
shown in Fig. 3. In general, low trace gas mixing ratios were indicative of
clean marine atmospheric boundary conditions, as pollutants are oxidized by
OH during the relatively long air transport time over the Mediterranean Sea
(more than 30 h), and without significant impact of direct anthropogenic
emissions.
Measured variables during the whole campaign from
7 July to 4 August 2014. (a) Meteorological data (temperature, T;
relative humidity, RH; wind direction, wd; wind speed, ws), O3 and CO indicate stable
conditions; in the lower panel the bar indicates the air mass origin: bright
blue represents westerly, while the brownish color represents northerly. (b) Observed
mixing ratios of HONO, NO2 and NO, and the
photolysis frequency JHONO and the HONO / NOx
ratio. The yellow and blue boxes reflect the dry and the humid periods,
respectively.
Ambient HONO mixing ratios ranged from below detection limit (< 4 pptv)
to above 300 pptv. Daily average HONO was 35 pptv (±25 pptv; 1σ standard deviation). The daily average
NO2 and NO mixing ratios were 140 ± 115 and 20 ± 35 pptv,
respectively, but showed intermittent peaks up to 50 ppbv when sampling air
was streamed from the diesel generator used to power the station, from the
access route or the parking lot by local winds (easterly, Fig. S2). These
incidents, which account for 4 % of the campaign time, were classified as
local air pollution events and were omitted from analysis. Mean O3 and
CO mixing ratios were 72 ± 12 ppb and 98 ± 11 ppbv,
respectively. OH radicals ranged from below detection limit
(1 × 105 molecules cm-3) during nighttime to
8 × 106 molecules cm-3 during daytime (see Fig. S3).
Daytime HO2/ OH ratio ranged from 100 to 150. The mixing ratios of
NO2, O3 and CO varied in unison and were significantly (p<0.05)
higher during periods when air masses originated from eastern Europe
(brownish bar in Fig. 3a, lower panel), indicative of air pollution and
shorter transport times compared to western Europe (NO2: northerly:
144 ± 130 pptv, westerly: 127 ± 106 pptv; O3: northerly:
74 ± 11 ppbv, westerly: 66 ± 12 ppbv; CO: northerly:
101 ± 9 ppbv, westerly: 90 ± 10 ppbv). In contrast, NO and HONO
mixing ratios were slightly higher when air masses came from western Europe
and over the sea (NO: northerly: 17 ± 35 pptv, westerly:
20 ± 44 pptv; HONO: northerly: 32 ± 26 pptv, westerly:
38 ± 22 pptv).
Diel variation of meteorological data (temperature, T;
relative humidity, RH), NO and NO2 mixing ratios, the
photolysis rate for HONO JHONO and HONO mixing ratios
(pink: measured; violet: daytime photostationary state (PSS); grey: nighttime
heterogeneous NO2 conversion) and HONO / NOx
ratio for (a) average for period when RH was above 60 % (blue box in Fig. 3)
and (b) average for dry period when RH was below 60 % (yellow box in
Fig. 3). Error bars represent standard deviation of diel mean.
Besides two different air mass origins, two periods with different behavior
of relative humidity were identified, as illustrated by blue and yellow boxes in
Fig. 3a and b. In both periods we found northerly and westerly air mass
origins. The diel profiles of trace gas mixing ratios and meteorological
variables of the humid period (blue box) are shown in Fig. 4a, and the ones of
the dry period (yellow box) in Fig. 4b. During the drier period HONO
concentrations were stable and low (6 pptv) during nighttime, while mean nighttime
HONO mixing ratios during the humid period (Fig. 4a) showed an expected slow
increase of about 20 pptv (from 20 to 40 pptv), as anticipated from
heterogeneous production and accumulation within a nocturnal boundary layer
characterized by a stable stratification and low wind speed (Acker et al.,
2005; Su et al., 2008b; Li et al., 2012). During both periods, but more
pronounced in the drier period, HONO rapidly increased by a factor of 2
within 2 h after sunrise and then slowly decreased until sunset.
Similar profiles were also observed for other trace gases, like isoprene or
DMS, which are transported in upslope winds. Strong HONO morning peaks and
high daytime mixing ratios suggest a strong daytime source, compensating the
short atmospheric lifetime (15 min) caused by fast photolysis.
Mean NO mixing ratios were close to the detection limit (5 pptv) at night
and increased after sunrise (06:00 local time, LT) to mean values of 60 pptv
(peak 150 pptv) at 09:00 LT, prior to declining for the rest of the day
until sunset (20:00 LT). In the absence of local NO sources low nighttime
values are a result of the conversion of NO to NO2 by O3, which was
continuously high (Hosaynali Beygi et al., 2011). The diel profiles of NO
mixing ratios followed closely those of HONO mixing ratios. This similarity
and their dependency on relative humidity are suggestive of a common source
for both reactive nitrogen species.
NO2 mixing ratios were somewhat lower during nighttime, but in general
the diel variability remained in a narrow range between 100 and 200 pptv.
Likewise, the diel courses of O3 and CO mixing ratios revealed
relatively low day–night variability in a range of 65–75 and 90–100 ppb,
respectively.
Discussion
Low-NOx conditions at this remote field site in photochemically aged
marine air were found to be an ideal prerequisite to trace as yet undefined
local HONO sources. On Cyprus, diel profiles of HONO showed peak values in
the late morning and persistently high mixing ratios during daytime, as has
been reported for some other remote regions (Acker et al., 2006a; Zhou et
al., 2007; Huang et al., 2002). This is not the case for rural and urban
sites, where atmospheric HONO mixing ratios are normally observed to
continuously build up during nighttime, presumably due to heterogeneous
reactions involving NOx and decline in the morning due to strong
photodissociation (e.g., Elshorbany et al., 2012, and references therein).
The diel HONO / NOx ratio (Fig. 4a, b, third panel) shows
consistently high values during the humid period (Fig. 4a) and significant
diel variation for the dry case (Fig. 4b) with higher values during daytime. The
ratio (average of 0.33 and peak values greater than 2) is higher than that
reported for most other regions, suggesting a strong impact of local HONO
sources. Elshorbany et al. (2012) investigated data from 15 different urban
and rural field measurement campaigns around the globe, and came up with a
robust representative mean atmospheric HONO / NOx ratio as low as
0.02. However, high values were observed at remote mountain sites, with mean
values of 0.23 (up to ≈ 0.5 in the late morning; Zhou et al., 2007)
or 0.2–0.4 at remote Arctic/polar sites (Li, 1994; Zhou et al., 2001; Beine
et al., 2001; Jacobi et al., 2004; Amoroso et al., 2010). Legrand et
al. (2014) observed HONO / NOx ratios between 0.27 and 0.93 during
experiments with irradiated Antarctic snow, depending on radiation wavelength,
temperature and nitrate content. Elevated HONO / NOx ratios at low
NOx levels show the importance of HONO formation mechanisms other than
heterogeneous NOx reactions.
Nighttime HONO accumulation
Between 18:30 and 07:30 LT HONO has an atmospheric lifetime of more than
45 min and [OH] is low, just about
1 × 105 molecules cm-3, so that the calculation of HONO
at photostationary state [HONO]pss (Reactions R1–R3) at
nightis not appropriate. Instead, nighttime HONO concentrations can
be estimated due to heterogeneous reaction of NO2 described in Eq. (1)
(Alicke et al., 2002, 2003; Su et al., 2008b; Sörgel et al., 2011b). Three
studies in different environments from a rural forest region in eastern Germany
(Sörgel et al., 2011b) and a non-urban site in the Pearl River Delta, China
(Su et al., 2008b), to an urban, polluted site in Beijing (Spataro et al.,
2013) found a conversion rate of about 1.6 % h-1
(1.1–1.8 % h-1).
[HONO]het=[HONO]evening+0.016h-1[NO2]Δt
[HONO]het denotes the accumulation of HONO by heterogeneous
conversion of NO2, [HONO]evening the measured HONO
concentration at 20:30 LT, [NO2] the measured average NO2
concentration between 20:30 and 07:30 LT, and Δt time span in hours.
Measured and calculated HONO mixing ratios are compared in Fig. 4 (upper
panel). During the humid period, during nighttime the estimated (according
Eq. 1; Fig. 4a, upper panel, grey line) and observed HONO mixing ratios are
in good agreement (R2=0.9). During the drier period the observed HONO
mixing ratios were lower than the ones calculated with a NO2 conversion
rate of 1.6 % h-1. Here the approach for the nighttime conversion
frequency by, e.g., Alicke et al. (2002, 2003), Su et al. (2008b) or
Sörgel et al. (2011b) (rate=HONOt2-HONOt1Δt⋅NO2‾) was used. The
7-day average conversion rate for the dry nights was 0.36 % h-1
(Fig. 4b, upper panel, black line), comparable to results of Kleffmann et
al. (2003) reporting a conversion rate of 6 × 10-7 s-1
(0.22 % h-1) for rural forested land in Germany.
As already mentioned above, it is apparent that HONO mixing ratios under
low-RH conditions during nighttime were much lower than under humid conditions,
and HONO morning peaks were most pronounced (compare Fig. 4a and b:
humid/dry). HONO (Donaldson et al., 2014a) and NO2 (Wang et al.,
2012; Liu et al., 2015) uptake coefficients have recently been reported to be
much stronger for dry soil and at low RH, respectively, which is in line with
HONO on Cyprus being close to the detection limit on nights with low relative
humidity. On the other hand, it has been shown on glass and on soil proxies
that the yield of HONO formation from NO2 on surfaces is low under dry
conditions but sharply increases at RH > 30 % (Liu et al., 2015) or
> 60 % (Finlayson-Pitts et al., 2003). On Cyprus the strong morning
HONO peaks after dry nights were accompanied by an increase in relative
humidity from 40 to 80 %. Deposited and accumulated NO2 on dry soil
surfaces could be released as HONO at high rates under elevated-RH
conditions. In contrast, in a humid regime HONO mixing ratios were
continuously high during nighttime and showed less pronounced morning peaks,
suggesting lower nighttime deposition of NO2 and lower HONO emissions in
the morning, respectively.
As morning HONO peak mixing ratios were most pronounced after dry nights on
Cyprus, our observations are to some extent contradictory to earlier results
that have proposed that dew formation on the ground surface may be
responsible for HONO nighttime accumulation in the aqueous phase, followed by
release from this reservoir after dew evaporation the next morning (Zhou et
al., 2002a; Rubio et al., 2002; He et al., 2006). We cannot rule out that the
latter could have contributed to nighttime accumulation of HONO during humid
conditions, as we had no means to measure dew formation at the site, and high
daytime HONO mixing ratios were observed under all humidity regimes. However,
kinetic models of competitive adsorption of trace gases and water onto
particle surfaces predict exchange behavior explicitly distinct from the
liquid phase (Donaldson et al., 2014a). The nitrogen composition in thin
water films (few water molecular monolayers) is complex, including HONO, NO,
HNO3, water–nitric acid complexes, NO2+ and N2O4
(Finlayson-Pitts et al., 2003). With only small amounts of surface-bound
water, nitric acid is largely undissociated HNO3 and is assumed to be
stabilized upon formation of the HNO3–H2O complexes (hydrates),
which have unique reactivity compared to nitric acid water aqueous solutions,
where it is dissociated H+ and NO3- ions (Finlayson-Pitts et
al., 2003). Likewise, HONO formation rates in surface-bound water are about
4 orders of magnitude larger than expected for the aqueous-phase reaction
(Pitts et al., 1984).
Diel HONO profiles very similar to those on Cyprus with a late-morning
maximum and late-afternoon/early-evening minimum have been observed at the
Meteorological Observatory Hohenpeissenberg, a mountain-top site in Germany
(Acker et al., 2006a) and by Zhou et al. (2007) at the summit of Whiteface
Mountain in New York State. For the latter study, formation of dew could be
ruled out as relative humidity was mostly well below saturation. Zhou et
al. (2007) argued that the high HONO mixing ratios during morning and late
morning can be explained by mountain up-slope flow of polluted air from the
cities at the foot of the mountain that results from ground surface heating.
On Cyprus the sea breeze, driven by the growing difference between sea and
soil surface temperature, brings air to the site which interacted with the
soil surface and vegetation and is loaded by respective trace gas emissions.
This is endorsed by the simultaneous increase of DMS and isoprene, markers
for transportation of marine air and emission by vegetation. In the late
afternoon, when the surface cools, down-welling air from aloft would
dominate, being less influenced by ground surface processes. Zhou et
al. (2007) could show that noontime HONO mixing ratios and average NOy
during the previous 24 h period were strongly correlated, much better than
instantaneous HONO / NOy or HONO / NOx, which is in line
with N accumulation on soil surfaces as discussed above.
HONO budget analysis for (a) the humid and (b) the
dry period. SOH+NO (black) stands for the formation rate of HONO
via the reaction of NO and OH, SHet_NO2 (yellow) is
the formation rate for the heterogeneous reaction of NO2 (conversion
rate a=1.6 % h-1; b=0.36 % h-1),
Lphot (green) and LOH+HONO (blue) are the loss
rates via photolysis, and the reaction with OH and Sunknown is
the unknown source. Error bars indicate standard deviation of diel mean.
Daytime HONO budget
During daytime (07:30 to 18:00 LT, with HONO lifetime being between 10 and
30 min), [HONO]PSS, the photostationary HONO concentration resulting
from gas-phase chemistry, can be calculated according to Eq. (2) (Kleffmann et
al., 2005):
[HONO]PSS=k1OH[NO]k2[OH]+JHONO,
where k1 and k2 are the temperature-dependent rate constants for
the gas-phase HONO formation from NO and OH and the loss of HONO by reaction
of HONO and OH, respectively (Atkinson et al., 2004; e.g., at 23.0 ∘C
a typical temperature during this study k1≈1.36×10-11 cm3 s-1; k2≈6.01×10-12 cm3 s-1). JHONO is the photolysis
frequency of HONO, which was measured with a spectroradiometer. [NO] is the
observed NO concentration. Since OH data were available only on a few days,
diel variations of [OH] were averaged (see Fig. S3).
As has been previously established by many other studies (Su et al., 2008a;
Michoud et al., 2014; Sörgel et al., 2011a), homogeneous gas-phase chemistry
alone fails to reflect observed HONO mixing ratios. Observed daytime values
were up to 30 times higher than calculated based on PSS, indicating strong
additional local daytime sources of HONO. Lee et al. (2013) argue that the
HONO PSS assumption might overestimate the strength of any unidentified
source if the transport time from nearby NOx emission sources to the
measurement site is less than the time required for HONO to reach PSS. In
this study, the missing source was calculated according to Su et al. (2008a)
(Eq. 3), where PSS was not assumed. Also in our measurements,
dHONO/dt was not equal to 0, as HONO was not at PSS.
SHONO=JHONO[HONO]+k2[OH][HONO]-k1[OH][NO]-khet[NO2]+Δ[HONO]Δt
[HONO] is the measured HONO concentration and khet the
heterogeneous conversion rate of NO2 to HONO, which was discussed above
to be 1.6 % h-1 during the wet period and 0.36 % h-1
during the dry period. Δ[HONO] /Δt is the observed
change of HONO concentration unequal to 0. The uncertainty of the calculated
missing source SHONO was estimated to be about 16 % based on
the Gaussian error propagation of instrument uncertainties of HONO, NO,
NO2, J and OH.
Linear correlation factors (Pearson correlation, R2) of HONO and
the unknown source SHONO to meteorological factors and different
NOx parameters.
During the whole campaign Time-of-day average HONOSHONOHONOSHONOT0.0060.1250.4880.214RH0.0770.005d0.0920.103Heat flux0.2610.3000.617c0.585cJNO20.2630.3950.718b0.672bNO0.2420.1540.857a0.600cNO20.0520.0780.620c0.496NO2⋅ RH0.1260.1110.638c0.505cNO2⋅ RH ⋅ aerosol surface0.0950.0920.2560.579cNO2⋅J0.1910.1640.828a0.813aNO2⋅ RH ⋅J0.2660.2210.850a0.807aNO2⋅ RH ⋅J⋅ aerosol surface0.2210.2040.806a0.814aSNO0.012-0.015dDuring the humid period During the dry period Time-of-day average Time-of-day average HONOSHONOHONOSHONOHONOSHONOHONOSHONOT0.0060.1160.0310.1230.1200.0160.453-0.004RH0.0000.081d0.010d0.146d0.3740.1930.730b0.603cHeat flux0.1100.2430.1840.591c0.502c0.3350.685b0.634cJNO20.1500.4650.2450.669b0.678b0.3200.829a0.664bNO0.1680.1350.4180.650b0.4870.3010.730b0.409NO20.0660.0650.3000.2670.0370.003d0.619c0.174NO2⋅ RH0.0840.0480.2940.1710.1610.0100.714b0.456NO2⋅ RH ⋅ aerosol surface0.0470.0720.1110.2500.2410.0850.557c0.551cNO2⋅J0.2140.2610.4270.845a0.3580.0160.872a0.603cNO2⋅ RH ⋅J0.2310.2440.4670.775b0.4340.0680.820a0.703bNO2⋅ RH ⋅J⋅ aerosol surface0.1400.1520.4650.795b0.4140.1300.664b0.631cSNO0.2940.720b0.0590.094
a Highly correlated: R2> 0.8 (in bold font).
b Moderately correlated: 0.65 <R2< 0.8 (in italic
font). c Poorly correlated: 0.5 <R2< 0.65 (in normal
font). d Anti-correlated.
Nevertheless, at the study site of Cyprus, the mean upwind distance between
the measurement site and the coastline was about 6 km, and the mean wind
velocity was about 3 m s-1. Accordingly, the respective air mass
travel time over land is estimated to be about half an hour, which is
somewhat longer than the daytime lifetime of HONO and might provide enough
time for the equilibrium processes. Furthermore and in strong contrast to
Lee et al. (2013), at the Cyprus site the concentrations of HONO precursors
(NO and OH) were extremely low, far too low to explain the observed HONO
concentrations. In the late morning (around 10:00 LT) the unknown source was
at its maximum, with peak production rates of up to 3.4×106 molecules cm-3 s1 and a daytime average of about
1.3×106 cm-3 s-1, which is in good agreement with
other studies at rural sites, like a mountain site at Hohenpeissenberg ((3±1)×106 cm-3 s-1, at NOx≈2 ppbv; Acker et
al., 2006a), a deciduous forest site in Jülich
(3.45 × 106 molecules cm-3 s-1, at NO ≈
250 pptv; Kleffmann et al., 2005) and a pine forest site in southwest Spain
0.74×106 molecules cm-3 s-1, at NOx≈1.5 ppbv; Soergel et al., 2011a) but smaller than at urban sites in Houston
(4–6 × 106 cm-3 s-1, at NOx≈6 ppbv;
Wong et al., 2012), Beijing (7×106 cm-3 s-1, at
NOx≈15 ppbv; Yang et al., 2014) and southern China
(5.25 ± 3.75 × 106 cm-3 s-1, at NOx≈20 ppbv (Li et al., 2012), or 1–4 × 107 cm-3 s-1,
at NOx≈35 ppbv (Su et al., 2008a)).
The contributions of gas-phase reactions and the heterogeneous reaction of
NO2 (conversion rate a 1.6 % h-1 and
b 0.36 % h-1) to the HONO budget are illustrated in Fig. 5
exemplarily. For both periods the contributions are quiet similar; just the
absolute values are different. To compensate the strong loss via photolysis, a
comparably strong unknown source is necessary as the heterogeneous NO2
conversion or the gas-phase reaction of OH and NO is insignificant.
In polluted regions with moderate to high NOx concentrations, HONO
sources have often been linked with [NO2] or [NOx] (Acker et al.,
2005; Li et al., 2012; Levy et al., 2014; Sörgel et al., 2011a; Wentzel
et al., 2010). Under the prevailing low-NOx conditions during CYPHEX
(< 250 pptv), correlation analysis (see Table 1) of SHONO
with [NO2] (R2=0.50) and [NO2] ⋅ RH (R2=0.51)
indicate no significant impact of instantaneous heterogeneous formation of
HONO from NO2. Better correlations of SHONO with
JNO2 (R2=0.67) and JNO2⋅ [NO2]
(R2=0.82) indicate a photo-induced conversion of NO2 to HONO as
already suggested by George et al. (2005) or Stemmler et al. (2006, 2007).
Lee et al. (2016) found even lower correlation with [NO2] (R2=0.0001) but similar good correlation with
JNO2⋅ [NO2] (R2=0.70) at an urban background
site in London. Other light-dependent reactions such as the photolysis of
nitrate might additionally contribute to high daytime HONO. It is unlikely
that aerosol surfaces played an important role in heterogeneous conversion of
NO2 as the mean observed aerosol surface concentration was only about
300 µm2 cm-3. Based on a formula for photo-enhanced
conversion of NO2 on humic acid aerosols which was derived by Stemmler
et al. (2007), a HONO formation rate of only 5.1×102 molecules cm-3 s-1 can be estimated. Likewise,
Sörgel et al. (2015) showed that HONO fluxes from light-activated
reactions of NO2 on humic acid surfaces at low NO2 levels
(< 1 ppb and thus comparable to concentrations observed in this study)
saturated at around 0.0125 nmol m-2 s-1. Therefore heterogeneous
aerosol surface reactions can be neglected as HONO sources at the prevailing
low NOx levels.
Likewise, the nitrate concentrations of highly acidic marine aerosols
particulate matter as measured by HR-ToF-AMS (PM1 fraction, mean of
0.075 µg m-3) were too low to account for significant
photolytic HONO production
(1.7 × 102 molecules cm-3 s-1 or 0.01 % of
SHONO) calculated by Eq. (4):
Sphoto_NO3-=[NO3-‾]⋅JNO3-,
with Sphoto_NO3- being the source strength of HONO by
photolysis of nitrate, [NO3-‾] the mean particulate
nitrate concentration and JNO3- the photolysis frequency
of nitrate (aqueous) at noon (3×10-7 s-1; Jankowski et al.,
1999).
Recently an enhancement of the photolysis frequency of particulate nitrate
relative to gaseous or aqueous nitrate was found (Ye et al., 2016). But even
with this enhanced rate of 2×10-4 s-1 no more than
1.1 × 105 molecules cm-3 s-1 (8 % of
SHONO) HONO would be produced.
Common daytime source of HONO and NO
During CYPHEX, good correlation was found between [HONO] or SHONO
and [NO] (R2=0.86 and 0.60, respectively), indicating that both may have
a common source. A missing source of NO can be calculated as shown in
Eq. (5).
SNO=k1[OH][NO]+k3[HO2][NO]+k4[O3][NO]+k5[RO2][NO]-JNO2[NO2]-JHONO[HONO]+Δ[NO]Δtk3 and k4 are the temperature-dependent rate constants for the
reaction of NO with HO2 and O3, respectively (Atkinson et al.,
2004; at 23 ∘C: k3≈8.96×10-12 cm3 s-1; k4≈1.68×10-14 cm3 s-1); k5 is the rate constant for the reaction
of NO and organic peroxy radicals which was assumed to be the same as for the
reaction NO + CH3O2 (7.7×10-12 cm3 s-1
at 298 K; Ren et al., 2010; Sander et al., 2011). Like [OH], [HO2]
was also measured only on a few days, and therefore mean diel data were used
(Fig. S3). Total [RO2] was estimated to be maximum
1.6 ⋅ [HO2] (Ren et al., 2010; Hens et al., 2014). Using a
RO2/ HO2 ratio of 1.2, the absolute values of SNO are
reduced by 0.3 to 5.5 %. The budget analysis for NO for both humidity
regimes is illustrated in Fig. S4.
For NOx, an unexpected deviation from the PSS, or Leighton ratio, of
clean marine boundary layer air has been observed previously, invoking a
hitherto unknown NO sink, or pathway for NO to NO2 oxidation, other than
reactions with OH, HO2, O3 and organic peroxides (Hosaynali Beygi
et al., 2011). On Cyprus, two different atmospheric humidity regimes can be
differentiated. Under dry conditions (RH < 70 %, yellow boxes in
Fig. 3) and higher NOx concentrations (> 150 pptv) SNO is
negative, implying a net NO sink of up to
6.4 × 107 molecules cm-3 s-1 resembling the
abovementioned PSS deviations in remote marine air masses (see Figs. 6 and
7). However, during humid conditions (RH > 70, blue boxes in Fig. 3)
SNO was positive with values of up to
5.1 × 107 molecules cm-3 s-1. Due to low and
invariant acetonitrile levels, anthropogenic activity and local biomass
burning can be excluded as an NO source at this specific site. A net NO
source during humid conditions is assumed to result from (biogenic) NO
emission from soil. As shown in Fig. 8, the SHONO and SNO
(time-of-day average, excluding 3 days as there are transition days (25 July
and 2 August) or the RH changed too quickly (15 July)) were highly correlated
(R2=0.72), indicative of both reactive N compounds being emitted from
the same local source. Both HONO and NO have been reported to be released
from soil, with a strong dependency on soil water content (Su et al., 2011;
Oswald et al., 2013; Mamtimin et al., 2016). The (dry-state) soil
humidification threshold level for NO emission is reported to be somewhat
higher than for HONO (Oswald et al., 2013), which might explain why a net NO
source was preferentially calculated for higher-relative-humidity conditions,
while for HONO a daytime source under all humidity regimes prevailing during
the campaign was found. Mamtimin et al. (2016) investigated HONO and NO
emissions of natural desert soil and with grapes or cotton cultivation soils
in an oasis in the Taklamakan Desert in the Xinjiang region in China. After
irrigation they did not find direct emission, but when the soil had almost
dried out (gravimetric soil water content: 0.01–0.3) emissions up to
115 ng N m-2 s-1 were detected. In addition they observed
soil-temperature-dependent emission of reactive nitrogen. Analyzing microbial
surface communities from drylands, Weber et al. (2015) observed highly
correlated NO–N and HONO–N emissions with Spearman rank correlation
coefficients ranging between 0.75 and 0.99. In this study, NO and HONO
emissions were observed in drying soils with water contents of 20–30 %
water holding capacity.
NO2 (color-coded) and RH dependence of the sources of NO
(SNO) and HONO (SHONO).
Diel profile of both unknown sources,
SHONO(a) and SNO(b), for all
data, humid (excluding transition days, 25 July and 2 August, and 15 July as
RH conditions changed too quickly) and dry periods. Error bars indicate
standard deviation of diel average.
Correlation of SHONO to light-induced NO2
reaction (for both periods; humid: blue triangle; dry: orange square), to NO
and SNO (only for humid period, excluding the transition days
– 25 July and 2 August – and the day with quickly changing RH – 15 July);
time-of-day-average data were used.
Average diel pattern of primary OH production from HONO,
O3 and VOC, shown as (a) production rate and (b) percentage contributions to primary OH production.
Even though we cannot make firm conclusions regarding the exact mechanism of
HONO formation, the abovementioned correlation analysis (and Table 1) reveal
that the instantaneous heterogeneous NO2 conversion is not a significant
HONO source. We propose that HONO is emitted from nitrogen compounds being
accumulated on mountain slope soil surfaces produced either biologically by
soil microbiota or from previously deposited NOy. This forms the major
daytime HONO source responsible for morning concentration peaks and
consistently high daytime mixing ratios at the Cyprus field site. While
biological formation is assumed to be more relevant for humid conditions,
physical NOy accumulation can be assumed to be stronger under dry
conditions, as uptake coefficients for a variety of trace gases were shown to
be significantly higher for dry surfaces, among them NO2 (Wang et al.,
2012; Liu et al., 2015), HONO (Donaldson et al., 2014a) and HCHO (Li et al.,
2016). The strongest HONO morning peaks observed after dry nights were
accompanied by an increase in relative humidity driven by the sea breeze
(Fig. 4b), so we consider HONO as being released preferentially under favorable
humid conditions.
Primary OH production
Many studies showed high contribution of HONO photolysis to the OH budget (up
to 30 % on average daily; Alicke et al., 2002; Ren et al., 2006).
Here, the primary OH production rates
are calculated based on the main OH-forming reactions, which are the
photolysis of O3 and subsequent reaction with water (Reactions R6, R7),
the photolysis of HONO (Reaction R2) and the reaction of alkenes with ozone
(Reaction R8).
O3⟶hv(< 340nm)O1D+O2O1D+H2O→2OHalkene+O3→OH+otherproducts
Reaction rates were taken from Atkinson et al. (2004) and Atkinson (1997).
The water pressure over water was calculated according to Murphy and
Koop (2005). Reactions of O(1D) and HO2 not forming OH are also
considered. OH formation yields of the reactions of alkenes with O3 were
taken from Paulson et al. (1999). Photolysis rates (J values) and
concentrations of relevant compounds were as measured on Cyprus. Isoprene,
α-pinene, β-pinene, Δ3-carene and limonene (VOC) were
taken into account as the most relevant alkenes.
The results of this study are shown in Fig. 9. All three production routes
show a clear diel profile with higher production rates during daytime. In the
night only the reaction of alkenes with O3 produced significant amounts
of OH (2 × 104 molecules cm-3 s-1). With sunrise
the other sources become more relevant. During daytime the photolysis of HONO
generates about 1.5×106 molecules OH cm-3 s-1,
which is about 10 times higher than the ozonolysis of alkenes at that time.
The maximum OH production rate by O3 photolysis during daytime is about
1.3 × 107 molecules cm-3 s-1. In the morning
(06:00–08:00 LT) and evening hours (19:00–20:00 LT) the contribution
of HONO photolysis to the primary OH production is on average 37 % (see
Fig. 9b) with peak values of 65 %, which is much higher than the
contribution of O3 photolysis at that time. During the rest of the day
the contribution of HONO decreases to 12 %. At noon the most dominant OH
source is the photolysis of O3 (more than 80 %), while the
contribution of the ozonolysis of alkenes is almost negligible (1–2 %).
A complete and detailed HOx budget analysis with CYPHEX data will be
published soon.
Conclusion
Nitrous acid was found in low concentrations on the east Mediterranean island
of Cyprus during summer 2014. Daytime concentrations were much higher than
during the night and about 30 times higher than would be expected by budget
analysis based on photostationary state. The unknown source was calculated to
be about 1.9×106 molecules cm-3 s-1 around noon. Low
NOx concentrations, high HONO / NOx ratio and low correlation
between HONO and NO2 indicate a local source which is independent of
NO2. Heterogeneous reactions of NO2 on aerosols play an
insignificant role during daytime. Emission from soil, caused either by
photolysis of nitrate or gas–soil partitioning of accumulated nitrite/nitrous
acid, is supposed to have a higher impact on the HONO concentration during
this campaign. Also the NO budget analysis showed a missing source in the
humid period, which correlates well with the unknown source of HONO,
indicating a common source. The most likely source of HONO and NO is the
emission from soil.
Even though the HONO concentration is only in the lower pptv level, it has a
high contribution to the primary OH production in the early morning and
evening hours.
Data availability
Readers who are interested in the data should contact the authors: Hang Su
(h.su@mpic.de) or Hannah Meusel
(hannah.meusel@mpic.de).
The Supplement related to this article is available online at doi:10.5194/acp-16-14475-2016-supplement.
Acknowledgements
This study was supported by the Max Planck Society (MPG) and the DFG Research
Center/Cluster of Excellence “The Ocean in the Earth System-MARUM”.
We thank the Cyprus Institute and the Department of Labor Inspection for the
logistical support, as well as the military staff at the Lara Naval
Observatory in Ineia for the excellent collaboration.
Furthermore we would like to thank Mathias Soergel for his technical support
on experimental setup of atmospheric HONO measurements.
The article processing charges for this open-access
publication were covered by the Max Planck Society.
Edited by: N. Mihalopoulos
Reviewed by: two anonymous referees
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