To characterize the role of dew water for the ground surface HONO distribution, nitrous acid (HONO) measurements with a Monitor for AeRosols and Gases in ambient Air (MARGA) and a LOng Path Absorption Photometer (LOPAP)
instrument were performed at the Leibniz Institute
for Tropospheric Research (TROPOS) research site in Melpitz, Germany, from 19 to 29 April 2018. The dew water was also collected and analyzed
from 8 to 14 May 2019 using a glass sampler. The high time
resolution of HONO measurements showed characteristic diurnal variations
that revealed that (i) vehicle emissions are a minor source of HONO at Melpitz
station; (ii) the heterogeneous conversion of NO2 to HONO on the ground surface
dominates HONO production at night; (iii) there is significant nighttime
loss of HONO with a sink strength of 0.16±0.12ppbvh-1; and (iv) dew water with mean NO2- of 7.91±2.14µgm-2
could serve as a temporary HONO source in the morning when the dew droplets
evaporate. The nocturnal observations of HONO and NO2 allowed the direct
evaluation of the ground uptake coefficients for these species at night:
γNO2→HONO=2.4×10-7 to 3.5×10-6, γHONO,ground=1.7×10-5 to
2.8×10-4. A chemical model demonstrated that HONO deposition
to the ground surface at night was 90 %–100 % of the calculated unknown HONO
source in the morning. These results suggest that dew water on the ground
surface was controlling the temporal HONO distribution rather than
straightforward NO2–HONO conversion. This can strongly enhance the OH
reactivity throughout the morning time or in other planted areas that provide a large
amount of ground surface based on the OH production rate calculation.
Introduction
Nitrous acid (HONO) is important in atmospheric chemistry as its photolysis
(Reaction R1) is an important source of OH radicals. In the troposphere, OH radicals
can initiate daytime photochemistry, leading not least to the formation
of ozone (O3) and secondary organic aerosol (SOA):
HONO+hν→OH+NO.
At present, the mechanisms of HONO formation have been and are still widely
discussed. In the absence of light, heterogeneous reactions of NO2
occur on wet surfaces (Reaction R2) and are considered to be an important source of
HONO according to both laboratory studies and field observations
(Acker et al., 2004):
2NO2+H2O→HONO+HNO3.
Finlayson-Pitts et al. (2003) proposed a mechanism
(Reaction R3) involving the formation of the NO2 dimer (N2O4)
especially during nighttime. However, this pathway is not important in the
real atmosphere (Gustafsson et al., 2008). The surface of soot
(Ammann et al., 1998; Arens et al., 2001; Gerecke et al., 1998) or light-activated soot (Aubin and Abbatt, 2007; Monge et al., 2010) contains
functionalities attached to the large carbonaceous structures or individual
condensed organic species, like phenol (Reaction R4) (Gutzwiller et
al., 2002) and light-activated humic acids (Stemmler et al.,
2006) which undergo electron transfer reactions with NO2 yielding HONO
(Reaction R5, in which HA, Ared and X are humic acid, the activation of reductive
centers and oxidants, respectively). This reaction is also postulated for
aromatics in the aqueous phase but only proceeds at a relevant rate at high
pH levels (Ammann et al., 2005; Lahoutifard et al., 2002).
Gustafsson et al. (2008) provide the evidence that the formation of
HONO proceeds through a bimolecular reaction of absorbed NO2 and H (Reaction R6) on
mineral dust in which H is formed from the dissociation of chemisorbed water.
However, Finlayson-Pitts (2009) indicated that this pathway is
probably not transferable from the laboratory to the real atmosphere. In addition to
the direct emission from vehicle exhaust
(Kurtenbach et al., 2001) and the homogeneous gas-phase
reaction of NO with OH (Reaction R7) (Pagsberg et al., 1997), some
other HONO formation mechanisms have been proposed, e.g., homogeneous reaction
of NO2, H2O and NH3 (Reaction R8) (Zhang and Tao, 2010), the
photolysis of nitric acid and nitrate (HNO3/NO3-) (Reaction R9) (Ye
et al., 2016; Zhou et al., 2011), and nitrite emissions from soil (Reaction R10)
(Su et al., 2011):
R32NO2(g)↔N2O4(g)↔N2O4(surface)↔HONO(surface)+HNO3(surface),R4NO2+{C-H}red→HONO+{C}ox,R5HA⟶hνAred+X;Ared+X→A′;Ared+NO2→A′′+HONO,R6NO2(ads)+H(ads)→HONO(ads)→HONO(g),R7NO+OH→HONO,R8NO2(g)+H2O(g)+NH3(g)→HONO(g)+NH4NO3(s),R9HNO3/NO3-+hν→HONO/NO2-+O,R10NO2-(aq)+H+(aq)→HONO(aq).
Several studies (Acker et al., 2004; He et al., 2006; Lammel and Perner,
1988; Lammel and Cape, 1996; Rubio et al., 2009; VandenBoer et al.,
2013, 2014) reported that HONO deposited on wet surfaces
can be a source for observed daytime HONO. He et al. (2006)
observed HONO released from a drying forest canopy, and their lab studies
showed that, on average, ∼90 % of NO2- was emitted as HONO
during dew evaporation. Rubio et al. (2009) found a
positive correlation between formaldehyde and HONO in dew and the
atmosphere.
The dominant loss of HONO is photolysis during daytime which forms OH
radicals (Reaction R1). An additional sink of HONO is the reaction with OH radicals
(Reaction R11). Due to the absence of solar radiation and the low OH concentration,
the main loss process of HONO during nighttime is dry deposition, which can
reach a balance with HONO production and vertical mixing to generate a
steady state of HONO mixing ratio:
HONO+OH→H2O+NO2.
Due to its significant atmospheric importance, HONO has been measured for
many years with various techniques (Febo et al., 1993; Huang et al.,
2002; Kanda and Taira, 1990; Platt et al., 1980; Schiller et al., 2001; Wang and
Zhang, 2000). The LOng Path Absorption Photometer (LOPAP) is a two channel in
situ HONO measurement instrument which detects HONO continuously by wet
sampling and photometric detection. LOPAP is very selective without sampling
artifact and chemical interferences (e.g., NO2, NO, O3, HCHO,
HNO3, SO2, PAN, etc.). In addition, the detection limit of LOPAP
can go down to 0.2 pptv (Kleffmann and Wiesen, 2008) by
optimizing parameters like (a) sample gas flow rate, (b) liquid flow
rates and (c) the length of the absorption tubing (Heland et
al., 2001). LOPAP was validated and compared with the most established and
reliable HONO instrumental differential optical absorption spectroscopy (DOAS).
Both were used in the field and in a large simulation chamber under various
conditions resulting in excellent agreement (Heland et al.,
2001; Kleffmann et al., 2006).
The Monitor for AeRosols and Gases in ambient Air (MARGA) is a commercial
instrument combining a steam-jet aerosol collector (SJAC) and a wet rotating
denuder (WRD) which can quantify the inorganic water-soluble particulate matter (PM) ions
(Cl-, NO3-, SO42-, NH4+, Na+,
K+, Mg2+, Ca2+) and corresponding trace gases (HCl, HONO,
HNO3, SO2, NH3). In recent years, MARGA measurements have been
performed worldwide, which has been summarized by
Stieger et al. (2018). Within the cited study, HONO
concentrations measured by a MARGA system and an off-line batch denuder
without an inlet system were compared. Although the slope between both
instruments was 1.10 with slightly higher MARGA concentrations on average,
both instruments biased equally in the measured concentrations resulting in
a high scattering with a coefficient of determination of R2=0.41.
The probable reason was the off-line analysis of the batch denuder sample since
the resulting longer interaction of gas and liquid phase during the
transport led to further heterogenous reactions. As both instruments are
based on the same sampling technique, the present study could be a good
starting point for an intercomparison between MARGA and LOPAP for HONO
measurements to find possible reasons in the denuder deviations.
In this study, we present parallel measurements of HONO using LOPAP and
MARGA in Melpitz, Germany, over 2 weeks in 2018. For further
investigations, dew water was collected and analyzed from 8 to
14 May 2019 using two glass samplers. In addition, other water-soluble
compounds, such as gaseous HNO3, NH3 and particulate
NO3-, SO42-, NH4+, Na+, K+,
Mg2+ and Ca2+, trace gases (NOx, SO2 and O3), and
meteorological parameters were also measured simultaneously. Our
observations provide a direct intercomparison between LOPAP and MARGA for
HONO field measurements, offer additional insights into HONO chemical formation
processes, and examine the relative importance of dew as a sink and source of
HONO.
ExperimentSite description
Measurements were performed at the research station of the Leibniz Institute
for Tropospheric Research (TROPOS) in Melpitz (51∘32′ N, 12∘56′ E). This rural field site is situated in a meadow
and surrounded by flat grass land, agricultural areas and forests. The
Melpitz site can mainly be influenced by two different wind directions: the west
wind of marine origin crossing a large area of Western Europe and the
city of Leipzig (41 km northeast) and the east wind crossing Eastern Europe
(Spindler et al., 2004).
MARGA instrument
The MARGA (1S ADI 2080, the Netherlands) used in this study has already been
described in Stieger et al. (2018). Hence, only
a little information is provided here. An inlet flow of 1 m3h-1 was
drawn into the sampling box after passing through an inner Teflon-coated
PM10 inlet (3.5 m; URG, Chapel Hill, USA). Within the sample box, the
sampled air laminarly passed a WRD, in which water-soluble gases diffuse
into a 10 mgL-1 hydrogen peroxide (H2O2) solution at
pH =5.7. Particles can reach the SJAC because of their smaller diffusion
velocities. Within the SJAC, the particles grow into droplets under
supersaturated water vapor conditions and were collected by a cyclone. The
gas and particle samples are both collected over the course of 1 h.
Then, the aqueous samples of the WRD (gas phase) and the SJAC (particle
phase) were successively injected into two ion chromatographs (ICs) with
conductivity detectors (Metrohm, Switzerland) by two syringe pumps for
analyzing the anions and cations. The volumes of the injection loops for the
anions and cations were 250 and 500 µL, respectively. The
Metrosep A Supp 10 (75/4.0) column and Metrosep C 4 (100/4.0) column were
used to separate anions and cations, respectively. Lithium bromide was used
as the internal standard for both gas- and particle-phase samples and was added
during the sample injection to the IC.
The detection limits and the blanks for the MARGA system were performed
before the intercomparison campaign in 2018. The detection limit of HONO was
determined to be 10 pptv. The blanks were analyzed when the system was set up
in the field to consider potential contamination. For blank measurements,
the MARGA blank measurement mode, which has a duration of 6 h, was used.
Within the first 4 h, the MARGA air pump was off, and the denuder and
SJAC liquids were analyzed. The first and second hour samples were discarded
as they still included residual concentrations. The evaluation of the blank
concentrations was performed for the third and fourth hour samples. No
discernable peaks above the instrument detection limits were identified in
both the gas- and particle-phase channels.
The precision for HONO quantification is below 4 %, indicating good
repeatability. To test the robustness of the ion chromatography within the
MARGA, standard solutions with defined NO2- concentrations of 70,
120 and 150 µgL-1 were injected in the IC system. The
correlation between both the predefined concentrations within the standard
solutions and the measured concentrations by the MARGA IC resulted in a
slope of 1.13 (R2=0.99). This value indicates slightly lower
measured NO2- concentrations, which might also be a result of
nonstable NO2- in freshly made liquid standard solutions.
LOPAP instrument
The LOPAP (QUMA, Germany) employed in this work was described in previous
studies (Bernard et al., 2016; Heland et al., 2001). Only a brief
description is given here. The LOPAP instrument consists of two sections: a
sampling unit and a detection unit. The ambient air was sampled in the
sampling unit, which is composed of two glass coils in series in which the first
coil (channel 1) accounted for HONO with interferences and the second coil
(channel 2) sampled only interferences assuming that more than 99 % of
HONO was absorbed into the acidic stripping solution (pH =0) to form diazonium
salt in channel 1. This salt reacts with a 0.8 mM
n-(1-naphthyl)ethylenediamine dihydrochloride solution to produce the final azo
dye which is photometrically detected by long path absorption in special
Teflon tubing (Heland et al., 2001; Kleffmann et al., 2006). During our
field campaign in Melpitz, both the acidic stripping solution and 0.8 mM
n-(1-naphthyl)ethylenediamine dihydrochloride solution were kept in the dark
and were not changed during the whole campaign period. The temperature of
the stripping coil was kept constant at 25 ∘C by a thermostat.
Automatic zero air (Alphagaz 2, 99.9999 %, Air Liquid GmbH) measurements were
performed for 30 min per 12 h measurement to correct for zero drifts. In
addition, calibrations using NO2- standard solution
(Heland et al., 2001) were applied at the beginning (17 April), middle (20, 24, 25 April) and end (29 April) of the campaign to derive the HONO mixing ratio. The detection
limit of LOPAP was approximately 1–2 pptv with a response time of 5 min. The
error of the HONO mixing ratio was estimated based on these detection limits and
a relative error of 10 %. The relative error is calculated by error
propagation of all systematic errors; i.e., uncertainties in the gas flow are ca. 2 % and in the liquid flow ca. 2 %, the error in the nitrite concentration
during calibration is 1 %, and errors for the pipettes/flasks used are 2 times
the specified errors of all volumetric glassware since all glassware
was not used exactly at 20 ∘C as recommended by the
manufacturer.
To investigate the possible sampling inlet and denuder artifacts of the
MARGA, two different positions were selected for LOPAP during the
measurement period (explained in the Supplement): (M1) sampling unit of LOPAP was
connected to the MARGA inlet in the back of the 2 m sampling tube and the
PM10 inlet of MARGA as shown in Fig. S1a in the Supplement (18 April 2018, 13:00 UTC, to 20 April 2018, 08:00 UTC), and (M2) the sampling unit of LOPAP was
settled in the same level as the sampling head of MARGA (Fig. S1b) (20 April 2018, 15:00 UTC, to 29 April 2018, 07:00 UTC).
Dew water collection and analysis
To evaluate the HONO emission from the dew water in the morning, the dew
water was collected 1 year later after the HONO comparison campaign and
was analyzed on 8, 11, 13 and 14 May 2019.
Similar conditions (grass height, dew formation and day length) were
observed to improve the evaluation. For dew sampling, a glass sampler was
used (as shown in Fig. S2). Two 1.5 m2 glass plates (Plates 1 and 2) were placed 40 cm above the ground with a tilt angle of approximately
10∘. A gutter was installed at the lower end of each plate to
collect the water running down. The water was trapped in 500 mL bottles. The
dew samplers were prepared each evening before a likely dew event would occur (low dew-point difference, clear sky and low winds). Each plate was rinsed
with at least 2 L ultrapure water. A squeegee removed the excess water.
Afterwards, the plates were cleaned with ethanol and were again rinsed with 2 L ultrapure water. The plate was splashed with ultrapure water and squeegeed
six times and the gutter was cleaned. The sample of the sixth splash was
collected as the blank (∼50 mL).
The dew water normally was collected from 18:00 to 05:00 UTC. In the
morning, the excess dew on the plate was squeegeed. To achieve the volume of
dew (Vdew), the bottles were weighed before and after sampling by a
balance. The pH was measured by a pH meter (mod. Lab 850, Schott
Instruments) on a subsample of the total volume. After sampling, the aqueous
solutions were filtered and stored in a fridge (∼6∘C). Within 6 h, the HONO analyses of the dew and blank samples were
performed by double injection in the MARGA in the manual measurement mode as
HONO may volatilize between sampling and analysis. For the other ions
(Cl-, NO3-, SO42-, Oxalate, Br-, F-,
Formate, MSA, PO43-, Na+, NH4+, K+, Mg2+ and Ca2+), the samples were analyzed with laboratory ion
chromatogram systems (mod. ICS-3000, Dionex, USA). Blanks from water, the
filter, the syringes and bottles were subtracted.
Aerosol measurements
The particle size distributions were measured in the size range from 5 nm to
10 µm with a dual mobility particle size spectrometer
(D-MPSS; TROPOS-type D-MPSS) (Birmili et al., 1999) and an aerodynamic
particle size spectrometer (APSS; model 3321, TSI Inc., Shoreview, MN, USA).
For the particle number size distribution measurements, the aerosol is
sampled through a low-flow PM10 inlet and dried in an automatic diffusion
dryer (Tuch et al., 2009). The measurements and
quality assurance are done following the recommendations given in
Wiedensohler et al. (2012, 2018). The MPSS-derived particle number size distribution was inverted
by the algorithm described in Pfeifer et al. (2014) and following the bipolar charge distribution of
Wiedensohler (1988).
Other measurements
Trace gases of NO-NO2-NOx, SO2 and O3 were measured with a
NOx analyzer (Thermo Scientific Model 42i-TL, Waltham, Massachusetts, USA),
SO2 analyzer (APSA-360A) and O3 analyzer (APOA-350E; both Horiba,
Kyoto, Japan) with a time resolution of 1 min. It should be noted that
NO2 was converted to NO within the NOx analyzer by a blue light
converter (BLC2, Meteorologie Consult GmbH, Königstein, Germany). The
provider for the replacement of the Mo converter in the 42i-TL analyzer is MLU
Messtechnik GmbH, Essen, Germany. Meteorological parameters like temperature
(T), precipitation and relative humidity (RH), as well as wind velocity and
direction, were measured by PT1000, a rain gauge (R.M. Young Company,
USA), the CS215 sensor (Sensirion AG, Switzerland) and a WindSonic by Gill
Instruments (UK), respectively. Global radiation and barometric pressure
were recorded by a net radiometer CNR1 (Kipp & Zonen, the Netherlands) and
a digital barometer (Vaisala, Germany), respectively.
Calculation of photolysis rate
The off-line National Center for Atmospheric Research (NCAR) Tropospheric Ultraviolet and Visible (TUV) transfer model
(https://www2.acom.ucar.edu/modeling/tropospheric-ultraviolet-and-visible-tuv-radiation-model, last access: 14 January 2019)
was used to estimate the photolysis rate of HONO (JHONO), NO2
(JNO2) and production rate of O1D (JO1D) at Melpitz station
scaled by the measured global radiation. Aerosol optical depth (AOD), total
vertical ozone column, total NO2 column, total cloud optical depth and
surface reflectivity (albedo) were taken from the NASA web page for the
period of measurement (https://neo.sci.gsfc.nasa.gov/blog/, last access: 7 July 2018).
Time courses of HONO as hourly measured by MARGA and 30 s
measured by LOPAP (a) and normalized hourly for LOPAP (b). (c) blue and
green lines represent the error weighted orthogonal regression analysis
between MARGA and LOPAP for two different comparison period of M1 and M2,
respectively. The error bar in the panel (c) indicates the measurement error
of HONO concentrations in LOPAP and MARGA. The HONO concentration of MARGA
in panel (c) is shifted 400 pptv for clarity.
ResultsIntercomparison of LOPAP and MARGA
The hourly HONO mixing ratio obtained from MARGA with the 30 s and
hourly averaged HONO mixing ratios from LOPAP are shown in Fig. 1a and b,
respectively. They indicate that the MARGA values were higher than the values
of LOPAP. In addition, the comparison between both instruments in Fig. 1a
shows a delay in the MARGA concentrations after reaching the maximum
concentrations in the morning. This pattern was also observed in previous
studies by Volten et al. (2012) and Dammers et al. (2017), who
compared miniDOAS instruments with wet denuder systems. Compared to rapid
responses of the miniDOAS, the denuder-based instruments showed offsets and
delays because of inlet memory artifacts by particles or water. Both groups
also suggested transport effects of the liquid samples from the sampling to
the analysis unit resulted in delays and slow responses.
The comparisons of the MARGA and LOPAP HONO measurements for period M1 and
period M2 in Fig. 1c result in slopes of 1.71 and 2.17 using an error-weighted
Deming regression, respectively. These results are consistent with the former
intercomparison of both instrument types in the Chinese field campaign
(Lu et al., 2010; Xu et al., 2019) in which the HONO mixing ratio measured
with the wet denuder–ion chromatography (WD/IC) instrument was affected by a
factor of 3 on average. Within the present work, we evaluated the
relative importance of the denuder artifact with the inlet artifact. The
heterogeneous reactions of NO2 with H2O, as well as NO2 with
SO2, in water as described by Spindler et al. (2003) or
volatile organic compounds (VOCs) with NO2 could explain the artifacts in the denuder solution
(Kleffmann and Wiesen, 2008), which could account for ca. 71 %
(M1, where both LOPAP and MARGA used the common MARGA inlet) of the ca. 117 % overestimated HONO measurements from MARGA. Additional artifacts
such as the heterogeneous formation of HONO due to the long MARGA inlet system
should be responsible for another ca. 46 % (the difference between slopes
M2 and M1). Hence, the results show that the use of massive sampling inlets,
even if they are coated with Teflon, should be avoided for any in situ HONO
instrument. As a result, we chose the LOPAP-measured HONO in the following
sections because of its high accuracy.
Mean and mean error as 2 times the standard deviation of the
measured HONO (LOPAP) and the other pollutants at Melpitz station during
daytime (D, 04:00–18:00 UTC) and nighttime (N, 18:00–04:00 UTC).
a HONO derived from LOPAP.
b Data obtained from the MARGA instrument.
Time series of HONO (LOPAP measurement), NO, NO2, O3,
global radiation, temperature (T), relative humidity (RH) and surface wind
in Melpitz from 19 to 29 April 2018. The gaps are mainly due
to the maintenance of the instruments. The yellow shading indicates two sets
of observations discussed in Sect. 3.3. The gray color in the HONO panel
indicates the measurement error of HONO concentrations.
The hourly time-resolved quantification of water-soluble ions in
PM10 (NO3-, SO42-, NH4+, Na+,
K+, Mg2+, Ca2+) and their corresponding trace gases (HONO,
HNO3, SO2, NH3) were measured by MARGA in Melpitz from 19 to 29 April 2018.
General results
Figures 2 and 3 show an overview of the measured HONO, NO, NO2,
O3, meteorological parameters, water-soluble ions in PM10
(NO3-, SO42-, NH4+, Na+, K+,
Mg2+, Ca2+) and their corresponding trace gases (HONO, HNO3,
SO2, NH3) in the present study. The daytime (D, 04:00–18:00 UTC)
and nighttime (N, 18:00–04:00) averages are also provided in Table 1. During
the 2 week measurements, the prevailing winds were from the southwest and
northwest sectors, indicating a possible influence of city emissions from
Leipzig, Germany, on the site. The strong wind (maximum 13 ms-1)
led to a low concentration of water-soluble ions in PM10 (NO3-,
SO42-, NH4+) and their corresponding trace gases
(HNO3, SO2, NH3) during the 24 to
29 April 2018 period. The air temperature ranged from 5 to 27 ∘C, and the RH showed a clear variation pattern with higher levels
during the night and lower levels during daytime. In addition, the low mixing
ratio of NO and NO2 with a diurnal average of 0.9±1.2 ppbv and
3.7±2.2 ppbv, respectively, were recorded. These observations
highlight the nature of our measurement site as a typical background
environment. The HONO concentration from the LOPAP measurements varied from
30 to 1582 pptv and showed diurnal variations (with average values of
162±96 and 254±114 pptv during daytime and nighttime,
respectively).
Größ et al. (2018) reported the
linear function of the global radiation flux vs. OH radical concentration
for the EUCAARI 2008 campaign at Melpitz:
[OH]=A⋅Rad,
with Rad being the global solar irradiance (in Wm-2) and [OH] being the
hydroxyl radical concentration. The proportionality parameter A is 6110 m2W-1cm-3. On the basis of such a correlation, we
derived the OH concentration during the period of this field measurement
with an average of (2.8±0.7)×106 during daytime.
Diurnal variations of HONO and related species during the
measurement period except for two sets of observations show in Fig. 5 at the
Melpitz site. The photolysis rate of HONO was obtained from the TUV model.
The shaded gray area indicates the nighttime period (18:00–04:00 UTC).
Case events for HONO (LOPAP) and related species at Melpitz site
during the day from 23 to 25 April 2018. The red color in the HONO
panel indicates the measurement error of HONO concentrations.
Diurnal variation of HONO, particles and trace gas species
The diurnal profiles of HONO and related supporting parameters are shown in
Fig. 4 for the whole period except for two sets of observations: (1) no HONO
peak in the morning of 23 April and (2) HONO peak observed at
00:00–02:00 UTC on 25 April (Fig. 5). Overall, the HONO increased
rapidly after sunrise and peaked at 07:00 UTC, then dropped quickly,
reached a minimum at around 10:00 UTC, and remained constant until 17:00 UTC. Such a
daytime pattern was also found in Spain for a site surround by forests and
sandy soils (Sörgel et al., 2011).
Sörgel et al. (2011) explained this by
local emissions, which are trapped in the stable boundary layer before its
breakup of the inversion in the morning based on a similar diurnal cycle for
NO and NO2, which is different from this work. In this work, the
NO2 mixing ratio decreased from midnight until noon, and NO peaked
at 05:00 UTC then remained at a low concentration (<1 ppbv) for 18 h of 1 d. However, three hypotheses could be expected to explain this HONO
morning peak. For hypothesis (a), the HONO morning peak might possibly be caused
by the photolysis of particle-phase HNO3/NO3- (Ye et al.,
2016; Zhou et al., 2003, 2011) since, as shown in Fig. 4a, e and
f, the early morning variation trend of HONO during daytime was similar to
the one of NH3 in the gas phase, as well as NO3- and
NH4+ in PM10. Hypothesis (b), as reported by
Stemmler et al. (2006), states that the photosensitized NO2 on humic
acid could act as a source of HONO during the daytime. For hypothesis (c),
this morning peak of HONO has been reported for Melpitz (4–14 April 2008) by Acker et al. (2004), who expected
that the storage of HONO on wet surfaces can be a source for observed
daytime HONO. To be precise, it was observed that dew was formed overnight during
our campaign in Melpitz. Gaseous HONO could be deposited in these droplets.
Due to evaporation after sunrise, HONO would be reemitted in the atmosphere
which would lead to a HONO morning peak. These hypotheses will be further discussed
in Sect. 4.
As shown in Fig. 4a and b, the HONO and NO2 concentrations started to
increase coincidentally at 16:00 UTC when the sunshine was weak. This
could be explained by the variation of the vertical mixing increasing the
level of all near-ground-emitted formed species or by the heterogeneous
conversion of NO2 to HONO during nighttime; both will be discussed in
Sect. 4. The HONO mixing ratio then decreased from 21:00 UTC to around
100 pptv even though the NO2 concentration was kept constant around 5–6 ppbv. This decrease during nighttime indicates the HONO loss process (dry
and wet deposition, trapped in the boundary layer, dew, etc.) that surpasses
the HONO formation from the NO2–HONO conversion. The diurnal cycle
of O3 reflects the balance between the photochemical formation of
O3 (e.g., NO2+hν) and O3 consumption (e.g., ozonolysis of
terpenes).
a Note that the blank NO2- concentration is below the
detection limit of 0.02 µgL-1.b Final NO2-= Raw NO2- – Blank NO2-.c pH was measured by a pH meter on a subsample of the total volume.
HONO in the dew water
Dew water formation on canopy surfaces could be an efficient removal pathway
of water-soluble pollutants. High solubility of HONO makes dew water an
efficient sink and a stable reservoir for atmospheric HONO. Actually, a lot
of dew water has been observed on the grass around the Melpitz station
during the sampling period of 19 to 29 April 2018. Hence, to
investigate the dissolved HONO in the dew water at Melpitz station, the dew
water was collected and analyzed from 8 to 14 May 2019 during the
same season as the HONO measurements. Many ions, e.g., NO2-,
Cl-, NO3-, SO42-, Oxalate, Br-, F-,
Formate, MSA, PO43-, Na+, NH4+, K+, Mg2+ and Ca2+, were analyzed using MARGA and laboratory IC, but our
discussion only focuses on NO2-. The sample parameters (time, pH
etc.) and NO2- concentration in the sample (µgL-1)
are shown in Table 2 from two glass plates (plate 1 and plate 2). The final
dew water NO2- was calculated by subtracting the blank
NO2- from the raw data of dew water analysis in MARGA. The pH of
dew water in Melpitz ranged from 6.30 to 7.00. It should be noted that the
dew water was frozen until 1 h after sunrise on 8, 13
and 14 May 2019 but not on 11 May 2019. On this day, a third
sample was collected from 03:30 to 05:20 UTC after collecting the
first sample (18:00–03:20 UTC). The NO2- concentration per square meter of the sampler surface (FNO2-) was calculated with the following
equation:
FNO2-=[NO2-]×VdewS×1000,
where [NO2-] is the sample concentration (in µgL-1),
Vdew is the sample volume (in mL), and S is the surface area of the glass
sampler (1.5 m2). As shown in Table 2, higher FNO2- was obtained
on 11 May when dew water was not frozen. On other days (8, 13 and 14 May) frozen dew water was observed,
which likely inhibited the dissolving of HONO. Hence, these frozen samples were
not considered in this paper. On 11 May, the final FNO2-
could be obtained by averaging the FNO2- of the sum (9.43 µgm-2) of the first and third samples with the second sample (6.40 µgm-2) on 11 May, resulting in a mean of 7.91±2.14µgm-2. This value will be used for the following
calculation and discussion.
DiscussionContribution of vehicle emissions
Because the Melpitz site is close to a main national road from Leipzig to Torgau
(Germany) that is within the main southwest wind direction, the contribution
of vehicle emissions to the measured HONO mixing ratio should be evaluated.
Generally, the HONO/NOx ratio is usually chosen to derive the emission
factor of HONO in the freshly emitted plumes
(Kurtenbach et al., 2001). As illustrated in Fig. S3,
NOx concentrations were normally lower than 15 ppbv, and NO/NOx
ratios were ∼0.4 in this campaign, suggesting the detected
air is a mixture of fresh and aged air during the measurement period.
Therefore, a substantial part of HONO is secondary. Additionally, following
the criteria of Li et al. (2018), the bad correlation between
HONO and NOx (R2≈0.35) suggests that the direct HONO
emissions from the vehicle-emitted plumes were less important in this work.
Nighttime HONO
The nighttime HONO is different to some reported literatures (Huang et
al., 2017; Li et al., 2012; Wang et al., 2017; Zhou et al., 2007). HONO
increased after sunset to a maximum at 21:00 UTC and decreased until
sunrise.
The ratio HONOcorr/NO2 and the NO2–HONO conversion
frequency during early nighttime.
The ratio of HONO/NO2 is generally used as an index to estimate the
efficiency of heterogeneous NO2–HONO conversion because it is less
influenced by transport processes than individual concentrations. However,
the ratio might be influenced when a large fraction of HONO is emitted from
traffic, but this is expected to be less important as shown in Sect. 4.1. Hence in this work, a low emission factor of 0.3 % was used to correct
the direct HONO emissions from vehicles (HONOcorr) (Kurtenbach et
al., 2001). Six conditions as listed in Table 3 are selected to calculate
the NO2–HONO frequency following the criteria of Li et
al. (2018).
Only the nighttime data in the absence of sunlight (i.e., 17:30–06:00 UTC) are used.
Both HONOcorr and HONOcorr/NO2 ratios increased
steadily during the target case.
The meteorological conditions, especially surface winds, should be
stable.
Figure S4 presents an example of the heterogeneous HONO formation occurring on
28 April 2018. In this case, the HONO mixing ratios increased
rapidly after sunset from 100 to 600 pptv. Together with the HONO
mixing ratio, the HONOcorr/NO2 ratio increased almost linearly
between 18:00 and 19:50 UTC. The slope fitted by the least squares regression
for HONOcorr/NO2 ratios against time can be taken as the conversion frequency of NO2 to HONO (khet).
The ratio of HONOcorr/NO2 ranged from 0.055 to 0.161 with a mean
value of 0.110±0.041 (Table 3) using the data during early nighttime
(17:30–00:00 UTC) in the Melpitz campaign. The mean values are within the
wide range of reported values of 0.008–0.13 in the fresh air masses from
most sampling sites (Alicke et al., 2002, 2003; Sörgel
et al., 2011; Su et al., 2008; VandenBoer et al., 2013; Wang et al., 2017; Zhou
et al., 2007) except for the study of Yu et al. (2009), who got a high value of 0.3. To our best knowledge, the present work
also presents a high NO2–HONO conversion frequency (khet) of
0.027±0.017h-1 compared to most of the previous studies at
urban sites, such as Alicke et al. (2002) in Milan
(0.012 h-1), Wang et al. (2017) in Beijing (0.008 h-1), and Acker and Möller (2007) in Rome (0.01 h-1).
However, our value is additionally comparable to
Li et al. (2012) with
0.024±0.015h-1, Alicke et al. (2003) with 0.018±0.009h-1, and Acker and Möller
(2007) with 0.027±0.012h-1, who also conducted rural
measurements in the Pearl River Delta (PRD) area in southern China,
Pabstthum in Germany, and Melpitz, respectively, surrounded by farmland
(grasses, trees, small forests). The higher value may suggest that a more
efficient heterogeneous conversion from NO2 to HONO is present in rural
sites than in urban sites.
Relative importance of particle and ground surface in nocturnal HONO
production
The particle surface density Sa was calculated as (0.4–9.9) × 10-4m2m-3 from the particle size distribution (Fig. S5a)
ranging from 5 nm to 10 µm of APSS and D-MPSS data by assuming the
particles are in a spherical shape for the whole day period of 19–29 April 2018. Due to the high RH (RH ∼100 %
during nighttime in Fig. S5b), the particle surface density Sa would be
strongly enhanced (1 magnitude) by the RH correction to be (0.5–1.9) × 10-3m2m-3 with a hygroscopic factor f (RH)
following the method of Li et
al. (2012) and Liu et al. (2008) and calculated as follows:
f(RH)=1+a×(RH/100)b,
where the empirical factors a and b were set to 2.06 and 3.6, respectively.
The formation of HONO through heterogeneous NO2 conversion on particle
surfaces (Sa) can be approximated following the recommendations in
Li et al. (2010) by considering 100 % HONO
yield on the particle surface (NO2+Org, soot, etc.):
khet=14γNO2→HONO_a×υNO2×SaV,
where υNO2 is the mean molecular velocity of NO2 (370 ms-1) (Ammann et al., 1998), Sa/V is the particle
surface to volume ratio (m-1) representing the surfaces available for
heterogeneous reaction, and γNO2→HONO_a is
the uptake coefficient of NO2 at the particle surface. Assuming the
entire HONO formation was taking place on the particle surface, the
calculated γNO2→HONO_a from Eq. (4) varied
from 2.8×10-5 to 3.8×10-4 with a mean value of
(1.7±1.0)×10-4. This number is 2–3 orders of
magnitude higher than typical uptake coefficients determined in the lab for
the uptake of NO2 in the dark on different substrates, e.g.,
Teflon, glass, NaCl, TiO2, soot, phenol, etc.: 10-6 to less than 10-8 (Ammann et al., 1998; Gutzwiller et al., 2002; Kleffmann et al.,
1998; Kurtenbach et al., 2001). Thus, this theoretical uptake coefficient
clearly shows that formation on particles is not important. In addition, the
weak correlations between HONOcorr (R2=0.566), HONOcorr/NO2 (R2=0.208) and Sa (Fig. S6) confirm that the HONO formed
on particle surfaces could be unimportant, as previously reported
(Kalberer et al., 1999; Sörgel et al., 2011; Wong et al., 2011).
As illustrated above, the heterogeneous NO2 conversion on ground
surfaces (including surfaces such as plants, building, soils, etc.)
contributes mainly to the nighttime formation of HONO, which can be approximated
by Eq. (5) following the method in the literature (Kurtenbach et al., 2001; Li
et al., 2010; VandenBoer et al., 2013, 2014) and also been
applied by Zhang et al. (2016) by considering a 50 %
HONO yield from Reaction (R2):
khet=18γNO2→HONO_g×υNO2×SgV,
where γNO2→HONO_g is the uptake coefficient
of NO2 at the ground surface, and Sg/V represents the ground surface to
volume ratio. As described by Zhang et al. (2016), the
leaf area index (LAI; m2m-2) was used to estimate the surface to volume ratio for
the vegetation-covered areas following the method in Sarwar
et al. (2008):
SgV=2×LAIH,
where H is the mixing layer height, which was calculated from the backward
trajectory analysis based on Global Data Assimilation System (GDAS) data under dynamic conditions (Fig. S7).
The mixing layer height ranged between 20 and 300 m from 17:00 until
around 00:00 UTC in April 2018 (Fig. S7). The LAI value is multiplied by a
factor of 2 to take the areas on both sides of the leaves into account. In
Wohlfahrt et al. (2001), the LAI for meadows with different
grass heights are given. Regarding the grass height of ∼30 cm
in April 2018, we used a factor of 6 in the present study. If the entire HONO
formation was taking place on the ground surface, the calculated γNO2→HONO_g varied from 2.4×10-7 to
3.5×10-6 with a mean value of 2.3±1.9×10-6. This value agrees well with the reported range of γNO2→HONO from 10-6 to 10-5 on the ground surface based
on the laboratory studies (Donaldson et al., 2014; VandenBoer et al.,
2015) and field campaign in Colorado, USA
(VandenBoer et al., 2013) during the
nighttime. As the S/V ratio of particles is typically orders of magnitude
lower than for ground surfaces, it is suggested that the heterogeneous
reactions of NO2 on the ground surface may play a dominant role for the
nighttime HONO formation.
In addition, the relationship of the NO2–HONO conversion frequency
(khet presented in Table 3) with the inverse of wind speed is
illustrated in Fig. S8a. As indicated in Fig. S8a, wind speed was
predominantly less than 3 ms-1 during the field campaign period in
Melpitz. The high conversion frequency of NO2 to HONO mostly happened when
wind speed was less than 1 ms-1, which confirms that HONO formation
mainly takes place on the ground. However, one point (in blue in Fig. S8a)
showed highest NO2–HONO conversion frequency (khet) when wind
speed was ca. 4 ms-1 according to the second set of observation
mentioned in Sect. 3.3 and Fig. 5. The likely reason for the temporary
HONO peak is the dew droplet evaporation after increasing wind speed.
HONO deposition on the ground surface
As illustrated in Figs. 4a and S4, between midnight and sunrise (19:00–04:00 UTC), the deposition of HONO becomes increasingly important as the absolute
amount of HONO decreased. Assuming a constant conversion frequency of
NO2 to HONO, khet, the HONO deposition rate (LHONO) can be
roughly estimated by the following:
LHONO=d[HONO]dt+khet×[NO2].
The strength of the HONO sink during night is on average 0.16±0.12ppbvh-1 and ranged from 0.04 to 0.45 ppbvh-1. This value is
similar with reported ones in the literature (He et al.,
2006).
The relationship of [HONO]/[NO2] with RH during nighttime (18:00–04:00)
is illustrated in Fig. S8a. A positive trend of the [HONO]/[NO2] ratio
along the RH was found when RH was less than 70 %. However,
[HONO]/[NO2] performs a negative trend with RH for values over 70 %.
The same phenomenon was also observed by Yu et al. (2009) in Kathmandu and Li et
al. (2012) in the PRD region, China. This finding can be associated with larger
amounts of water on various ground surfaces (plants and grasses) when
ambient humidity approached saturation, leading to an efficient uptake of
HONO.
Assuming all the extra HONO is removed through deposition on the ground
surface, the change in HONO in the time interval of 22:00–04:00 UTC is
parameterized using a combination of Eq. (7) and the following equation:
LHONO=14γHONO,ground×[HONO]×υHONOH,
where γHONO,ground is the HONO uptake coefficient on the ground
surface, υHONO is the mean molecular velocity
of HONO with 3.67×104cms-1, and H is the mixing layer
height calculated from the backward trajectory analysis ranging between 20
and 150 m with an average of ca. 55 m from 22:00 until 04:00 UTC in April
2018. This approach yielded a γHONO,ground uptake
coefficient in the range of 1.7×10-5 to 2.8×10-4 with an average of (1.0±0.4)×10-4, which is
similar to data found in Boulder, Colorado, ranging from 2×10-5 to 2×10-4
(VandenBoer et al., 2013).
As observed by several studies (He et al., 2006; Rubio et al.,
2009; Wentworth et al., 2016), the effective Henry's law solubility of HONO
is highly pH-dependent (from borderline soluble at pH =3 to highly
soluble at pH ≥6), as would be expected for a weak acid. The pH of
collected dew water during nighttime in May 2019 was 6.3–7.0 (Table 2),
for which the effective Henry's law solubility of HONO would be high. The amount
of HONO in this dew water was quantified using MARGA and ranged between 42
and 165 µgL-1, which is higher than the NO2- in
dew waters in Santiago, Chile (Rubio et al., 2009). This could
strongly support the obtained HONO uptake coefficient on the ground surface.
These field-derived surface parameters of nighttime HONO production from
NO2 and surface deposition of HONO are valuable to the model
evaluation. However, it should be noted that the measured pH of collected
dew from the glass plate might differ compared to the pH of dew found on
soil or vegetated surfaces. The chemical nature of the material with which
the water is in contact can influence the effective pH.
A simple resistance model based on the concept of aerodynamic transport,
molecular diffusion and uptake at the surface (presented in the Supplement) as proposed
by Huff and Abbatt (2002) was used to evaluate the factor(s)
controlling the potential applicability of the γ coefficients
calculated here for the uptake of NO2 and deposition of HONO. As shown
in Fig. S9, the deposition loss of HONO is potentially limited by a
combination of aerodynamic transport, molecular diffusion and reaction
processes. However, the HONO uptake will be transport-limited if the real
uptake coefficients are greater than or equal to 2.8×10-4 and wind speed is
less than 0.5 ms-1. In addition, molecular diffusion could play an
important role for HONO uptake on the surface. Regarding the uptake of
NO2 on the ground surface, the range of NO2 uptake coefficients of
2.4×10-7 to 3.5×10-6 obtained in the present
work indicates a limitation only in the reactive uptake process. The
consistency between our findings and the values of these parameters in
models (Wong et al., 2011; Zhang et al., 2016) suggests that the broad
scale applicability of these field-derived terms for the surface conversion of
NO2 should therefore be possible. However, those value of γ
found for HONO (γHONO,ground=1.7×10-5 to
2.8×10-4) require further exploration from various field
environments and controlled lab studies.
Daytime HONO
HONO concentrations started to increase after sunrise and peaked at 07:00 UTC (Fig. 4); during that time, it also underwent photolysis, eventually
reaching a steady state between 10:30 and 16:30 UTC. Throughout the day, HONO
was observed to reach an averaged minimum mixing ratio of 98±15 pptv.
Since NO and NO2 do not have the same diurnal cycle as HONO (Fig. 4), Reactions (R2) and (R7) are not expected to be responsible for this HONO morning peak but
could contribute to the daytime HONO for the period of 10:30–16:30 UTC.
Observed average HONO atmospheric concentration (black line,
±1σ in shaded area) and the model calculated HONO concentration
including different HONO production and loss processes. The particle size spectrometer presents model
results by assuming an instantaneous photo-equilibrium between the gas-phase
formation (Reaction R7) and gas-phase loss processes (Reactions R1 and R11) of HONO. Model 1
includes Reactions (R1), (R7) and (R11). Model 2 includes Reactions (R1), (R2), (R7) and (R11) and surface
deposition from 00:00 to 00:00, whereas Model 3 describes
Reactions (R1), (R2), (R7) and (R11) and surface deposition from 17:00 to 08:00. Model 3 is used as the base to investigate the effect of Reaction (R9) (Model 4), Reaction (R5) (Model 5) and the
combination of Reactions (R5) and (R9) and dew HONO emissions (04:30–07:00) (Model 6).
Photostationary state in the gas phase
The measured diurnal daytime HONO could be compared to model results by
assuming an instantaneous photo-equilibrium between the gas-phase formation
(Reaction R7) and gas-phase loss processes (Reactions R1 and R11), which is described by the
following expression (Kleffmann et al., 2005):
HONOpss=k7OH[NO]JHONO+k11[OH].
OH concentration was estimated from the linear function of the global radiation
flux vs. OH radical concentration as described in the previous section and
shown in Fig. 6. JHONO was calculated using the TUV model as described in
Sect. 2.6. The rate constants of NO+OH (k7) and HONO+OH
(k11) used are 7.4×10-12cm3molecule-1s-1 (Burkholder et al., 2015) and 6.0×10-12cm3molecule-1s-1 (Atkinson et al., 2004), respectively. As a
result, shown in Fig. 6, the [HONO]pss (PSS, violet curve) could not
explain the sudden HONO increase after sunrise, but it indicates a HONO peak
around 04:40 UTC according to the relatively high NO concentration.
However, some studies (Michoud et al., 2012; Sörgel et al., 2011)
already discussed that the stationary state of HONO can be only reached
during noontime. Hence, a model calculation (named Model 1) was also used to
discuss the HONO contribution from the gas-phase reaction of NO with OH
radicals:
d[HONO]dt=k7[OH][NO]+khet[NO2]-JHONO[HONO]-k11[HONO][OH],
where khet derived in this work is 0.027 h-1, and [NO] and [NO2] are
averaged concentrations from field measurements. The results are shown in
Fig. 6 (orange line, Model 1). It is reasonable to indicate that
Reaction (R7) only contribute 30 %–55 % to the HONO increase in the early
morning (04:30–07:30 UTC). Reaction (R7) can continually contribute 50 % of the
measured HONO from 10:30 to 16:30 UTC. However, regarding the large
uncertainty of [OH] (a factor of 2), the “unknown HONO sources” exist but
could be non-crucial. Basically, the additional HONO contribution rate could
be estimated from the following equation:
Punknown=d[HONO]dt+JHONO[HONO]+k11[OH][HONO]-k7[OH][NO].
An additional source of 91±41pptvh-1 was derived beside the OH
reaction with NO according to a HONO mixing ratio of 98±15 pptv for the
time period of 10:30 to 16:30 UTC. This could be well explained by the
photochemical processes such as Reactions (R5) and (R9) and will be discussed deeply in
the next section.
Evidence for nighttime-deposited HONO as a morning source
As observed in our field measurements and shown in Fig. 2, the HONO
concentrations always presented a strong increase from 04:00 to 07:00 UTC,
which induces three hypotheses as also mentioned in Sect. 3.3: (a) photolysis of gas-phase and particulate nitrate, (b) photosensitized
conversion of NO2, and (c) dew on ground surfaces serving as a HONO sink
during the night and becoming a morning source by releasing the trapped
nitrite back into ambient air.
To identify this HONO source, the chemical box model as expressed in Eq. (12)
was extended with additional processes. The heterogeneous reaction of NO2
on the wet surface (Reaction R2) and HONO deposition on the ground surface were
firstly used to quantify the contributions of the well-known HONO production
and loss processes. In addition, the HONO deposition on the ground surface
independent of RH (24 h, named Model 2) and with an RH dependence
(nighttime 17:00–08:00 UTC, named Model 3) are also discussed:
d[HONO]dt=k7[OH][NO]+khet[NO2]-JHONO[HONO]-k11[HONO][OH]-14γHONO,ground[HONO]υHONOH.
Both the surface production of HONO through the NO2 heterogeneous reaction
and subsequent loss by ground surface deposition are already termed in Eqs. (5)
and (8), respectively. Here, khet is 0.027 h-1, and γHONO,ground is (1.0±0.4)×10-4, which have been calculated from
the present observations. These values are applied to the model calculation
to simulate the diurnal cycle of HONO. As shown in Fig. 6, both Model 2
(blue line) and Model 3 (green square) cannot explain the HONO morning peak,
but Model 3 can reproduce well the nighttime HONO, indicating that surface
loss of HONO is an important sink to consider when the RH was saturated.
Hence, Model 3 was used as basic run for the following model calculation.
To investigate the contribution of photolysis of nitric acid and nitrate
(HNO3/NO3-) (Reaction R9) on the diurnal HONO based on hypothesis (a), the following model calculation (Model 4, pink line) was made:
d[HONO]dt=k7[OH][NO]+khet[NO2]+JHNO3[HNO3/NO3-]-JHONO[HONO]-k11[HONO][OH]-14γHONO,ground[HONO]υHONOH.
Here gas-phase HNO3 and particle NO3- are summed up, and the
photolysis frequency JHNO3 was derived from the TUV model by multiplying by
an enhanced factor of 30 due to a faster photolysis of particle-phase
HNO3 (Romer et al., 2018). As a result, the
photolysis of HNO3/NO3- (Model 4, pink line) could not
reproduce the HONO morning peak shown in Fig. 6. However, it could
reproduce well the HONO for the time period of 10:30 to 16:30 UTC.
To investigate the contribution of the photosensitized conversion of NO2
(Reaction R5) on the diurnal HONO based on hypothesis (b), the following model
calculation (Model 5) was performed:
d[HONO]dt=k7[OH][NO]+khet[NO2]+14γaSaV+γgSgVυNO2JNO2[NO2]-JHONO[HONO]-k11[HONO][OH]-14γHONO,ground[HONO]υHONOH.
Here γa and γg are the light-enhanced NO2
uptake coefficients, both 2.0×10-5
(Zhang et al., 2016), on both the aerosol surface and
ground surface, respectively. JNO2 was multiplied by the light intensity divided by 400 when the light intensity was greater than or equal to 400 Wm-2. As shown
in Fig. 6 (Model 5, cyan line), the photosensitized NO2 on the aerosol
and ground surface could not reproduce the HONO morning peak. This favors
the third hypothesis that dew evaporation processes release HONO resulting
in the sudden morning peak.
Example of HONOunknown99.5-RH as a function of time
(zero point from time 04:30 UTC) to estimate the temporary HONO emission
rate from dew water (kemission). The blue line is the linear least square
analysis of HONOunknown99.5-RH vs. the internal time to obtain the
minimum (e.g., 22 April for the low slope) and maximum (e.g., 24 April for the high slope) of kemission.
Indeed, as shown in Fig. S10, the HONO morning peak always happens according
to a rapid decrease in RH between 04:30 and 09:00 UTC. However, there is one case that
happened at 01:00 UTC on 25 April 2018 possibly due to an upcoming
strong wind which decreased the RH and evaporated the dew water on the
ground surface. It should be noted that this HONO morning peak was never
observed during this field measurement period without a rapid RH decrease as in the
case of a dry ground surface, as was observed during the morning of 23 April 2018. To figure out the relationship between temporary HONO
emission from dew water and decreasing RH, the following equation was
defined:
kemission=dHONOunknown99.5-RHdt=HONOunknown99.5-RH(t2)-HONOunknown99.5-RH(t1)(t2-t1),
where HONOunknown=HONOmeasure-HONOModel4 was
calculated for each day of the whole campaign period. The kemission values could
be obtained from the linear least square analysis of
HONOunknown99.5-RH vs. the internal time of the HONO morning peak
(04:30–07:00 UTC), as shown in Fig. 7. The maximum and minimum of
kemission are obtained as 0.026±0.008 and 0.006±0.001pptv%-1s-1, respectively, with an average of
0.016±0.014pptv%-1s-1, as presented in Table 4.
The average value was used in the following model calculation to reproduce
the diurnal cycle of HONO:
d[HONO]dt=k7[OH][NO]+khet[NO2]+JHNO3[HNO3/NO3-]+14γaSaV+γgSgV⋅υNO2JNO2[NO2]+kemission⋅(99.5-RH)-JHONO[HONO]-k11[HONO][OH]-14γHONO,ground[HONO]υHONOH.
In Fig. 6, Model 6 (red line) shows that the amount of deposited HONO
could represent the amount of HONO during the morning peak. In Fig. S11, the
measured atmospheric HONO mixing ratio and the calculated HONO mixing ratio
using Model 6 with a minimum dew HONO emission (kemission=0.006pptv%-1s-1) and a maximum dew HONO emission
(kemission=0.026pptv%-1s-1) are shown. HONO
emission from the dew water evaporation represented at least 90 % and
likely in excess of 100 % of the calculated unknown HONO morning peak,
which may continually serve as a HONO source for the whole daytime as long as
water evaporates depending on the weather conditions.
Summary of the temporary HONO emission rate from dew water,
kemission, from 19 to 29 April 2018.
Periodkemission (pptv%-1s-1) MinMax21 Apr 20180.00540.035722 Apr 20180.00480.031424 Apr 20180.00570.019226 Apr 20180.00670.030227 Apr 20180.00480.021528 Apr 20180.00790.017Mean0.006±0.0010.026±0.008Total average0.016±0.014HONO emission from dew water evaporation in the morning
The hypothetical morning HONO mixing ratio (pptv) due to the complete dew
water evaporation could be estimated from the following equation by taking
the measured dew nitrite and the mixing layer height:
[HONO]=α×Sg×FNO2-H×Sg=α×FNO2-H.FNO2- is the NO2- concentration per square meter of the glass
sampler surface. The mean FNO2- from 11 May 2019 was used for
the calculation. Sg represents the surface area of the flat ground
(analog to the surface area of the glass sampler), and α is the enhanced
factor for Vdew (dew water sample volume of the glass sampler in Eq. 2)
due to the larger cold surfaces from grass which can get in contact with
humid air than the flat glass sampler. The α value was calculated as
2×LAI to take the areas on both sides of the leaves and the
vegetation-covered areas on the ground into account, and a factor of 6 for
LAI was assumed and used in Sect. 4.2.2. However, regarding the
possibly different grass height during the HONO field measurement and dew
measurements in April 2018 and May 2019, respectively, we would use a range
of 1–6 for LAI in this section. During the HONO peak at 06:00 or 07:00 UTC, the
mixing height ranged between 175 and 600 m, while the value ranged from 20 to 200 m at 00:00–05:00 UTC. Hence, the overall concentration increase
from this source would be 377–2264, 189–1132, 76–122, 38–226 and 13–76 pptv
if all of the deposited HONO is released into the overlying air column for
mixing heights of 20, 40, 100, 200 and 600 m, respectively. Since the
released HONO was subjected to photolysis, using a JHONO from the TUV model
scaled by global radiation (Sect. 2.7), a maximum [HONO] of 176–1053,
88–527, 35–211, 18–105 and 6–35 pptv for the mixing heights of 20, 40, 100, 200
and 600 m, respectively, would be contributed from the surface nitrite
release at 07:00 UTC after the process started at 04:00 UTC. For a
reasonable 100 m mixing height, this would account for 5 %–30 % of the
observed HONO morning peak in Fig. 6. This low percentage might be a result
of the different sampling time of dew measurements compared to HONO
measurements, and further studies are required for the exact quantification.
Although the above calculations may be simplified well, the results do
suggest that the release of the deposited HONO on wet/moist canopy surfaces
may contribute to the morning HONO concentrations in the overlying
atmosphere right after dew evaporation.
Indeed, few field studies (He et al., 2006; Rubio et al., 2009) have
reported that dew water can serve as a sink and a temporary reservoir of
atmospheric HONO. Previously, the role of dew as a nighttime reservoir and
morning source for atmospheric NH3 has been reported by
Wentworth et al. (2016). Our results suggest that
nocturnally deposited HONO forms a ground surface reservoir which can be
released on the following morning by dew evaporation. Therefore, a
significant fraction of the daytime HONO source can be explained for the
Melpitz observations.
The OH production rates from the photolysis of HONO and O3 at Melpitz station from 19 to 29 April 2018. The OH
concentration is also shown as the yellow area plot, which was calculated from
the global radiation flux measurement: [OH]=A⋅Rad, which is taken from Größ
et al. (2018).
Impact on the primary OH sources
HONO serves as an important primary source of OH during daytime in the
troposphere (Kanaya et al., 2007; Kleffmann et al., 2005; Villena et al.,
2011). Seiler et al. (2012) reported that the HONO is almost the
only source of OH radicals in the early morning. The morning peak of HONO is
mainly released from the dew evaporation and could imply a strong supply of
OH radicals which, hence, enhances the atmospheric oxidizing capacity in the
atmosphere around Melpitz. Here, the net rate of OH radicals from the HONO
photolysis was calculated and compared with that from ozone photolysis,
which is typically proposed as the major OH radical source in the atmosphere
when water vapor is not limited.
R12O3+hν→O(1D)+O2(λ<320nm),R13O(1D)+H2O→2OH,R14O(1D)+M→O(3P)+M(M=N2).
Other OH sources, such as the photolysis of oxidized VOCs, peroxides and
ozonolysis of unsaturated VOCs, are not considered due to the lack of
measurement data for these radical precursors. The net rate of OH production
from HONO photolysis (PHONO→OH) was calculated by the source strength
subtracting the sink terms due to the reactions of Reactions (R7) and (R11). The OH production
rate (PO3→OH) from O3 photolysis can be calculated by using the
method proposed by Su et al. (2008) and Li
et al. (2018):
18PHONO→OH=JHONO[HONO]-k7[NO][OH]-k11[HONO][OH],19PO3→OH=2J(O1D)[O3]k13[H2O]k14M+k13[H2O],
where J(O1D) was obtained from the TUV model scaled by the global
radiation. The temperature dependences of k13 and k14 are taken
from Jet Propulsion Laboratory/NASA evaluation number 18 (Burkholder et
al., 2015). As shown in Fig. 8, the photolysis of HONO produced similar
amounts of OH compared to the photolysis of ozone at the mean daytime
(09:00–14:00 UTC), which is (7.2±2.0)×105moleculecm-3s-1. PO3→OH was, as expected, highest during the highest J values
and negligible at sunrise and sunset. PHONO→OH had a similar trend
after noontime but presented a strong OH production around 07:00 UTC
due to the HONO morning peak. These results demonstrate the significant role
of HONO in an atmospheric oxidizing capacity, especially for areas that
experience frequent dew formation. In addition, the OH concentration
calculated from the global radiation flux measurement was also shown in the
yellow color in Fig. 8. The different trend of calculated OH concentration
compared to PHONO indicates that the morning OH concentration could be
highly underestimated.
Conclusion and atmospheric implications
The intercomparison of MARGA and LOPAP for the HONO measurement was applied
from 19 to 29 April 2018 at the Melpitz site. A higher HONO
mixing ratio (ca. 117 %) was obtained from MARGA compared to that of
LOPAP caused by heterogeneous reactions within the MARGA WRD or a potential
sampling inlet artifact.
The maximum dew water NO2- concentration per square meter of the glass
sampler surface was determined to be 7.91±2.14µgm-2 in
May 2019. Thus, under consideration of photolytical losses and homogeneous
mixing, the maximum contribution to the HONO morning peak from dew water
evaporation could be calculated, and it ranged from 1053±45 to 35±1 pptv for mixing heights of 20 to 600 m, respectively.
Well-defined diurnal cycles of HONO with concentration peaks in the early
morning and in the evening are found. The high time resolution of HONO
measurements revealed that (i) vehicle emissions are a negligible HONO source
at the Melpitz site, (ii) HONO formed from the heterogeneous reaction of
NO2 on the ground surface is the dominant nighttime source with a high
NO2–HONO conversion frequency of 0.027±0.017h-1, and (iii) significant amounts of HONO (0.16±0.12ppbvh-1) are deposited on
the ground surface at night. The accurate observations of HONO and NO2
allowed for the direct estimation of the ground uptake coefficients for these
species at night: γNO2→HONO_g=2.4×10-7 to 3.5×10-6 and γHONO,ground=1.7×10-5 to 2.8×10-4.
The ground uptake coefficients of NO2 and HONO are within the ranges
of laboratory and model coefficients. The range of HONO uptake coefficient
values calculated in this investigation is potentially limited by a
combination of transport and diffusion to the ground surface.
A chemical model utilizing observational constraints on the HONO chemical
system and known sources and sinks supports the hypothesis that dew water on
the ground surface, especially on leaf surfaces, behaves as a sink at night
and a temporary reservoir for atmospheric HONO in the morning. The dew
evaporation had a negative relationship with the RH in the atmosphere, and,
hence, the HONO emission rate was estimated to be 0.016±0.014pptv%-1s-1 depending on the RH after sunrise
(start from 04:00 UTC). Furthermore, the formation and evaporation of dew on
the ground surface influence significantly the air–surface exchange of HONO
and, thus, its temporal distributions in the atmospheric boundary layer in
the morning and night. The OH production rate from the photolysis of HONO
compared to that from the photolysis of O3 showed that this dew emission
of HONO can strongly enhance the OH reactivity throughout the morning time and,
hence, plays a vital role in the atmospheric oxidation.
Data availability
The compiled datasets used to produce each figure within this paper are
available as Igor Pro files upon request.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-20-13069-2020-supplement.
Author contributions
YR wrote the paper with input from all authors. BS and GS analyzed the MARGA
and dew data and wrote the paper. YR and BG conducted the HONO measurements
using LOPAP. TT and AW were responsible for the particle measurements. AM and
HH designed the experiments and led the campaign. All coauthors commented
on the paper.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
For the laboratory analysis and the preparation of
solutions, we thank Anett Dietze, Anke Rödger and Susanne Fuchs. For support
especially in the field, we thank Rene Rabe and Achim Grüner. We also thank
the TROPOS mechanical workshop for the construction of the dew sampler.
Financial support
This research has been supported by the German Federal Environment Agency (UBA) research foundation (grant nos. 351 01 093 and 351 01 070) and the European Union (EU) for the transnational access (TNA) under ACTRIS-2. The comparison of HONO measurements with MARGA and LOPAP at TROPOS research site Melpitz (MARLO) is part of the project that has received funding from the European Union's Horizon 2020 research and innovation program (grant no. 654109). The
CNRS team (Orléans-France) acknowledges the support from Labex Voltaire
(grant no. ANR-10-LABX-100-01) and the ARD PIVOTS program (supported by the Centre-Val de
Loire regional council). Europe invests in Centre-Val de Loire through the
European Regional Development Fund.
Review statement
This paper was edited by Hang Su and reviewed by two anonymous referees.
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