an online monitoring system for atmospheric nitrous acid (hono) based on stripping coil and ion...

JOURNAL OFENVIRONMENTALSCIENCES

ISSN 1001-0742

CN 11-2629/X

www.jesc.ac.cn

Available online at www.sciencedirect.com

Journal of Environmental Sciences 2013, 25(5) 895–907

An online monitoring system for atmospheric nitrous acid (HONO) based onstripping coil and ion chromatography

Peng Cheng1, Yafang Cheng1,2, Keding Lu1, Hang Su2, Qiang Yang1, Yikan Zou3, Yanran Zhao3,Huabing Dong1,4, Limin Zeng1, Yuanhang Zhang1,∗

1. State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering,Peking University, Beijing 100871, China. E-mail: [email protected]

2. Multiphase Chemistry Department, Max Planck Institute for Chemistry, Mainz 55128, Germany3. School of Chemical Engineering & Environment, Beijing Institute of Technology, Beijing 100081, China

4. State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy ofSciences, Beijing 100029, China

Received 17 December 2012; revised 06 January 2013; accepted 01 February 2013

AbstractA new instrument for measuring atmospheric nitrous acid (HONO) was developed, consisting of a double-wall glass stripping coil

sampler coupled with ion chromatography (SC-IC). SC-IC is featured by small size (50 × 35 × 25 cm) and modular construction,

including three independent parts: the sampling unit, the transfer and supporting unit, and the detection unit. High collection efficiency

(> 99%) was achieved with 25 μmol/L Na2CO3 as absorption solution even in the presence of highly acidic compounds. This instrument

has a detection limit of 8 pptv at 15 min time resolution, with a measurement uncertainty of 7%. Potential interferences from NOx,

NO2+SO2, NO2+VOCs, HONO+O3, HNO3, peroxyacetyl nitrite (PAN) and particle nitrite were quantified in laboratory studies and

were found to be insignificant under typical atmospheric conditions. Within the framework of the 3C-STAR project, inter-comparison

between the SC-IC and LOPAP (long path liquid absorption photometer) was conducted at a rural site in the Pearl River Delta.

Good agreement was achieved between the two instruments over three weeks. Both instruments determined a clear diurnal profile

of ambient HONO concentrations from 0.1 to 2.5 ppbv. However, deviations were found for low ambient HONO concentrations (i.e. <

0.3 ppbv), which cannot be explained by previous investigated interference species. To accurately determine the HONO budget under

illuminated conditions, more intercomparison of HONO measurement techniques is still needed in future studies, especially at low

HONO concentrations.

Key words: interferences; intercomparison

DOI: 10.1016/S1001-0742(12)60251-4

Introduction

Among the reactive nitrogen species (NOy = NO + NO2

+ PAN + HNO3 + HONO + organic nitrate + HO2NO2 +

NO3, etc.), nitrous acid (HONO) plays an important role

in atmospheric chemical processes. Photolysis of HONO

(Reaction (1)) (R1) is a significant (or even dominant)

primary source of hydroxyl radical, the most important

oxidant in the troposphere.

HONO + hv (< 400 nm) −→ NO + OH (1)

HONO is also an indoor pollutant, which is harmful

to people’s health through the formation of nitrosamines

(Pttts et al., 1985; Febo and Perrino, 1991), and the re-

action with nicotine leading to so-called third-hand smoke

* Corresponding author. E-mail: [email protected]

(Sleiman et al., 2010). Furthermore, HONO is an important

ingredient of the global nitrogen cycle, contributing to

climate change (Kulmala and Petaja, 2011).

A large unknown daytime source of HONO is de-

duced by experimental budget analysis (Kleffmann et al.,

2005; Su et al., 2008a; Zhang et al., 2008; Li et al.,

2012). The proposed HONO source mechanisms were

mainly in the form of heterogeneous reactions on both

the ground and aerosol surfaces, such as heterogeneous

hydrolysis of NO2 (Finlayson-Pitts et al., 2003; Ramazan

et al., 2004) on different humid surfaces (R2), reduction

of NO2 with photo-sensitized mineral surfaces like TiO2

(Gustafsson et al., 2006; Ndour et al., 2008) and soot

particles (Ammann et al., 1998; Kalberer et al., 1999),

reduction of NO2 involving reducing organic compounds

(Gutzwiller et al., 2002) like diesel exhaust organics (R3),

896 Journal of Environmental Sciences 2013, 25(5) 895–907 / Peng Cheng et al. Vol. 25

hydrocarbons on soot (Monge et al., 2010) and humic

acids (R4) (Stemmler et al., 2006, 2007), and photolysis

of deposited HNO3/nitrate on surfaces (R5) (Zhou et al.,

2003), the photolysis of nitrophenol (Bejan et al., 2006),

etc. Recently, a new study pointed out that biogenic nitrite

in soil is able to release HONO (R6) and provide the source

strength to sustain the observed HONO concentrations in

the rural Pearl River Delta (Su et al., 2011). In addition,

the photolysis of NO2 under visible light (R7) (Li et al.,

2008) was also considered to be of significance for certain

areas or time periods. Overall, a general and quantitative

picture of the daytime HONO source is still missing for

the troposphere.

2NO2(ads) + H2O(ads) −→ HONO(g) + HNO3(ads) (2)

NO2 + HC(red) −→ HONO + HC(ox) (3)

NO2 + red(ads) −→ HONO(g) + ox(ads) (4)

HNO3/NO−3 (ads) + hv −→ HNO2(ads) + O(3P)(ads) (5)

NO−2 (aq) + H+(aq) ←→ HNO2(aq) ←→ HONO(g) (6)

NO2+hv (> 420 nm) −→ NO*2,NO*

2+H2O −→ OH+HONO (7)

To further explore the daytime HONO source mech-

anism, more comprehensive field and laboratory studies

need to be performed, including direct HONO measure-

ments. However, since HONO is a trace gas with mixing

ratio down to several tens of pptv in daytime due to its rapid

photolysis during daytime, one key task of these studies is

to develop instrumentation that can accurately and precise-

ly determine HONO concentrations with high sensitivity.

In the last three decades, many techniques were tested and

developed, including both spectroscopic and wet-chemical

approaches. Spectroscopic methods detect HONO by

its unique specific optical absorption spectrum, and in-

clude Fourier transform infra-red spectroscopy (Ndour

et al., 2008), differential optical absorption spectroscopy

(DOAS) (Stutz et al., 2010), tunable diode laser absorption

spectrometry (Schiller et al., 2001), and cavity ring down

spectroscopy (Wang and Zhang, 2000). In principle, the

spectroscopic techniques are quite selective, and should

be interference-free. In reality, a number of problems

exist. Firstly, the spectroscopic techniques are not sensitive

enough for the typical HONO concentrations observed

during daytime. Secondly, the spectroscopic techniques

are strongly influenced by aerosol concentrations when

they are in open path form; and are perturbed by surface

reactions when in cavity form. Moreover, Kleffmann et al.

(2006) found that the widely applied spectroscopic method

DOAS underestimates the actual HONO concentrations

due to the HONO impurities in the NO2 reference spectra.

The wet-chemical techniques determine the corresponding

aqueous-phase analyte of the water-soluble gases after col-

lection, and then the concentration of the gaseous pollutant

can be calculated. For example, there are conventional

filter techniques (Appel et al., 1981) and dry denuder

techniques (Febo et al., 1993), different types of wetted

diffusion denuders coupled with ion chromatography (IC)

systems (Vecera and Dasgupta, 1991; Slanina and Wyers,

1994; Zellweger et al., 1999; Zhang et al., 2003; Acker

et al., 2005), coil scrubber/HPLC-technique (Huang et al.,

2002), etc. Compared to spectroscopic methods, the wet-

chemical techniques are cheaper and much more sensitive,

with detection limits down to several pptv. But they often

suffer from sample artifacts and chemical interferences

generated during the process of sampling and analysis.

Moreover, some of them have drawbacks such as long

sample integration times and heavy maintenance work.

Continuous improvements have been made in these re-

spects. One of the most successful techniques is the long

path liquid adsorption photometer (LOPAP) (Heland et al.,

2001), in which the sampling artifacts can be effectively

minimized with an external sampling module by excluding

any inlet tubing, and the interferences are removed by a

two-channel instrument concept. Besides these two kinds

of methods, chemical ionization mass spectrometry is

the most recent development, by which HNO3, HONO

and some other gases can be detected individually with

different ionization sources (Roberts et al., 2010). It is

sensitive, selective and fast, but it requires expensive and

delicate system components, and also suffers from wall-

effect artifacts. Additionally, indirect measurements like

laser-induced fluorescence (which detects OH followed by

HONO photolysis) (Liao et al., 2006) and chemilumines-

cence (detects NOx followed by a denuder) (Brauer et

al., 1990) were also tested in previous studies; however

these were not so successful due to their high cost and low

sensitivity.

In China, HONO was first measured by SJAC-MOBIC

(Steam Jet Aerosol Collector Combined with Mobile Ion

Chromatograph System) in 2001 (Zhou et al., 2002). In

the follow-up studies, several different techniques were

successfully applied to determine the ambient HONO

concentration in a few field measurements (Su et al.,

2008a, 2008b; Li et al., 2012; Qin et al., 2009; Dong et al.,

2012), including DOAS, LOPAP, and a modified version of

SJAC, namely, GAC-IC (Gas and Aerosol Collector with

IC). Typical daytime HONO concentrations were about

several hundred pptv in a rural site of Pearl River Delta and

1 ppbv in Beijing (Lu et al., 2010). Accordingly, a strong

unknown daytime HONO source for Pearl River Delta

and Beijing could be deduced through the HONO budget

analysis as well as from observations performed in other

countries. The intercomparison between GAC and SJAC

indicated that a significant interference could be caused by

a long sampling line under irradiation (Su et al., 2008a),

and the intercomparison between GAC and LOPAP further

demonstrated the advantage of a short inlet line offered by

the external coil sampler of LOPAP (unpublished results of

the PRIDE-PRD2006 campaign). As part of a continuous

No. 5 An online monitoring system for atmospheric nitrous acid (HONO) based on stripping coil and ion chromatography 897

effort following the developments of the wet chemical

methods based on ion chromatography detection, in the

present study, a new in situ wet-chemical technique based

on a glass stripping coil sampler-ion chromatography com-

bination (SC-IC) was developed and evaluated. A series of

experiments were carried out to optimize this instrument

to minimize the potential interferences and maximize the

measurement sensitivity. Moreover, extensive lab studies

were performed to investigate the potential interferences

of SC-IC. Finally, our SC-IC instrument participated in

a three-week field campaign (3C-Star) in the Pearl River

Delta during Autumn 2008. Good agreements of the ob-

served HONO concentrations were achieved between our

SC-IC and a commercial LOPAP instrument. Intercompar-

ison of the two instruments was also investigated.

1 SC-IC instrument

1.1 Measurement principle

In this work, the ambient HONO is first collected in a

stripping coil and converted to NO2− (R8) in the liquid,

and then detected by an ion chromatograph.

HONO←→ H+ + NO−2 (8)

Based on the spiral structure of the coil, efficient mixing

of gas-liquid phases is achieved with much smaller volume

and significantly reduced residence time (ca. 40 msec)

compared to that of the denuder technique. Therefore, the

influence of the possible heterogeneous reactions that pro-

duce HONO within the sampling processes is minimized.

According to Lee and Zhou (1993) and Zhou et al. (1999),

the ideal collection efficiency of HONO, namely β, is a

function of the effective Henry constant of HONO (H∗),temperature (T ), gas constant (R), the gas flow rate (Fg)

and liquid flow rate (Fl).

β =FlH∗RT

Fg + FlH∗RT(9)

Here, the effective Henry constant, can be further defined

as (Zellweger et al., 1999)

H∗ = H × (1 + Ka/[H+]) (10)

where, Ka is the ionization constant of HONO, H is the

Henry constant and [H+] is the acidity. According to Eqs.

(9) and (10), we deduced that a high β of 99% could be

achieved when pH ≥ 6.2 under experimental conditions of

Fg/Fl � 104, T � 293 K. However, an alkaline solution

(pH > 7) would increase the accommodation probability

of other reactive nitrogen compounds. As a compromise,

we chose a solution of 25 μmol/L Na2CO3 (pH = 6.9 in

equilibrium with 370 ppmv CO2 in atmosphere) (Su et al.,

2008a) as the absorbing solution for SC-IC.

The liquid sample is further injected into a sample

loop of known volume, and then onto an ion exchange

column with a buffered solution (eluent). Anions in the

sample are retained by the stationary phase on the column

based on coulombic (ionic) interactions, but can be eluted

afterward with specific retention times. After passing an

anion suppressor, the separated NO2− is finally detected

by the conductivity detector. From the nitrite concentration

in the liquid sample (cNO−2 ), the ambient mixing ratio of

HONO can be calculated by the following equation:

cHONO =cNO−2 × Fl × R × T × φ

MNO−2 × Fg × P× 106 (11)

where, P (Pa), MNO−2 (g/mol) denote atmospheric pres-

sure, and molecular weight of NO−2 , respectively. φ =

αBrstd/αBrsamp

is the calibration factor of the IC system,

which comes from the internal NaBr standard addition,

where αBrstdand αBrsamp

denote the area of Br− detected

by IC in the standard solution and the liquid sample,

respectively.

1.2 Technical setup

As shown in Fig. 1, our instrument contains three separated

and enclosed units: The sampling unit, transferring and

supporting unit and detection unit. Of the sampling unit,

the sampler is a five-turns spiral coil (30 mm average

turn diameter) enclosed by a double-wall glass cylinder,

which is further protected by a small and ruggedized box

with dimensions of 15 × 12 × 12 cm (length, width and

height). The coil consists of a glass tube (2 mm inner

diameter) and a gas-liquid separator at the end. A water

bath is utilized to keep the coil at a constant temperature

(293 ± 0.1 K). Furthermore, the protecting box is also

designed to provide additional thermal insulation and light

shielding, and the latter feature prevents photochemical

reactions from taking place in the sampling unit. Gas

and liquid exchange between the other two units and the

sampling unit are achieved by long tubes (in current setup

about 4 m) with thermal insulation and light shielding.

Stripping coil

Computer IC

20°C water bath

Dryer MFCPump

Sec. bottle

Valve

Filter

Col. vial Abs. solution

Peristaltic pump

Fig. 1 Schematic drawing of stripping coil-ion chromatography (SC-

IC). The sampling unit, transferring and supporting unit and detection

unit are enclosed by red, blue and pink frames, respectively. MFC: mass

flow controller.


Consequently, the sample unit is quite movable referred to

the main instrument, so that it can be placed outside and the

length of the inlet line can be minimized (down to about

3 cm). In this case, the potential interferences generated

by heterogeneous reactions on the inlet surfaces can be

excluded.

The transferring and supporting unit contains a security

bottle, a plastic bottle with scrubbing solution, a collection

vial, two peristaltic pumps, a membrane air pump, a

mass flow controller (MFC), and a power supply. Devices

containing or transferring liquid were separated and insu-

lated for safety. Electric components were integrated and

controlled by Transistor-Transistor Logic signals from the

IC system. In a measurement cycle, the ambient air is

sampled into the coil drawn by a membrane pump, mixing

with the scrubbing solution driven by a peristaltic pump.

The gas and liquid mixture flows through the coil to the

gas-liquid separator, and then is pumped out by the same

peristaltic pump. The downward flow from the gas-liquid

separator is collected and stored in the collection vial, in

which air bubbles can escape from the liquid. The liquid

samples in the vial are further pumped into an IC system

by another peristaltic pump every 15 min. Experiments

confirmed that the current sample is not perturbed by the

retention of the previous one.

Of the detection unit, an IC system (ICS-90s, Dionex,

USA) is utilized to quantify the nitrite (NO2−) in the

liquid sample. The IC system was specially designed to be

small and compact for application in field campaigns. It is

equipped with a 4 × 25 mm guard column (IonPac AG14)

followed by a 4 × 250 mm analytical column (IonPac

AS14). A solution of 3.5 mmol/L Na2CO3 and 1.0 mmol/L

NaHCO3 is used as eluent at a flow rate of 1.0 mL/min,

resulting in a 15 min chromatographic time cycle, which

determines the highest time resolution of the system.

1.3 Characterization of SC-IC system

1.3.1 HONO sourceWe built up a stable and high-purity HONO generator

to characterize and optimize the performance of the in-

strument. In this generator, HONO is generated via the

reaction between sodium nitrite and excess dilute sulfuric

acid in a coil. The produced HONO is further entrained by

a concurrent zero air flow supplied by a zero air generator

(Model 1011, Thermo, USA). Consequently, stable HONO

gas flow with concentrations from 23 to 128 ppbv was

generated by changing the flow rate of the reagent solution

(Fig. 2).

1.3.2 Collection efficiency: gas and liquid flow ratesThe ratio of gas and liquid flow rates (Fl/Fg) is an impor-

tant parameter to characterize different samplers used in

wet-chemical techniques. With a small ratio, the ambient

HONO may not be completely absorbed; while with a

high ratio, the solution might be too dilute and result in

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

20

40

60

80

100

120

140

HO

NO

gen

erat

ed (

ppbv)

Flow rate of reagent solution (mL/min)

y = 91.172x + 8.5123

R2 = 0.9982

Fig. 2 HONO concentrations generated in the HONO source as a

function of reagent flow rate. Concentration of sodium nitrite and sulfuric

acid was 1 ppm and 1‰ respectively. Flow rate of the carrier gas was 3

L/min, the coil temperature was controlled by water bath to be 293 K.

poor detection sensitivity. In addition, it is necessary to

consider minimizing the gas-liquid contact time to inhibit

heterogeneous reactions. However, reducing the gas-liquid

contact time cannot be simply achieved by increasing

the flow rate, due to the boundary conditions set by the

subsequent separations. According to Sauer et al. (1999),

Zhou et al. (1999), Heland et al. (2001) and our previous

experiences, a sampling gas flow rate of 2 L/min was fixed

by a MFC (D07-7B, Sevenstar, China), while the flow

rate of the liquid was adjusted to derive the optimal ratio.

The effective collection efficiency, βe was determined by

measuring HONO concentrations with two identical coil

samplers in a series connection. The signals of the two

coils were detected by the IC system simultaneously. βe

was then calculated by the following equation:

βe = (1 −C2/C1) × 100% (12)

where, C1 and C2 were the concentrations of nitrous acid

collected by the first coil and the second, respectively.

Figure 3 shows a series of experiments performed with

40 ppbv gaseous HONO generated by our standard HONO

source at 293 K. It can be found that complete absorption

(βe > 99%) was achieved when the liquid flow rate was

above 0.19 mL/min. For this reason, the final sampling

conditions of Fg = 2 L/min and Fl = 0.2 mL/min were

adopted for later experiments. The return flow from the coil

sampler was slightly smaller (< 5%) than that pumped into

the coil, indicating that the evaporation during gas-liquid

interaction is negligible.

In polluted urban environments, the acidic components

in the atmosphere will increase the acidity of the scrubbing

solution significantly, which might cause a reduction of the

collection efficiency (Zellweger et al., 1999). Since SO2

is the typical and major acidity provider, we investigated

the impact of SO2 on the collection efficiency. For a

test HONO concentration of 40 ppbv, the βe was slightly


0.00 0.05 0.10 0.15 0.20 0.25

40

50

60

70

80

90

100

110

Coll

ecti

on e

ffic

iency

(%

)

Flow rate of scrubbing solution (mL/min)

Fig. 3 Collection efficiency of HONO in the sampling coil as a

function of the flow rate of the scrubbing (Fl) solution at 293 K. The

applied sample gas flow rate is 2 L/min and the corresponding HONO

concentration is 40 ppbv.

reduced from 99.98% to 99.77% when 120 ppbv SO2

was added. This experiment confirms that the ambient

HONO can be completely collected in our coil in polluted

environments.

1.3.3 Blank, limit of detection and accuracyThe instrumental blank of the IC, reagent blank and system

blank were all investigated. For anionic species analyzed

with IC, the background noise was calculated by the

variance of the baseline at the retention time of nitrite

ion by injecting ultra-pure water (18.2 MΩ, MilliQ) into

the IC; the reagent blank was obtained by injecting the

scrubbing solution into the IC; the last one was obtained by

sampling zero air. The three blanks were almost identical

within ± 10%, corresponding to a gaseous mixing ratio of

HONO of ± 4 pptv. This result ensured that there were no

detectable impurities in the reagent or in the zero air.

The limit of detection (LOD) of the SC-IC instrument

was derived as 3 times the standard deviation of system

blank signals (3σ). The LOD of 8 pptv for HONO was

deduced from the system blank measurements. This is

close to Trebs’ (2004) work (12 pptv), in which HONO

was collected by a wet-annular denuder and detected by

an IC system. This detection limit is sufficient for the

HONO measurements in most atmospheric environments.

However, on a long term run, the LOD may be degraded

due to the unstable and noisy baselines caused by aging of

the ion exchange columns and anion suppressor.According to Eq. (11), the systematic error of the HONO

measurement is estimated by the sum of the uncertaintiesof the air flow rate, the liquid flow rate, Br− signals instandard and samples, and the mixing ratio of nitrite inthe liquid sample (coming from the uncertainty of theslope of the calibration fit). The measurement accuracywas estimated to be 7% following the Gaussian ErrorPropagation (Trebs et al., 2004):

σs =

√(σFg

Fg)2 + (

σFl

Fl)2 + (

σαBrstd

αBrstd

)2 + (σαBrsamp

αBrsamp

)2 + (σslope

slope)2 (13)

where, σs is the relative system uncertainty, and σx (x = Fg,

Fl, Brstd, Brsamp, slope) are the absolute standard deviations

of the corresponding parameters.

1.4 Calibration

We routinely performed calibration of the SC-IC system

with 1000 mg/L (NO2−) NaNO2 stock solution (Merck),

which was found to be stable for more than 3 months when

stored in a refrigerator (277 K). A gradient of working

standard solutions with concentration range of 5–100 μg/L

(corresponding to gaseous HONO mixing ratios from 0.28

to 5.6 ppbv) were prepared by two-stage dilution of the

stock solution. The correlation for the linear fit of the

calibration line was 0.9987 (R2, Fig. 4). In addition, a small

amount of NaBr was added to the scrubbing solution as

an internal standard to calibrate the IC. Calibrations of

MFC (Fg) were performed every week with a Gilibrator

(Gilibrator2, Sensidyne, USA). Daily calibrations of the

liquid flow (Fl) were performed, due to the fast aging of

silicone tubes and the need for the tubes to be replaced

when necessary.

0 20 40 60 80 100 120

0

20

40

60

80

100

y = 0.8816x + 0.0012

R2 = 0.9987

C

(×

10-

3 m

g/L

)

IC signals (×10-3 μS.min)

NO

2

-

Fig. 4 Calibration of the IC system by NaNO2 with direct injection to

the sample loop of IC.

2 Interferences

Extensive laboratory studies were performed to explore the

possible interference signals contributed by reactive nitro-

gen compounds – NO, NO2, nitrate, PAN and mixtures

NO2 + SO2, NO2 + VOCs, HONO + O3 under experi-

mental conditions. In these experiments, the impurity of

HONO in the interfering species was carefully removed

beforehand with another stripping coil. All tubing and

connectors were kept as short as possible to inhibit any

heterogeneous reactions or retention on the inner walls of

the system. A NOx Analyzer (ML9841B, Monitor Labs,

Australia), O3 Analyzer (49C, Thermo, USA) and SO2

Analyzer (43C, Thermo, USA) were used in these probe

experiments.


2.1 NOx, NO2+SO2, NO2 + VOCs

Since high concentrations of NOx from tens of ppbv to

hundreds of ppbv can be present in polluted areas, the

heterogeneous reactions of NOx and water in the sampling

processes have long been considered to be a major source

of measurement artifacts in wet chemical techniques. We

prepared a series of NO/NO2 mixtures of 100–300 ppbv

with a NO gas standard (86 ppmv NO in N2, Huayuan Gas-

es) and a Gas Dilution Calibrator (GasCal1100, Ecotech,

Australia), containing an ultraviolet lamp so that NO2 can

be quantitatively produced by the reaction of NO with O3.

300 ppbv NO was sampled at first, and the HONO signals

were just around the system blank (Fig. 5). Secondly, a

series of NO2 mixtures with mixing ratios of 100–300 ppbv

were sampled. A tiny amount of HONO was detected,

and a linear correlation with a slope of 0.03‰ between

the mixing ratios of HONO and NO2 was found (Fig.5). Our result is better than a similar instrument using

a 28-turn coil sampler, of which a slope of 0.1‰ was

observed toward NO2 injected through scrubbing solutions

with similar pH (Zhou et al., 1999).

Formation of nitrous acid from dissolved NO2 and SO2

in the liquid phase has been a substantial interference

in wet-chemical methods, especially in alkaline solutions

(Spindler et al., 2003; Littlejohn et al., 1993). The reaction

of NO2 and SO2 mainly takes place on the wet surface of

the sampler and is relatively slow (Spindler et al., 2003),

thus this kind of interference can be effectively avoided

by using a small inner surface for the stripping coil and

a short gas-liquid contact time. For our SC-IC instrument,

the HONO signals were almost identical within instrument

uncertainties before and after a high level (100 ppbv) of

SO2 was added in the presence of high (300 ppbv) NO2

concentrations (Fig. 5).

The reaction of NO2 with adsorbed hydrocarbons dur-

ing the sampling of HONO could be another source of

interference (Gutzwiller et al., 2002). Ethene, ethyne,

toluene, and n-butane at mixing ratios of 250, 230, 215 and

200 ppbv, respectively, were added with 300 ppbv NO2.

Measurement results indicated that there was no additional

HONO formation via in situ reaction between NO2 and

0

4

8

12

16

20

HO

NO

sig

nal

s (p

ptv

)

Limit of detection

System blank

NO sampled NO2 sampled NO2 and SO2

sampled

NO2 and VOCs

sampled

300 ppbv

100 ppbv

200 ppbv

300ppbv

300 ppbv NO2

100 ppbv SO2

300 ppbv NO2

250 ppbv ethene

230 ppbv ethyne

215 ppbv toluene

200 ppbv n-butane

Fig. 5 Interferences in HONO measurement from NO, NO2, NO2 +

SO2, NO2 + VOCs.

VOCs (Fig. 5).

2.2 HNO3 and peroxyacetyl nitrite (PAN)

The wet-chemical methods also suffer from interference

from some other nitrogen-containing compounds. Zhou

et al. (2002b) found that photolysis of adsorbed nitric

acid/nitrate on the inner surface of sample lines may be

a potential interference for ambient HONO measurement.

Since no inlet tubing was used in the SC-IC instrument,

and the sampling unit was designed to be light-shielded,

any interference signal contributed by ambient HNO3

is avoided in principle. We generated gas-phase HNO3

following the method of Canosa et al. (1988). We first

prepared a mixture of nitric acid and sulfuric acid (1:2),

and secondly bubbled through this mixture (kept at –12°C,

by an ice-glycol bath) a small flow of N2 gas (1–5 sccm),

finally diluted with 3 L/min zero air. The generated HNO3

was monitored by ion chromatography after sampling with

a stripping coil. It was found that no nitrite signal was

detected when a high concentration (47 ppbv) of HNO3

was generated and sampled.

PAN is slightly soluble in water and can be hydrolyzed

to form nitrite (Frenzel et al., 2000). A system to generate

PAN similar to that of Nielsen et al. (1982) was built to test

its impact on HONO measurement. In this method, 2.5 mL

peracetic acid was added into 25 mL n-tridecane, then 2

mL 98% H2SO4 was added. After 5 min reaction, 0.5 mL

fuming HNO3 was added slowly. The mixture was cooled

with constant stirring during the entire procedure. After

another 5 min, it was poured into 25 mL of ice water in a

separatory funnel, in which the tridecane layer containing

PAN was separated from water-soluble impurities. The

PAN-tridecane solution was desiccated with MgSO4 and

stored at low temperature until use. A steady flow of

high-purity N2 gas (5–20 sccm) was passed through the

PAN-tridecane solution kept at 273 K and then diluted

with 3 L/min zero air. The PAN concentration in the

diluted gas was monitored by a NOx analyzer, since the

conversion efficiency of PAN was greater than 90% for

the Mo converter (Zhou et al., 1999). The generated gas

flow with 45 to 190 ppbv PAN was sampled by the SC-

IC instrument. The interference strength of PAN toward

HONO was determined to be 0.63‰. Considering typical

high PAN concentrations (several ppbv) present in polluted

urban areas, the propagated HONO interference signal is

just several pptv; thus it can be neglected.

2.3 HONO + O3

Since HONO might be oxidized by O3 and H2O2 in the

liquid phase (Damschen and Martin, 1983), this was a

problem for some early off-line techniques with long sam-

ple integration times (Vecera and Dasgupta, 1991; Sickles

and Hodson, 1989). However, some recent studies found

that the effect of ozone and H2O2 on HONO measurement

was negligible (Heland et al., 2001; Zhou et al., 1999;


Boring et al., 2002) if the contact time between sample

air and scrubbing solution was short. The result was also

confirmed by our SC-IC instrument. No oxidation of NO2−

to NO3− was observed when 300 ppbv O3 was added into

a 40 ppbv HONO gas flow.

2.4 Particulate nitrite

The contribution of particulate nitrite to the HONO mea-

surement by the SC-IC system was determined by two

factors: the first one is the particle uptake rate in the

coil and the other one is the nitrite abundance in the

particulate phase. The particle uptake mechanisms in the

coil include diffusion, inertial impaction, interception and

turbulent deposition. Interception and impaction deposi-

tions are the important ones for large particles, while

diffusion deposition is important for ultra-fine particles.

The role of ultra-fine particles can be neglected since they

do not contain significant aerosol masses. By contrast,

more attention needs to be paid to particles larger than 500

nm. The turbulent gas flow in the stripping coil results in

an enhancement of uptake of large aerosols due to their

impaction or interception on the surface of the stripping

coil.

In this study, aerosols were generated by a particle

generator (Atomizer 9302, TSI) with a solution of NaNO2

(NO2−, 1000 mg/L) under a pressure of 4.1 × 104 Pa

and diluted to 10 L/min with zero air. With a SMPS–

Scanning Mobility Particle Sizer (DMA3081-CPC3776LF,

TSI, USA), the number and mass size distributions of

the generated aerosols were determined (Fig. 6). These

aerosols were mono-dispersed with a maximum of number

concentration at 117.6 nm, while the major part of total

aerosol mass was contributed by particles larger than 400

nm.

The generated nitrite particles were fed into the stripping

coil sampler, with a Teflon filter (Whatman, UK) placed

downstream of the coil to collect the remaining particles.

The filter samples were extracted by ultrapure water and

10 100 1000

0

1

2

3

4 dN/dlog(Dp)

dM/dlog(Dp)

-1

0

1

2

3

4

5

6

7

8

9

Diameter (nm)

dM

/dlo

g(D

p)

(×10

3 μ

g/m

3)

dN

/dlo

g(D

p)

(×10

5 c

m-3)

Fig. 6 Number (N) and mass size (M) distribution of particles generated

for the particulate nitrite interference experiments. Mass concentration

253 μg/m3, relative humidity in the range of 21%–29%, temperature

298 K.

analyzed by the IC system. According to the experimental

results, the absorbed particles in the coil were calculated

to be (22 ± 9)% (mass) of the total particles generated.

Similar tests were also conducted with the coil sampler of

a LOPAP instrument. Broske et al. (2003) reported only

1% losses of SOA (secondary organic aerosol) particles

with diameters of 50–800 nm, and Kleffmann et al. (2007)

reported 1.7%–2.2% losses of NH4NO3 particles with

diameters from 50–900 nm in the sampling coil of LOPAP.

The difference in absorbed mass fraction of the test parti-

cles between SC-IC and LOPAP is considered to be due to

the different structures of the coils, different particle com-

ponent and size distributions, etc. In another study (Huang

et al., 2002), the summed relative detection sensitivity from

both aerosols and gaseous precursors in gaseous HONO

measurements by coil sampler was estimated to be (6 ±2)%. Since particulate nitrite is only a minor fraction of

the total nitrite budget in the atmosphere, we think the

coil sampler they used had a particle uptake rate similar

to our SC-IC. With respect to the HONO measurement

done with denuders, the absorbed mass fraction of the test

particles was determined to be of 3%–5% (Lane et al.,

1988; Koutrakis et al., 1990; Sioutas et al., 1994). Because

in the denuder technique it is much easier in principle for

particles to pass through than with of a coil sampler, we

think the ratio of the particle absorption determined for our

instrument is plausible.

Table 1 summarizes the above investigated HONO

measurement interferences in our SC-IC instrument con-

tributed by a series of reactive nitrogen compounds or their

mixtures with ambient acidic compounds, reactive hydro-

carbons, and oxidants. In the left column, the interference

is given as the measured HONO signal divided by the

mixing ratio of the corresponding interfering compound.

For some of the experiments, the signals were too small to

be detected, they were thus labeled as lower than limit of

detection (< LOD). In the right column, an extrapolation to

real values in a field campaign (3C-Star) was performed.

The possible HONO interference signals under such am-

Table 1 Summary of the investigated HONO interferences in our

SC-IC instrument

Parameter Interference

Relative detection Absolute interference

sensitivities compared signal estimated for a

to HONO campaign (3C-Star)

NO < LOD < LOD

NO2 0.034‰ < LOD

NO2+SO2 0.034‰ < LOD

NO2+VOCs 0.034‰ < LOD

HNO3 < LOD < LOD

HONO+O3/O3 < LOD < LOD

PAN 0.63‰ < LOD

Particle (22 ± 9)% (NaNO2) 11 pptv (average in daytime)

18 pptv (average in night)

Limit of detection (LOD) = 8 pptv herein, the laboratory results and an

extrapolation to a field campaign.


bient conditions were derived as the product between the

relative detection sensitivity of a certain parameter and its

observed values. The results listed in Table 1 indicate that

interferences from the gas phase are negligible while that

contributed by particulate nitrite could be of significance

(22 ± 9)%. It is thus necessary, as a part of method

validation, to estimate the effect of nitrite in aerosols

on HONO measurement under atmospheric conditions.

Lammel and Cape (1996) have summarized some results

of particle measurements in the field, and concluded that

particulate nitrite is only a minor fraction of the total

nitrite in the atmosphere, and no relationship between gas-

phase HONO and particulate nitrite should be expected, in

respect that the effective Henry’s law constant of HONO is

quite small under the prevailing acidic aerosol conditions

(Park and Lee, 1988). Particulate nitrite concentrations

measured by a GAC instrument in the Beijing and Pearl

River Delta regions were also in a very low range, e.g.,

below the detection limit or less than 10% of gaseous

HONO (Personal communication with Dong). But excep-

tions existed indeed. Strong correlation with mean ratios of

HONO to aerosol nitrite of 1:1.6, 0.65:0.35 were observed

by Simon and Dasgupta (1995), and Acker et al. (2005),

respectively. In addition, a ratio of HONO/nitrite between

0.07 and 10.58 with an average of 1.68 was observed by

Zellweger et al. (1999). Although Zellweger thought the

high ratio of HONO/nitrite reflects breakthrough of HONO

in the denuder, this assumption has not been verified.

In conclusion, the effect of particulate nitrite on HONO

measurement is generally weak but may be a problem in

some exceptional cases. In addition, since HONO can be

dissolved in fog water with concentrations up to hundreds

of μmol/dm3 both in rural and urban conditions (Lammel

and Cape, 1996; Acker et al., 2008), interferences of foggy

droplets can be significant. However, as we estimated for

the ambient conditions during the 3C-Star campaign, the

contribution of particulate nitrite therein was marginal.

3 Field measurements and intercomparisonwith LOPAP

In the framework of an intensive campaign (Synthesized

Prevention Techniques for Air Pollution Complex and

Integrated Demonstration in Key City-Cluster Region, 3C-

Star) in the Pearl River Delta of China from October

to November, 2008, the SC-IC system was applied to

measure the ambient HONO concentrations at Kaiping

(22.32◦N, 112.53◦E) super site, a typical rural area about

120 km southwest of Guangzhou. It is downwind of

the central polluted Peal River Delta area due to the

East Asian monsoon in that season. Online monitoring

of NOx, NOy, SO2, O3 and PM2.5 was performed by

a series of commercial instruments (Table 2). PAN was

measured by a self-manufactured instrument based on

the gas chromatography-electron capture detector (GC-

Table 2 Instruments for atmospheric trace gas measurements at

Kaiping in autumn 2008

Species Instrument Time resolution Detection limit

NOx Photolytic 1 min 50 pptv

NOy Thermo 42i-Y 1 min 50 pptv

SO2 Thermo 43C 30 sec 1 ppbv

PAN GC-ECD 5 min 5 pptv

PM2.5 TEOM 1400a 30 sec 0.1 μg/m3

Nitrite in PM2.5 GAC-IC 30 min 0.023 μg/m3

ECD) method; particulate nitrite in PM2.5 was measured

by a self-developed GAC instrument (Dong et al., 2012).

Figure 7 shows an overview of the time series of HONO

and ancillary parameters which are potential interfering

species for ambient HONO measurement from October 29

to November 19, 2008.

Since the LOPAP instrument has been successfully used

for HONO measurements in both field and laboratory

studies for a decade, it was utilized as the reference

instrument for an intercomparison with our developed SC-

IC instrument in this field campaign. Another reason for

choosing LOPAP is that SC-IC and LOPAP are both in

situ wet-chemical techniques equipped with a coil sampler

and without additional inlet tubes. The LOPAP used at

the Kaiping site is a modified version of the commercial

LOPAP instrument (QUMA GmbH, Wuppertal). Detailed

information about the instrumental setup can be found

in Li et al. (2012). The external sampling units of both

instruments were fixed on the roof (1.5 m above the roof

surface) of a three-story building side by side. System

calibration of the SC-IC and gas flow rate calibration

were performed every five days, and liquid calibration was

conducted every day.

Overall, reasonably good agreement of the measured

HONO concentrations was achieved between the instru-

ments (Fig. 7a). The observed HONO concentration varied

from tens of pptv to several ppbv. For most of the days a

clear diurnal variation was presented. The averaged diurnal

profiles of observed HONO concentrations by LOPAP and

SC-IC are shown in Fig. 8. The data measured by both

instruments showed good agreement during both day and

night; of which a steady accumulation of HONO during

nighttime was characterized, reaching a maximum (about

1 ppbv) in the morning, and then constantly decreased to

0.2–0.3 ppbv around sunset due to photolysis. The signifi-

cant daytime HONO concentrations (0.4–0.5 ppbv around

noontime) implied a large contribution to daytime OH

initiation processes since the HONO photolysis frequency

is larger than that of O3 by more than fifty times.

To compare the observed HONO concentrations by

SC-IC and LOPAP in a more quantitative way, we first

synchronized the data of LOPAP and SC-IC into 15 min

intervals, and then applied a linear regression analysis

with both “bivariate” method and “standard” method as

proposed by Cantrell et al. (2008) (Fig. 9). With the “stan-


0

1

2

3

0

20

40

60

0

2

4

6

80

20

40

60

Oct 28 Oct 30 Nov 1 Nov 3 Nov 5 Nov 7 Nov 9 Nov 11 Nov 13 Nov 15 Nov 17 Nov 19

0.0

0.2

0.4

0.6

0.80

50

100

150

200

HONOSC-IC

HONOLOPAP

HO

NO

(p

pb

v)

a

NOy NO2

NO

y,

NO

2 (

ppbv)

b

PA

N (

pp

bv

)

d

SO

2 (

ppbv)

c

N

itri

te i

n P

M2

.5 (

μg/m

3)

PM

2.5 (

μg/m

3)

f

Date

e

Fig. 7 Time series of observed HONO (black: SC-IC and red: LOPAP), NO2, NOy, SO2, PAN, PM2.5, nitrite in PM2.5 during Oct 29 to Nov 19, 2008

at Kaiping.

0 4 8 12 16 20 24

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

HONOSC-IC

HONOLOPAP

HO

NO

(p

pb

v)

Time (hr)

DaytimeNight Night

Fig. 8 Mean diurnal profiles of the observed HONO concentrations by

SC-IC and LOPAP from Oct 29 to Nov 19, 2008 at Kaiping. Individual

HONO measurement results were averaged into one hour intervals. Error

bars of each data point denote the ambient variability in corresponding

time intervals. The nighttime is colored by medium grey.

dard” method (default options applied in several popular

analyzing software programs such as Excel, Origin, etc.),

the distances between the fitted line and the data are min-

imized only in the y-direction (only y-error is considered).

Following this approach, the slope of regression is 1.02

± 0.01 and the intercept is 0.09 ± 0.01 ppbv. However,

use of such an approach is normally limited to conditions

where the error of the x-variable is much smaller than that

of the y-variable. For our purpose, both the errors of the

x-variable and y-variable shall be accounted for. Thus, we

further applied the “bivariate” method, which minimizes

the perpendicular distances between the fitted line and the

data. The yielded slope of regression is 0.86 ± 0.02 and the

intercept is 0.26 ± 0.02 ppbv. The difference of the regres-

sion slope characterizes the difference of the calibration

standard applied for both instruments. Nevertheless, within

the combined accuracy as stated for LOPAP (10%) and

SC-IC (7%), the agreement is quite good. Sensitivity tests

were performed with the “bivariate” method to diagnose

the possible impact of the measurement errors on the

regression results. It was found that the calculated slope

and intercept were relatively constant toward the prior

case when similar measurement errors were estimated for

both instruments. However, if we assume big differences

existed between the measurement errors of SC-IC and

LOPAP, the calculated slope and intercept would change

significantly. Interestingly, the diagnosed slope approaches

unity when the applied measurement error of LOPAP is


0.0 0.5 1.0 1.5 2.0 2.5

0.0

0.5

1.0

1.5

2.0

2.5

HO

NO

SC

-IC (

pp

bv

)

HONOLOPAP

(ppbv)

N = 1440

Fig. 9 Comparison of the HONO concentration measured by the SC-IC

and LOPAP instruments. The error bars represent the measurement error

of SC-IC (7% + 8 pptv) and LOPAP data (10% + 6 pptv). The solid line

denotes the linear fitting results with the “bivariate” method (slope: 0.86

± 0.02, intercept: 0.26 ± 0.02 ppbv), while the dashed line denotes that

with the “standard” method (slope: 1.02 ± 0.01, intercept: 0.09 ± 0.01

ppbv, R2 = 0.84).

much larger than that of SC-IC, while assuming a larger

measurement error of SC-IC will lead to a contrary result.

It seems that the difference of the calculated slope with the

“bivariate” and “standard” methods can be attributed to the

deteriorated measurement precision of the correspondingly

used LOPAP instrument.

We further explore the causes of the deviations by the

functional dependence of the relative difference of HONO

measurement by SC-IC and LOPAP, HONOSC-IC−HONOLOPAP

HONOLOPAP

toward potential interfering species in Fig. 10. Firstly, as

analyzed in Fig. 10a, good agreement between LOPAP

and SC-IC is achieved at HONO mixing ratios above 0.3

ppbv. But the offset between LOPAP and SC-IC does

show a significant influence on the relative difference

of HONO measurements by the two instruments in the

lower mixing ratio range. Secondly, negative correlations

between the relative difference and mixing ratios of NO2,

NOz and NO2×SO2 can be acknowledged, which excludes

significant interfering effects coming from these species

(Fig. 10b, c, d). Finally, a small positive systematic differ-

ence is determined for both low and high particulate nitrite

and PM2.5 values without a trend, which indicates that the

interference is not significantly caused by a compound or

mechanism related to the aerosol phase (Fig. 10e, f). These

abstracted results based on field measurements are consis-

tent with the laboratory interference studies discussed in

Section 3. However, the significant relative difference (up

to 50%) of observed HONO at conditions of low HONO is

still unresolved, which corresponded to the late afternoon

time intervals (around 16:00) over the mean diurnal profile

(Fig. 8).

Due to the poor detection limit of DOAS, intercompar-

ison between wet-chemical techniques and spectroscopic

techniques was mainly performed for HONO on the level

of ppbv. Actually, very few intercomparison studies about

HONO measurements at conditions of low HONO (few

hundreds pptv) have been performed, especially in the

field. The accurate and precise detection of HONO at few

hundreds pptv or lower concentration levels still needs

to be improved. In the meantime, more investigations are

required to elucidate the possible interference mechanisms

for low HONO concentrations as well (e.g., under chamber

experiments with a better-controlled environment).

4 Conclusions

A new instrument for gaseous HONO measurement based

on Stripping Coil and Ion Chromatograph (SC-IC) was

developed as part of a continuous effort following the

earlier field studies performed in the Pearl River Delta

and Beijing. The instrument is featured by modular and

compact construction, high mobility, simple operation and

low cost, thus can be used as auto-monitoring system for

HONO in both field and laboratory experiments.

We characterized and optimized our SC-IC instrument

with a stable and high-purity HONO generator. The SC-IC

instrument showed a HONO collection efficiency of 99%

even at air masses containing highly acidic compounds, a

detection limit of 8 pptv (3σ) and a time resolution of 15

min. The uncertainty of HONO measurement with SC-IC

is estimated to be about 7% (1σ).

Possible measurement interferences caused by NO,

NO2, NO2+SO2, NO2+VOCs, HNO3, HONO+O3, PAN,

and Particulate nitrite for the developed SC-IC instrument

were explored in a series of laboratory experiments. Negli-

gible amounts of HONO concentrations smaller than the

instrument detection limit were observed for high con-

centrations of gases (both single compound and mixtures)

except NO2. The interference contributed by NO2 was

determined to be 0.034‰ which is also negligible under

most atmospheric conditions. The particle mass absorption

ratio in the coil sampler was determined to be (22 ± 9)%.

Considering that particulate nitrite is normally a small

fraction of the total nitrite in atmosphere, the interference

from particulate nitrite is thus also negligible.

Finally, the developed SC-IC instrument was validated

against the widely accepted LOPAP technique. Reasonably

good agreement was achieved between SC-IC and LOPAP

with a regression slope of 1.02 (standard method) and 0.86

(bivariate method). At low HONO concentrations (< 0.3

ppbv), deviations appeared between SC-IC and LOPAP.

However, we found this difference was not related to the

known interfering species or mixtures observed. In a recent

study: Formal Intercomparison of Observations of Nitrous

Acid (FIONA) (Rodenas et al., 2012), it was found that


1.0

0.5

0.0

-0.5

-1.0(HO

NO

SC

-IC -

HO

NO

LO

PA

P)/

HO

NO

LO

PA

P

0.0 0.5 1.0 1.5 2.0 2.5HONO (ppbv)

1.0

0.5

0.0

-0.5

-1.0(HO

NO

SC

-IC - H

ON

OL

OP

AP)/

HO

NO

LO

PA

P

1.0

0.5

0.0

-0.5

-1.0

(HO

NO

SC

-IC - H

ON

OL

OP

AP)/

HO

NO

LO

PA

P

1.0

0.5

0.0

-0.5

-1.0

(HO

NO

SC

-IC - H

ON

OL

OP

AP)/

HO

NO

LO

PA

P

1.0

0.5

0.0

-0.5

-1.0

(HO

NO

SC

-IC - H

ON

OL

OP

AP)/

HO

NO

LO

PA

P

1.0

0.5

0.0

-0.5

-1.0

(HO

NO

SC

-IC - H

ON

OL

OP

AP)/

HO

NO

LO

PA

P

0 10 20 30NO2 (ppbv) NO2 (ppbv)

0 5 10 15 20

0 200 400 600 800 1000

NO2 × SO2 (ppbv2)

0.0 0.2 0.4 0.6

Pariculate nitrite (μg/m3)

0 50 100 200 300

PM2.5 (μm2)

a

d e f

b c2

1

0

-1

-2

2

1

0

-1

-2

2

1

0

-1

-2

2

1

0

-1

-2

2

1

0

-1

-2

2

1

0

-1

-2

HO

NO

LO

PA

P (

ppbv)

HO

NO

LO

PA

P (

ppbv)

HO

NO

LO

PA

P (

pp

bv

)H

ON

OL

OP

AP (

ppbv)

HO

NO

LO

PA

P (

ppbv)

HO

NO

LO

PA

P (

ppbv)

Fig. 10 Functional dependence of the relative difference between HONO concentrations measured by SC-IC and LOPAP toward HONO, NO2, NOz,

NO2×SO2, particulate nitrite, PM2.5. In (a), the data were equally divided into 6 bins when x-parameters < 1 and 3 bins when x-parameters > 1. In (b,

c and f), the data were equally divided into 6 bins of different x-parameters. In (d), the data were equally divided into 4 bins when x-parameters < 200

and 4 bins when x-parameters > 200. In (e), the first bin represents those data sets without particle nitrite detected, while the other 5 bins were equally

distributed. The circles and squares denote the averaged values of the relative difference between HONO concentrations measured by SC-IC and LOPAP

and the HONO concentrations detected by LOPAP, respectively. The vertical and horizontal bars represent 1σ errors of the corresponding y-variables

and x-variables.

such deviation can appear between two LOPAP instru-

ments or between LOPAP and DOAS techniques. Overall,

further investigations are still required to ensure reliable

HONO measurement at low HONO concentrations.

Acknowledgments

This work was supported by the National Natural

Science Foundation of China (Major Program: No.

21190052, 40675072, and Innovative Research Group: No.

41121004), this study was also supported by the Strategic

Priority Research Program of the Chinese Academy of

Sciences (No. XDB05010500). The authors would like

to thank Hu Wei and Chen Chen for their assistance in

particle generation and measurement, and Gao Tianyu for

their assistance in PAN production. We also thank Theo

Brauers and Rolf Haseler from IEK-8 Forschungszen-

trum Julich for providing the LOPAP instrument and the

corresponding technical support. The support of the 3C-

Star Science team both during the field campaign and

afterwards is acknowledged.

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an online monitoring system for atmospheric nitrous acid (hono) based on stripping coil and ion...

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