Photolysis of Particulate Nitrate as a Source of HONO and NOx

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Photolysis of particulate nitrate as a source of HONO and NOx Chunxiang Ye, Ning Zhang, Honglian Gao, and Xianliang Zhou Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017

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Environmental Science & Technology

Photolysis of particulate nitrate as a source of HONO and NOx Chunxiang Ye1, 2,*, Ning Zhang3, Honglian Gao3, and Xianliang Zhou1, 3*

1

2

Wadsworth Center, New York State Department of Health, Albany, NY 12201

State Key Laboratory of Environmental Simulation and Pollution Control, College of

Environmental Sciences and Engineering, Peking University, Beijing, 100871, China 3

Department of Environmental Health Sciences, State University of New York, Albany, NY 12201

* Correspondence to: Chunxiang Ye ([email protected]) Xianliang Zhou ([email protected])

A manuscript submitted to Environmental Science and Technology

ACS Paragon Plus Environment


1

ABSTRACT

2

Photolysis of nitric acid on the surface has been found recently to be greatly enhanced from that

3

in the gas phase. Yet, photolysis of particulate nitrate (pNO3) associated with atmospheric

4

aerosols is still relatively unknown. Here, aerosol filter samples were collected both near the

5

ground surface and throughout the troposphere on board NSF/NACR C-130 aircraft. The

6

photolysis rate constants of pNO3 were determined from these samples by directly monitoring

7

the production rates of nitrous acid (HONO) and nitrogen dioxide (NO2) under UV light (> 290

8

nm) irradiation. Scaled to the tropical noontime condition on the ground level (solar zenith angle

9

= 0 o), the normalized photolysis rate constants ( ) are in the range from 6.2 × 10-6 s-1 to 5.0

10

× 10-4 s-1 with a median of 8.3 × 10-5 s-1 and a mean (± 1 SD) of (1.3 ± 1.2) × 10-4 s-1. Chemical

11

compositions, specifically nitrate loading and organic matter, affect the rate of photolysis.

12

Extrapolated to ambient pNO3 loading conditions, e.g. ≤ 10 nmol m-3, the mean value is

13

over 1.8 × 10-4 s-1 in the suburban, rural and remote environments. Photolysis of particulate

14

nitrate is thus a source of HONO and NO2 in the troposphere.

15 16

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17

INTRODUCTION

18

Gaseous nitrous acid (HONO) is an important precursor of hydroxyl radical (OH) in the

19

troposphere. On average, 20-80% OH formation is attributable to HONO photolysis near the

20

ground surface in various surroundings (1-4). Many mechanisms have been proposed to be

21

HONO sources near the ground surface, including direct emissions from soil (5, 6) and from

22

combustion processes (7, 8), gas-phase reactions involving oxides of nitrogen (NOx = NO + NO2)

23

within the air parcel (9-13), heterogeneous reactions of NOx (7, 9, 14), and photolysis of

24

HNO3/nitrate on the ground surfaces (15-18). Some of these HONO sources are still not well

25

understood and thus HONO budget constraint has been of great challenge (19).

26

Recent aircraft-based HONO measurements have showed substantial levels of HONO

27

during the day in the air masses decoupled from various ground HONO sources, suggesting

28

existence of volume HONO sources to sustain the observed HONO concentration against its fast

29

photolysis (10, 11, 20, 21). Zhang et al. (21) suggested the photo-enhanced heterogeneous

30

reductions of NO2 on organic components of aerosols and photolysis of particulate nitrate to be

31

the two potential HONO sources, but did not quantify their relative contributions due to the lack

32

of relevant measurements concerning these two mechanisms. Another airborne HONO

33

measurement further indicated that the low levels of both NOx and aerosol surface density had

34

excluded the heterogeneous reactions of NO2 on aerosols as an important volume HONO source,

35

and a novel gas phase reaction between HO2·H2O and NO2 was proposed (10). Although the

36

proposed gas phase reaction accounts for most of the observed volume HONO source (10), it has

37

been shown to be greatly overestimated by a different airborne study (11). Evidence from our

38

recent airborne measurement in the marine boundary layer (MBL) suggests that photolysis of

39

particulate nitrate is a major volume HONO source (20).

2



40

Photolysis of gaseous HNO3 is a relatively slow process, with a photolysis rate constant

41

of ~7 × 10-7 s-1 under the typical tropical noontime conditions (22, 23). Therefore, HNO3 is

42

traditionally considered to be the end-product of tropospheric reactive nitrogen (22). However,

43

photolysis of surface adsorbed HNO3 and/or nitrate has been found to be much faster, with a rate

44

constant ranged from 1 × 10-5 s-1 to 3 × 10-3 s-1 on the surfaces of natural and artificial materials

45

and in urban grime (15, 18, 24). HONO has been shown to be the dominant product on most of

46

the natural surfaces and NO2 as the dominant product on metal and other artificial surfaces (18).

47

Photolysis of HNO3/nitrate on surfaces, including forest canopy and snowpack, has been

48

proposed to be HONO and NOx sources for the overlying atmosphere (16, 17, 19, 25-27). While

49

the photolysis of particulate nitrate (pNO3) has been shown to be a major volume HONO source

50

in the MBL (20), this mechanism has not been examined in different atmospheric environments,

51

and thus its importance as sources of HONO and NOx and its role in the reactive nitrogen cycling

52

is still unknown.

53

Here we report laboratory determined pNO3 photolysis rate constants using ambient

54

aerosol samples collected on Teflon filters from various environments both near the ground

55

surface and throughout the troposphere on board NSF/NACR C-130 aircraft, and examine the

56

importance of this photolytic process as HONO and NOx sources in the troposphere.

57

EXPERIMENTAL SECTION

58

Aerosol samples.

59

(Sartorius, pore size 0.45 µm, 47 mm diameter), with the upper filter collecting both aerosol and

60

gaseous HNO3 and the underlying filter collecting only gaseous HNO3. The underlying filter was

61

used to correct the gaseous HNO3 adsorbed in the upper filter (see below, Eq. 1). Aerosol

62

samples collected on the ground sites were termed as the “ground” samples. These “ground”

Each aerosol sample was collected using two Teflon filters stacked together

3


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samples were collected in urban (Wadsworth courtyard, Downtown Albany, New York; 1 m

64

above ground), suburban/rural (Delmar, New York, depending on wind directions, 2 m above

65

ground) and remote areas (Whiteface Mountain summit, New York, 4 m above ground) in

66

various seasons of from 2008 to 2010. The sampling flow rates ranged from 2 to 15 L min-1, the

67

sampling times from 5 hr to 166 hr, and the overall sampling volumes of air from 1 m3 to 57 m3,

68

to collect a very wide range of particulate nitrate loadings on the Teflon filters, from 2.2 nmol to

69

568 nmol per filter. In addition to these “ground” samples, some “aloft” aerosol samples were

70

collected throughout the troposphere onboard NSF/NCAR C-130 aircraft in Southeast US during

71

the NOMADSS (Nitrogen, Oxidants, Mercury and Aerosol Distributions, Sources and Sinks)

72

field campaign in the summer of 2013. One aerosol sample was collected during each flight,

73

between a half hour after takeoff and a half hour before landing, with a sampling time of 6 - 8

74

hours and sampling flow rate of 3 L min-1. Each aerosol filter sample was placed in a pre-marked

75

petri dish, wrapped in a piece of aluminum foil, and stored in a freezer before use. The sampling

76

locations and sampling date for “ground” samples were summarized in Table 1. For “aloft”

77

samples, each usually was collected from a wide geographic area. The flight track is available in

78

our project data archive (http://data.eol.ucar.edu/master_list/?project=SAS).

79

Photochemical experiment. The photochemical experiment has been described elsewhere (18,

80

20). Briefly, the aerosol filter was removed from the freezer and placed in a cylindrical Teflon

81

reactor with quartz window on the top, and then subjected to UV light (λ > 290 nm) in a

82

photochemical safety cabinet. Zero air, at 50% RH and 20 (±1) °C, was introduced at one end

83

near the bottom of reactor and lead out at another end near the top of the reactor. HONO and

84

NO2 produced in the reactor and carried out by the air flow were sampled successively by two

85

coil samplers connected in series, with the first coil sampling HONO with DI water and second

4



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86

coil sampling NO2 with acetic acid modified sulfanilamide and N-(1-Naphthyl) ethylene-diamine

87

(SA/NED) solution. Both HONO and NO2 were converted into azo dye derivative and detected

88

by two long path absorption photometer (LPAP) systems. Each LPAP system consisted of a

89

miniature fiber optic spectrometer (USB2000, Ocean Optics), a 1-m liquid-waveguide capillary

90

flow cell (LWCC-3100, WPI) and a tungsten light source (FO-6000, WPI). The effective light

91

intensity in the flow reactor was measured using a nitrate actinometer with a fluorescence

92

detector (HITACHI, l-7480) (23). The particulate nitrate on the Teflon filter was extracted with

93

10 - 15 ml 1% NH4Cl buffer solution (pH = 8.5) after photolysis experiment, converted to nitrite

94

with a Cd column, and then measured with a LPAP system. The photolysis loss of nitrate during

95

about 10 min light-exposure experiment was corrected by adding the integrated production of

96

both HONO and NO2 (Eq. 1). The overall production rate of HONO and NO2 is divided by

97

nitrate amount to obtain the photolysis rate constant (Eq. 2), and further normalized to the typical

98

tropical summer conditions on the ground (solar zenith angle ϴ = 0°) with an aqueous nitrate

99

photolysis rate of 3.0 × 10-7 s-1 (23) to obtain the normalized photolysis rate constant (Eq. 3):

100

N ,

= N + P + P

101

=

102

= × -./×0/12

!"#$%#&,'($$&'#

(Eq. 1) (Eq. 2)

*&+,

(Eq. 3)

103

where N is the nitrate amount in nmol extracted from the Teflon filter, 34567869,:;779:6 is

104

the corrected nitrate amount expected on the filters prior to the photochemical experiment,

105

“P + P ” is the overall production rate of HONO and NO2 in nmol s-1, is the

106

experimental determined photolysis rate constant in s-1, 1 in the clean MBL (20, 53). As a result of a much faster photolysis rate

348

for pNO3 compared to that for HNO3, the total nitrate (TN = HNO3 + pNO3) is expected to be

349

more photochemically reactive and shorter lived, with a lifetime,a b , defined by equation (Eq. 9):

350

a b = O

b

P×*,

(Eq. 9)

351

Assuming a TN/pNO3 ratio of 10 and a of 1.8 × 10-4 s-1, a b is estimated to be ~ 15 hr

352

for the total nitrate in the troposphere at noontime due to pNO3 photolysis, which is at least one

353

order of magnitude shorter than the photolysis lifetime of gaseous HNO3. Inclusion of this

354

photolytic mechanism would thus improve the simulations of reactive nitrogen chemistry and

355

tropospheric zone formation in regional and global models.

16



356

ACKNOWLEDGEMENT

357

This research is funded by the NSF grants (ATM-0632548, AGS-1216166). Any opinions,

358

findings, and conclusions or recommendations expressed in this paper are those of the authors and

359

do not necessarily reflect the views of NSF.

360

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1 2

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Table 1. Summary of sample information, photolysis rate constant ( ), and HONO to NO2 production ratios. The parallel samples, collected over the same periods of time but at different flow rates, are marked with symbols of *, #, $ and &.

Location Albany, NY

Sampling Time 1/8/09 10:00 – 1/9/09 10:00 1/24/09 10:30 - 1/24/09 10:30

11/10/10 13:30 -18:00 11/10/10 13:30 -18:00

$

11/10/10 18:10 - 11/11/10 10:40 11/10/10 18:10 - 11/11/10 10:40

10/22/10 9:30 -10/23/10 9:30

&

10/28/10 9:30 -10/29/10 9:30

&

10/28/10 9:30 -10/29/10 9:30

10/26/10 9:30 -10/27/10 17:30 10/26/10 9:30 -10/27/10 17:30

212.9 344.7 568.5 130.2 210.3 350.8 1.6

4.1

11/10/10 18:10 - 11/11/10 10:40

10/22/10 9:30 -10/23/10 9:30

36.8

1.1

*

$

58.2

57.6

*

10/9/10 8:30 – 10/10/10 8:30

530.4

29.4

6/8/09 9:30 – 6/12/09 9:30

10/9/10 8:30-18:30

14.4

14.7

6/8/09 9:30 – 6/10/09 10:30

Delmar

[pNO3] ×10-9 mol m-3

14.4

6/8/09 11:00 – 8/9/09 11:30

#

Absorbance ×10-3

10.8

2/8/09 15:00 – 2/9/09 15:00

#

Nitrate 10-9 mol

14.4

2/8/09 15:00 – 2/9/09 9:00

#

Volume m-3

5.5

1.0

12.3

4.0

34.9

38.2 31.0 32.9 12.1 49.0 81.5 ̶ 4.7 2.5 6.4

14.8 31.9 39.5 8.9 7.1 6.1 1.5 1.3 12.3 8.7

HONO/NO2

s-1

-5

2.2×10

-5

2.3×10

-5

1.0×10

-6

6.2×10

-5

7.8×10

-5

5.2×10

-5

4.2×10

-4

1.3×10

-4

1.1×10

-4

1.1×10

-5

8.2×10

-5

14.9

138.1

26.2

9.3

6.8×10

1.7

2.2

10.4

1.3

4.3×10

2.9

21.8

2.9

11

5.8

29.3

2.9

22.8

5.8

42.7

3.8

30.7

7.7

51.2 25


4.6 6.3 12.1 9.5 16.5 9.6 17.9

7.5 3.8 5.5 7.8 7.5 8.1 6.7

-4 -4

1.7×10

-4

1.8×10

-4

1.5×10

-5

7.4×10

-5

8.2×10

-5

6.1×10

-5

7.9×10

6.3 4.8 4.0 5.6 5.5 6.4 5.0 10.9 5.7 5.4 6.5 5.8 4.9 3 13.1 11.7 11.3 9.3 11.2 9.5

Page 27 of 31


Whiteface Mountain

-4

8/21/08 15:40 - 8/22/08 12:40

9.5

3.2

8

0.33

1.0×10

9/2/08 13:35 - 9/5/08 13:40

21.6

17.6

38

0.81

5.0×10

9/5/08 13:50 - 9/12/08 11:45

49.8

Aloft samples 6/24/13 15:30 – 22:00 UTC

33

0.78

6/27/13 15:30 – 21:30 UTC

6.5

0.97

6/29/13 15:30 – 20:45 UTC

1.2

0.93

7/4/13 15:30 – 21:45 UTC

2.1

1.1

7/11/13 15:30 – 21:30 UTC

3.1

1.1

7/12/13 21:00 – 7/13/13 3:15 UTC

3.6

1.1

7.5

3 4

26


33

0.66 8.3 1.2 2.2 2.8 4.0 6.8

-4 -5

8.3×10

-5

1.3×10

-4

1.3×10

-4

2.8×10

-4

3.1×10

-4

2.6×10

-4

1.2×10

32.7 7.3 10.9 1.7 1 2.6 3.6 2.1 2


1 2

3 4 5

Figure 1. Absorbance signals of HONO and NO2 during the light-exposure of an aerosol sample

6

collected on June 27, 2013 on board C-130 aircraft. The blue dotted line fits the initial HONO

7

production rate (open blue circles) and the blue solid line fits the subsequent HONO production

8

rate (solid blue circles).

27


Page 28 of 31

Page 29 of 31


1.00E-01 100

P, x10-12 mol s-1

(a) 10 1.00E-02 1.00E-03 1

Albany winter Albany summer Albany fall Delmar WFM Aloft

1.00E-04 0.1 1.00E-05 0.01

1

10

100

1000

1.E-03 10-3

JNpNO3, s-1

(b) 1.E-04 10-4

1.E-05 10-5

1.E-06 10-6

1

9

10

100

1000

NpNO3, 10-9 mol

10

Figure 2. The total production rate of HONO and NO2 (P) (a) and the photolysis rate constant

11

(JNpNO3) (b) as a function of light-exposed nitrate loading (NpNO3). The black solid lines are the

12

regression curves of Eq. (4) (panel (a), r2 = 0.71) and Eq. (5) (panel (a), r2 = 0.70). The dotted

13

lines connect pairs of parallel samples at the same location and time, but with different sampling

14

flow rates. The data point in dotted circle has been excluded from the fitting. “Albany winter”,

15

“Albany summer”, “Albany fall”, “Delmar”, “WFM” and “Aloft” represent aerosol samples

16

collected in the city of Albany in winter, summer and fall, in the village of Delmar, at the summit

17

of White Face Mountain, and onboard C-130 aircraft, respectively.

28



Page 30 of 31

1.E-03 10-3

JNpNO3, s-1

1.E-04 10-4

1.E-05 10-5

1.E-06 10-6 1.E-01 0.1

1.E+00 1

1.E+01 10

1.E+02 100

[pNO3], x10-9 mol m-3 18 19 20

Figure 3. The photolysis rate constant (JNpNO3) as a function of particulate nitrate concentration

21

([pNO3]). The black solid line is the regression curve of Eq. (6) (r2 = 0.69). The legends are the

22

same as in Figure 2a.

23

29


Page 31 of 31


24

25 26

Figure 4. Relationships between the total production rate of HONO and NO2 (P) and light

27

absorbance at 300 nm (Abs) in the extraction solution (a) and between the photolysis rate

28

constant (JNpNO3) and the ratio of light absorbance at 300 nm in the extraction solution to nitrate

29

loading (Abs/NpNO3). The black solid lines are the regression curves (r2 = 0.60 in panel (a) and r2

30

= 0.65 in panel (b)). The legends are the same as in Figure 2a.

31

30


Photolysis of Particulate Nitrate as a Source of HONO and NOx

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