Mechanisms for surface ozone depletion and recovery during polar sunrise

Pergamon

Atmospheric

PII: S1352-2310(96)00264-6

MECHANISMS FOR SURFACE RECOVERY DURING S. I,. GONG,

Ewironmnt Vol. 31, No. 7, pp. 969-981, 1997 Copyright Q 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 13%2310/97 s17.00 + 0.00

OZONE DEPLETION POLAR SUNRISE

AND

J. L. WALMSLEY, L. A. BARRIE and J. F. HOPPER

Atmospheric Environment Service, 4905 Dufferin Street, Downsview, Ont., Canada M3H 5T4 (First received 26 January 1996 and injnal form 25 August 1996) Abstract-As part of the Polar Sunrise Experiment (PSE 94) in April 1994, vertical profiles of ozone concentration, temperature and wind speed above an Arctic Ocean icefloe were obtained to investigate boundary layer ozone depletion. They show sustained periods of depleted surface ozone (~1 ppbv) in a layer 300 tl>400 m deep. A one-dimensional model was applied to the data in an attempt to determine the magnitude of chemical destruction rates. The rate of change of ozone corresponding to the combined result of horizontal advection and a volume or a surface sink was calculated to be in the range of - 0.001-0.0~31ppbv s- 1 while the surface deposition velocity was estimated, for most cases, to be in the range of 0.006-0.016 cm s-l. Generally, at this fixed observational point, air mass changes and associated horizontal aljvection were dominant factors in controlling ozone change. If weak chemical sink rates are to be separated from strong advection changes, future studies will have to take special care to define the horizontal gradient of ozone. Copyright 0 1997 Elsevier Science Ltd Key word index: Arctic ozone depletion, dry deposition.

model to these profile measurements to investigate the rate of surface ozone depletion and recovery in the Arctic Ocean boundary layer.

1 INTRODUCTION

Surface ozone depletion episodes during the period of polar sunrise have been observed since 1986 (Bottenheim et al., 1986, 1990; Barrie et al., 1988, 1989, 1994b; Mickle et al., 1989; Oltmans et al., 1989; Anlauf et al., 1994; Solberg et al, 1994). Efforts (Barrie et al., 1988; Finlayson-Pitts et al., 1990; McConnell et al., 1992;

2. THEORY

2.1. Volume ozone source/sink

Fan and Jacob, 1992) have been made to investigate the mechanisms of these depletion episodes which are often characterized by a decrease in surface ozone concentration from about 40 ppbv to less than the instrumental detection limit ( N 1 ppbv) and a recovery in several hours or days. The advection of surface ozone depleted boundary-layer air from the Arctic Ocean has been identified as the cause of observed ozone depletion at sites located inland in the Arctic (Barrie et al., 1989). Despite previous extensive investigations of ozone depletion at polar sunrise (Bottenheim, 1993; Barrie et a/., 1994a), little observational evidence exists about the rate of ozone depletion and about whether the depletion occurs uniformly throughout the atmospheric boundary layer (a volume sink), or at the ice/snow surface (a surface sink). During Polar Sunrise Experiment (PSE 94) at Ice Camp Narwhal (83” 53.95’N, 63” 16.82’W) (Fig. 1) 22 vertical profiles of ozone, temperature, pressure, wind speed, wind direction and relative humidity were measured during the period of 11-24 April 1994 (Hopper et al., 1996). The intent of this paper is to apply a one-dimensional

The model applied to the data is as follows:

g+v.vc=g Ktz)E+s,, [ 1

(14

where C is the ozone concentration, v is the horizontal wind velocity, K(z) is the vertical eddy diffusion coefficient of ozone and So, is the volume ozone source/sink term. v. VC is the advection contribution to the ozone change. Since the ozone concentration as a function of time (t) and altitude (z) is established from measurements, equation (la) can be rewritten in a discrete form with observables on the right-hand side and unknowns on the left-hand side:

(1’4 which is the net rate of ozone change due to chemical sinks and advection. In order to estimate So, from (lb), the horizontal advection term v’ VC has to be negligibly small or experimentally determined. The assumption that advection is negligible can be met in periods of low wind speeds or horizontal homogeneity

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970

where r = 0.74, CI~= 4.7, t12 = 6.35, yi = 15, y2 = 0.74 with rc = 0.35. Wieringa (1980) re-analyzed the Kansas data to obtain r = 1, c~i = 6.9, c(~ = 9.2, yi = 22, yz = 13 with IC= 0.41. A comparison of different constants for the above was made by Walmsley (1988), who found that results were not strongly dependent on which of the two sets of constants was chosen. In this study, the Busiginer et al. (1971) constants are used. Assuming that u* and O* are constant in the surface layer, equation (2) may be integrated between two elevations zi and z2 (zi < z2) to give

Greenland

80

100.

00 -

70’

80’

Fig. 1. The observation site, Ice Camp Narwhal, near Alert.

of ozone concentration. of neglecting horizontal

Later, we explore the validity advection.

2.2. Eddy diflusiuity In order to calculate the vertical turbulent diffusivity in equation (lb), the friction velocity (u*) and Monin-Obukhov length (L) are required. They are obtained from the wind speed and temperature (or potential temperature) profiles. According to Businger et al. (1971) and Businger (1973), in the surface boundary layer (SBL), the mean vertical profiles of wind speed (u), potential temperature (0) are related by:

F* ; =&

(0,

Kz

a0

B

z

*

4%(i)

=

(2)

where ICis the von Karman’s constant, z is the height above the surface, u* is the friction velocity and 0, is the scaling temperature. u* and f3* are defined by:

J z

u*=

-, P

8*=

--.

H Pc,b

(3)

Here r is the surface stress, p is the air density, cp is the specific heat of air at constant pressure, and H is the surface heat flux. &,,([) and &([) are the dimensionless profile functions for momentum (m) and heat and tracers (h), which are functions of the dimensionless stability parameter, c = z/L, where L = u:T/k-go, is the MoninObukhov length. Fitting (2) to their Kansas Experimental data, Businger et al. (1971) recommended the following forms and constants for 4, (c) and & (0: &=

1 +a15, = (1 - y1[)-1’4,

4h = r(1 + a20 = r(1 - yZ[)-l”,

i>o [ < 0

(4a)

i>O (4b) [ < 0

Au k.-_=ln n*

Z 0 Zl

- tirn (12) + tilll (ii)

(5a)

A8 K-=ln Q*

Z 0 Zl

- $ll (12) + $ll (ii)

(5b)

where Au and A8 are the differences in wind speed and potential temperature between zi and z2, with ‘Vi)

= j (1 - 4) di/i.

The expressions for Y under various conditions can be found in Walmsley (1988). Theoretically, with two levels of data for wind speed (An) and potential temperature (A@, u* and O* can be obtained by solving equations (5a) and (5b). Once u*, f3* and L are known, the vertical eddy diffusion coefficient K(z) can be estimated by (Businger and Arya, 1974)

(6)

where f is the Coriolis parameter. Equation (6) was proposed for use under stable conditions. Walmsley (1973) used a very similar formulation for both stable and unstable stratifications. The eddy viscosity profile (Walmsley, 1973) has a shape quite similar to the Leipzig profile (Lettau, 1950). A similar form was used successfully by Agee et al. (1973). One advantage of equation (6) is that a single expression for K(z) extends beyond the surface boundary layer (SBL) into the planetary boundary layer (PBL). Use of equation (6) for different stability conditions is unlikely to result in large errors. Furthermore, most of the valid profiles were stable or neutral during the PSE 94 experiment. 2.3. Surface depletion -

dry deposition

Ozone and other atmospheric constituents can be removed at the surface by dry deposition. In some cases, dry deposition may be the most significant removal process. Transfer from a reference level to the surface consists of three distinct steps (Seinfeld, 1986; Wesely, 1989; Walmsley and Wesely, 1996): aerodynamic (R,), surface (&,) and transfer (R,) component. The total deposition velocity is related to these three resistances by vd=R-‘=(R,+R,+R,)-‘.

(7)

Surface ozone depletion and recovery The first step, aerodynamic, involves the transport of species through the surface layer to the immediate vicinity of the surface by turbulent diffusion which depends on meteorological conditions. It is given for neutral or moderately stable conditions as R&r, za) = v&n

u (z,) zo)-l =X

(8)

where z, and z,, are the reference height and roughness length, respectively. ri(zJ is the wind speed at z,. The boundary layer resistance (Rb) is due to the transport across the laminar sublayer just adjacent to the receptor surface. It is expressed as follows (e.g., Seinfeld, 1986): Rdzo, z,) = ASc=lu,,

(9)

where A and CIare determined experimentally, SC the Schmidt number of ozone and z, is a height below that of the roughness length za. For ozone, c[ = 213 and A = 5 is used. The transfer component (R,) is governed by uptake by physical process such as adsorption or by chemical destruction. For a reactive substance such as ozone, this is often the rate limiting step. In theory, if the transfer resistance l(R,) is infinity, the dry deposition velocity is zero, i.e. no surface sink. Since the surface property at the ice camp was not measured during the experiment, it is difficult to quantify the R,. However, the deposition velocity can be related to the ozone surface flux F as fc~llows: F = v,j C(z,)

(lOa)

where C(z,) is the concentration of ozone at some reference height z,. Within the layer z,, i z < zr, deposition is assumed to be a one-dimensional, steady-state, constant flux process occurring without entrainment. The flux can therefore be expressed as

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with a standard balloon-sonde system (Hopper et al., 1996). Upon examination, most of the wind profiles were found to have unrealistically high wind speeds at the lowest levels (O-400 m). This problem is attributed to low-sensitivity of the portable radio receiver antenna used during PSE 94, which prevented a good lock onto the reference radio navigation transmitter until the sonde was several hundred meters above the surface. Use of these data produced K(z) values up to 3 orders of magnitude higher than the value expected under stable meteorological conditions [K(z) normal range: 0.5-5 m2 s- ‘1. Instead, K(z) was calculated from wind speed measured at a height of 2.85 m using an anemometer at the surface meteorological station (Hopper et al., 1996). To obtain K(z) from this one level wind speed, the surface roughness length (z,,) is required. Uncertainties in using the 2.85 m wind and surface roughness length to calculate K(z) depend on the nature of the terrain where the observation was made and the accuracy of zO.The measurement was done over a flat ice floe surface with no large-scale obstructions in most directions. An ice ridge about 200 m distant in the south to southwest (160-260”) sector may have perturbed the flow. Situations having winds from that sector were ignored in the calculation of K(z). Since z0 affects K(z), the ozone source/sink term So, depends on the choice of z0 which in turn depends on the surface type. For a flat, snow-covered surface, the value ranges from 3 x 1O-3 (Cook, 1985) to 3 x lo-’ m (Bintanja et al., 1995). In the next section, we will explore the sensitivity of dry deposition velocity to z0 values. Finally, the Sonde measurements at about 50 m and the surface tower (2.53 m) observations of temperatures were used to calculate the potential temperature difference (AQ)in (5) to obtain u* and 0* which are subsequently utilized to estimate K(z).

4. RESULTS AND DISCUSSION

Since Do, is usually much less than K(z), hereafter we neglect Do,. By equating equations (10a) and (lob) and utilizing the calculated K(z) and observed concentration profiles near the surface, the transfer resistance (R,) can be estimated as

R,=_C(z,)-R,-Rb.

(11)

K(z) g

3. IDATA SCREENING

It is apparent from equations (5) and (6) that an accurate calculation of vertical eddy diffusivity of ozone relies on the quality of the measurement data for wind speed and temperature. Profiles of wind speed and direction were obtained for each sounding

4.1. Ozone and eddy d$fiisivity Figure 2 is an altitude-time contour plot of ozone concentration at the Ice Camp Narwhal during PSE 94 for a period between 11 and 24 April 1994. The contour was produced by gridding the 20 valid observational profiles of ozone into 400 x 400 resolution grids in an altitude-time domain. Figure 3a shows the observed surface ozone for the same period which can be divided into three intervals according to the observed ozone concentrations from the surface up to 1000 m. The first interval began on 11 April when depleted surface ozone up to 300 m was observed and persisted until 19 April when surface ozone started to increase (recover). During the second interval, 19-22 April, the surface ozone concentration reached the normal surface value of - 40 ppbv. The rest of the experimental period was classified as the third interval, when surface ozone was again depleted.

S. L. GONG

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Ozone

at Narwhal Period

et al.

Ice Camp

[ppbv] Period

1

Period

2

3

1600

-

1200

Ed < .Z = a

1000

600

600

400

200

11

12

13

14

16

16

17

16

19

20

21

22

23

24

26

Date Fig. 2. Altitude-time contour plot of ozone at Ice Camp Narwhal. The data are gridded from 22 profile measurements during 11-25 April 1994. The three periods in the figure represent the time within three separate air masses.

In a close look at meteorological conditions in the experimental period, this deplete-recover-deplete cycle seems correlated to wind speed and direction changes. Figure 3 shows the continuous ozone, wind speed and direction measurements at the surface tower (2.85 m) for the same period. The vertical dashlines with triangles on the top indicate the times when profile measurements were made. The two horizontal dashed lines in Fig. 3b represent the wind direction sector (160-260”) in which the surface wind measurements were possibly disturbed by a pressure ridge at a distance of about 200 m upwind. In the following analysis of the eddy diffusivity, calculations were not made when the wind was from that sector. The diamond points are the hourly averaged wind speed used in the eddy diffusivity calculation. Using equation (6), eddy diffusivity profile was estimated for each valid profile measurement of wind and temperature and gridded to form the contour plot in Fig. 4. In view of the distribution of eddy diffusivity and ozone concentration, a possible mechanism is proposed to interpret the ozone measurement. In interval 1 (Fig. 3), because of a relatively low wind speed, the eddy diffusivity was very small. Thus, little vertical mixing took place and the surface-depleted ozone layer remained unchanged. Although, there

was a high wind period around 15 April and a downward movement of ozone was observed from 600 to 400 m above the ground, this vertical mixing [K(z) N 0.0063 m2 s- ‘1 was not strong enough to penetrate to the ground. The recovery of ground level ozone on 19 April coincided with a wind speed increase and direction change associated with a change of air mass. Due to the high wind, a large increase in the eddy diffusivity occurred [K(z) N 0.75 m2 s-‘1 (Fig. 4). Ozone was brought down from aloft by strong turbulent mixing and reached ground level. This high ground-level ozone characterizes interval 2. Another important possible source of ozone change is horizontal advection. The depletion event in interval 3 was due to advection. At mid-day of 22 April, a new air mass moved in from the west and displaced the high surface ozone with depleted concentrations up to a height of 400 m in less than a day. In our analysis, the contribution of advection has been neglected, as discussed in Section 2.1. During the air mass change on 19 April, advection may have acted together with turbulent mixing to produce high ground-level ozone. This raises the question of whether advection contributed to the high ground ozone as well. Unfortunately, there are no data from

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Surface ozone depletion and recovery

Surface Ozone, Wind Speed and Direction at Narwhal Interval 1 (a)

#,

40.8,

!,

I,

I,

Interval 2

I_

I,

8,

n!,

I,

I,

Interval 3

I,

Y+o,

r-

30 -

3 20 2 _ iz w 0 lo-

I Aprll

Apr’lP

I

I Aprl3

IIIII Aprl4

v

I Aprl5

Aprl6

Profile Date

III

I,

Aprl7

,,I,

AprlB

+

Aprl9

,,,,I,,,Apr20

ApRl

Apr22

Apt23

Ap124

ApRL

Wind Speed for K Calculation

(b)

360 Direction

I

I I

8270

&rll

Apr.12

Aprlt

Aprl4

Aprl5

Aprl6

Aprl7

AprlU

Aprl9

Apr20

Apr21

Apr22

Apr23

Apr24

Ape5

Date Fig. 3. Surface measurements of ozone concentration, wind speed and direction for the same period as in Fig. 2: (a) surface ozone concentration in three different intervals; and (b) surface wind speed and direction with profile date. A number beside the dashed vertical line indicates the profile number such as Nar 01, Nar 08 and Nar 22.

PSE 94 that allow us to quantitatively estimate the significance of advection. However, we can qualitatively select situations when and where advection is likely to be at a minimum.

4.2. Volume depletion and surface sink of ozone A volume sink corresponds to chemical destruction in the atmosphere and a surface sink corresponds to

S. L.

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Eddy Diffusivity [m2 s-l] Interval2 7500 7000

1600

6500 6000 5500 5000 4500 do00 3500 .3000 .2500 .2000 .1500 .lOOO .0500 .0250

600

.0125 .0063 .0031 .oooo

0 Aprll

Aprl2

Aprl3

April

April

Aprl6

April

AprlO

AprlD

AprPO

Apr21

Apr22

Apr23

Apr24

Apr25

Date Fig. 4. Altitude-time contour plot of eddy diffusivity calculated from the valid profiles of wind speed and temperature measurements..

chemical destruction in the surface snow. The volume sink can occur throughout the entire air column. It is represented by So, in equation (lb) and can be calculated if the horizontal advection is negligible. In the layer below the reference level zr, the surface sink is represented by ud in equation (7). The objective of this section is to explore whether any information about ozone sinks can be determined from the simple models outlined above and the ozone profile observations over the Arctic ocean. A. Volume ozone sourcejsink. The volume source/sink (So,) can be estimated in periods of negligible advection from equation (lb) by using the observed ozone concentration and calculated eddy diffusivity profiles for the same period. Figure 5 shows the left-hand side of equation (1b); this is, the net rate of ozone changes due to both volume source/sink and advection, i.e. Ao, = So, - v. VC for the entire period of the experiment. Aoj was constructed from both the ozone profile observations, which determine the AC/At and AC/Az of right-hand side in equation (lb), and a theoretical calculation of K(z) in equation (lb), which is based on the wind and temperature profile measurements. In order to minimize the effect of horizontal advection, which may be large when associated with frontal passages and accompanying air mass changes, we chose to apply equation (lb) to situations that were

within an air mass and at low wind speeds. As shown in Figs 2 and 3b, a period from 16 to 18 April in interval 1, and a period from 23 to 24 April in interval 3 are within two different air masses, respectively. The two periods are marked in Figures 2 and 5 as period 1 and 3. The rate of ozone change for each period was calculated up to 1000 m in altitude (Fig. 6). If horizontal advection is negligible, as assumed, the rate of ozone change can be approximated as the volume source/sink rate (So,). A positive value represents ozone production and a negative value represents ozone depletion. For period 1, the values are negative near the surface below 60 m, indicating a weak surface sink of ozone. However, between 80 and 300 m, the rate of ozone change is positive and implies an ozone source. In fact, ozone was observed to increase between 80 and 300 m in period 1. This increase is due to non-negligible horizontal advection which brought ozone to the observation site. In contrast, the rate of ozone change in period 3 is negative from the surface up to 1000 m. Period 3 was preceded by an air mass change which led to ozone depletion from the surface up to 400 m and the depletion trend was still observable in the period. The negative change rate of ozone agrees with the observation. Since surface ozone up to 200 m in this period was completely depleted, dry deposition was unlikely to play a role in destroying ozone. This decrease may be associated with depletion

Surface ozone depletion and recovery

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et a[.

Averaged Ozone Change Rate for Periods 1 & 3

Fig. 6. The averaged ozone change rate for two separate periods. It is calculated at each altitude by averaging ozone change rate for that period.

by advection, i.e. transfer of ozone-poor air into the region. Above 400 m, the curves in periods 1 and 3 display some similarity in shape but differ in magnitude. One portion in interval 2 from 18002 19 April to 20002 April needs further exploration and is marked as the steady-state period in Fig. 3a. During this period, ozone-rich air was introduced into the region, and the ozone-depleting processes were still active at the surface and/or from surface to about 200 m. Based on the partially depleted ozone mixing ratio (26 ppbv) that was observed, it appears that Camp Narwhal was in the midst of the formation of the depleted layer. Trajectory calculations showed that advection was direct and constant from the northern Greenland. No depletion was observed during this time at the Background Air Pollution Monitoring (BAPMoN) laboratory approximately 6 km from the main Alert station, so that it can be assumed that air leaving Greenland had the tropospheric mixing ratio of approximately 45 ppbv observed at BAPMoN (Hopper et al., 1996). Ozone was consumed during the passage of air on the snow-covered surface between Greenland and the ice camp. Using a box model to simulate the ozone depletion between Greenland and the ice camp, Hopper et al. (1996) estimated the ozone depletion rate of 1.0 x 10’ molecules crnm3 s-l ( - 3.4 x 10e4 ppbvs- ‘). This estimate is an average ozone change rate between Greenland and the ice camp, which was attributed to a volume source, such as OH, Br and Cl, or a surface sink. The vertical mixing was not taken into consideration between the box and the free troposphere and no differentiation can be made between volume and surface sinks from the model.

At this period of time, it appeared that even within the same air mass the surface ozone concentration gradient was still formed due to the depletion process. The magnitude of such advection [v *VCI calculated from the observation of ozone concentrations, distance and the wind speed between Greenland and the ice camp was 4.7 x 10e4 ppbv s- ‘. It should be noted that this was an averaged advection rate between Greenland and the ice camp and not necessarily the advection rate to the camp. Since no measurements were available between them, this averaged rate was used to represent the advection rate to the ice camp as a first approximation.

S03

VVC 4.7~10~ ppbv i’

AC/At = 0

18 19 Date + Time [GMT]

Fig. 7. A schematic representation of a steady-state column over the ice camp during interval 2.

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Surface ozone depletion and recovery

100 m was about 2.0 x 10m4 ppbv s-l. Using averaged advection contribution, 4.7 x 10m4 ppbv s- ‘, the ozone depletion rate at this column was estimated to be 2.7 x 1O-4 ppbvs-’ for this period. Since the column was well mixed up to 100 m, the surface ozone gradient was not available at the measurement height of 2.85 m to estimate the dry deposition velocity, which hampered the effort to differentiate between volume and surface sink contribution to the ozone change rate. To verify the significance of advection for other periods when no advection information was available, equation (lb) is applied with the assumption of no volume source/sink, i.e. So, = 0. With the measured wind speed profiles, an upper estimate of the magnitude of the horizontal ozone gradient can be obtained

In order to estimate the ozone depletion rate at the ice camp during thi,s period, a steady-state column up to 100 m at the ice camp was isolated from the measurements (Fig. 7). Due to wind-induced ( N 5.6 m s- ‘) vertical mixing, the concentration in this column was kept almost constant (26 ppbv) for the duration. Above the column, an ozone gradient existed which transferred ozone down to the column by eddy ditTusion. In view of Fig. 8, Profiles Nar 12 to 13 can be approximated by such steady-state column, even though the column height may vary. Under these conditions, (1b) is reduced to:

( >

so,=v.vc-$K(z)2

(14

where the eddy diffusion term, (A/AZ) (K(z) AC/AZ), was calculated from the ozone profiles and eddy diffusion coefficients during this period. The averaged eddy contribution tN3the column during this period at

by (13)

Nar07

0

1

2

NarO8

3

Ozone

4

5

6

4

5

[ppbv]

6

7

Ozone

NarO9

8

9

10

7

8

30

31

[ppbv]

NarlO

1 4

5

6

7

Ozone

8

9

10

2

3

(ppbv]

4

(a)

24

25 Ozone

26

6

Nar13

NarlP

1 23

5

Ozone [ppbv]

27 [ppbv]

28

29

‘25

26

27 Ozone

28

29 [ppbv]

Fig. 8. Ozone profiles below 100 m. The x-axis scale is set the same for all plots [Ax = 6 ppbv]. scales of x and y axis are the same for all plots, the slopes are inter-comparable.

Since the

S.

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Nar14

Narl5

I25

1’ 23

26

27

28

29

31

24

(b)

18

26

27

28

Ozone [ppbv]

Narl6

Narl7

25

26

27

’

J

28

29

34

35

36

37

38

Ozone [ppbv]

Ozone [ppbv]

Narl8

Narl9

I17

25

Ozone [ppbv]

’ 24

30

19

20

21

22

23

Ozone [ppbv]

0

”

’

1

2

1

”

’

3

4

29

30

39

40

/

b

5

6

Ozone [ppbv]

Fig. 8. (Continued)

where VC is expressed as ppb/lOO km and UH is the wind speed. This rough estimate yields a value for IVCI,,, < 30 ppb/lOO km, most of which are less than 5 ppb/lOO km except for some periods of air mass change. Leaitch et al. (1994) conducted airborne measurements of ozone in the high Arctic (69”N-83”N) during 6-16 April, 1992. According to their Flights 8 and 9, a value of IVCl - 10 ppb/lOO km was extracted for a constant temperature layer at a height of - 300 m. The short distance interval (about 100 km) and constant temperature seem to indicate that the flights were within the same air mass. The present upper estimates are the same order of magnitude as those of Leaitch et al. (1994). This indicates that even within an air mass, advection may not be negligible and our approach is not able to extract the volume source/sink from the estimated rate of ozone change. In another flying interval of Flight 9 (Leaitch et al., 1994), however, the observed jVC/ was well below 1 ppb/lOO km. Within this type of air mass,

advection is probably negligible compared to the volume depletion rate. Since there are no concurrent measurements of both vertical ozone profiles and VC at the ice floe in PSE 94 and since we cannot be certain that advection is negligible compared to So,, we cannot derive reliable estimates of volume depletion rate. B. Surface ozone deposition velocity (vd). Negative ozone change rate (depletion) was observed for the two periods (Fig. 6) near the surface. The depletion process can also be demonstrated by the ozone vertical gradient near the surface (Fig. 8). Except for the profiles having a complete surface ozone depletion, most of them display a positive slope (dC/dz > 0) during ozone recovery periods from profiles Nar 07 to Nar 10. The positive ozone slope implies that there is a net ozone flux towards the surface and this flux has to be balanced by an ozone sink at the surface and/or a volume sink between the reference height (z,) and the surface. Negative slope is also observed (profile Nar

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Surface ozone depletion and recovery

15) from the reference height up to 20 m in the period of high surface ozone occurrence. The characteristics of the ozone concentration gradients in Fig. 8 can be explained by the corresponding eddy diffusivity variations during that period. For profiles Nar 07 to Nar 10, small eddy diffusivity prevailed and little vertical mixing took place. Consequently, a consistent positive ozone gradient was observed for that period. However, profiles Nar 12 to Nar 16 were subject to a gradually increased turbulent mixing due to large wind speeds and thus the ozone gradients were brought to zero nea.r the surface. In this period, turbulent mixing is the dominant mechanism to determine the ozone gradient compared to dry deposition. The definition for dry deposition velocity in equation (10a) is valid only in a constant flux layer. In order to justify the luse of equation (lOa) for our ozone profiles, the logarithm of altitude has been plotted vs ozone concentration for each profile (Fig. 8). A linear plot indicates that a constant flux is approached. In Fig. 8, a straight line was fitted to the bottom few points for each graph. The slope of the straight line, Alog z/AC, is used 1.0 compute AC/AZ at the reference height. In what follows, we explore the simple situation of a sink parameterized by a dry deposition velocity at the reference height, equation (lOa). Since, for the PSE 94 Ice Camp experiment, the lowest measurement point for ozone is a.t 2.85 m, the ozone concentration and the extrapolated slope at this height are used for the dry deposition velocity calculation by equating equations (10a) and (lob) and rearranging:

(13)

‘(“)’

In equation (13) we have assumed that advection is negligible compared with eddy diffusion. The conceptual model of ozone transport across the reference height is shown in Fig. 9. The dry deposition velocity of ozone is calculated for two separate periods: a recovery period repres-

8’

J

Fdawn

..

ented by profiles Nar 07 to Nar 10 and a depletion period by Nar 17. The correlation coefficients for the regression of these profiles are larger than 0.8. Therefore, the slopes are considered to represent a constant flux layer. Upper and lower limits of vd are obtained by assuming z,, = 3.0 x 10e3 m and z. = 3.0x lo- 5 m, respectively. These limits reflect the sensitivity of dry deposition velocity to the surface roughness length (zO). The average dry deposition velocity of ozone from profiles Nar 07 to Nar 10 is estimated in the range of 0.006-0.016 ems-‘. These values for a snow surface are more than one magnitude lower than those obtained (0.4 and 0.6 cm s-l) by Droppo (1985) in a flat grassland near Champaign, Illinois but are very close to the values measured by Wesely et al. (1980) over snow surface, e.g., 0.012-0.082 cm s- ‘. A much larger value of dry deposition velocity was estimated from profile Nar 17, which was made at the beginning of a period of ozone depletion. These values were in the range 0.2-0.3 cm s- ‘, which is still about half the value given by Droppo (1985). Measurements made over snow surface (Galbally and Roy, 1980) suggests a surface resistance of 800-24,000 sm-‘, which corresponds to a dry deposition velocity in the range of 0.004-O. 125 cm s- ‘. This is very close to the value derived from present study. Using (11) and two other contributions (R, and Rb) from the dry deposition velocity, the transfer resistance (R,) of ozone is estimated. It is found that the transfer component (RJ is the rate limiting step for ozone dry deposition onto the ice/snow surface of the Arctic. By the same analysis of Droppo’s (1985) data, the transfer component (R,) is also found to be the limiting process of ozone deposition on the grassland. From a theoretical point of view, the transfer resistance should depend on the surface physical and chemical properties. Consequently, the difference in surface type between the Arctic and the grassland must cause the significantly different dry deposition velocity. From the calculated dry deposition velocity of 0.2-0.3 ems-’ in the depletion period, an estimate

,j

Fh

=

vdC,= K,

dC

z

.~~~_.~~~~~.~..~...____~

Fig. 9. Schematic transport of ozone by dry deposition across the reference height.

S. L. GONG et al.

980

of ozone lifetime in a layer of depth H can be made by: dC dt=

--

vd c

--f c

=

H

Coe-uIH(‘)

(14)

Thus ozone would have an e-folding lifetime r of H/vd. For an ozone layer of H = 400 m depth (Fig. 2), the above range of vd (0.2-0.3 cm s-l) would result in a r in the range 1.5-2.3 days. Compared to the observational depletion time of about 1 d for the surface ozone on 22 April 1994 (Fig. 2), this seems to suggest that the dry deposition of ozone on the ice/snow surface in the Arctic played a role in the surface ozone depletion in that period. On the other hand, if the dry deposition velocity (0.006-0.016 cm s- ‘) for the recovery period is used, the lifetime of ozone in a layer of 400 m depth is from 29 to 77 d. This indicates that dry deposition was negligible in that period. It is noted that the dry deposition velocity of 0.2-0.3 ems-’ was calculated from profile Nar 17 when ozone concentration was high [ - 35 ppbv]. However, lower deposition velocities of 0.006 to 0.016 cm s- ’ were observed for four consecutive profiles of Nar 07-Nar 10 (Fig. 8) when ozone concentration was low [ - 5 ppbv]. This may suggest that the dry deposition velocity is dependent on ozone concentration provided that entrainment and horizontal advection did not distort this profile. It should be emphasized that the ozone depletion rates estimated in this study are associated with a flat area covered by freshly fallen snow on the ice floe on which the observations were made. Arctic boundary layer air encounters many other types of surfaces such as refrozen leads, open leads, pressure ridges and wind swept snowpack areas whose sea-salt surface composition may be very different than in this study. In other words, the low depletion rates observed here may not be typical. In concluding this section, it is appropriate to suggest that, in order to separate ozone depletion from advection, an experimental design with two or more ground level observational points separated by 50-100 km is necessary. Failing that horizontal aircraft transects would be desirable. In addition, a 10 m (or higher) tower with the ability to obtain accurate simultaneous measurements of ozone, wind and temperature gradient and accurate measurements of wind and temperature in the boundary layer are needed. Since there is an indication from the current analysis that the dry deposition velocity may vary between ozone recovery and depletion periods, it is also desirable to take ground samples at the same time to identify any significant changes of ground chemical properties between these two periods.

5. CONCLUSIONS

The recovery of ground-level ozone concentrations after an event of complete depletion was correlated

with vertical the eddy diffusivity in the surface boundary layer. Throughout the experimental period, stable atmospheric conditions prevailed and wind shear was the main cause of vertical mixing. It was found that the transition of ozone change (depletion/recovery) was usually accompanied by an air mass change. During the recovery period, an air mass change occurred with strong wind speeds which resulted in a high vertical eddy diffusivity. The associated turbulent vertical mixing entrained the ozone down from the free atmosphere and enabled the ground-level ozone to recover. Three factors contribute to the depletion of groundlevel ozone: (1) advection of ozone-poor air during an air mass change, (2) volume depletion and (3) surface dry deposition. The change rate of ozone concentration during a period of relative steady meteorological conditions was estimated to be 2.7 x 10m4 ppbvs-’ and was attributed to both (2) and (3). Lack of horizontal ozone gradient information prevents us from accurately determining the magnitude of the volume depletion sink for ozone from the PSE 94 data. Further experiments are required to obtain advection as well as vertical profiles of ozone simultaneously. However, on several occasions when advection was likely at a minimum, dry deposition deduced from flux gradient relationships in the surface boundary layer yielded dry deposition velocities in the range 0.006 to 0.3 ems- ‘. These pertain to one type of surface found on the Arctic ocean, namely a snowcovered icefloe surface. The ozone lifetime of 29-77 d in a 400 m layer over this surface deduced during low ozone conditions ( - 5 ppbv) exceeds the observation time scale of about 1 d for a complete surface ozone depletion. However, lifetimes of 1.5-2.3 d deduced for a 35 ppbv ozone event are more consistent with indirect observational evidence of depletion rates. Acknowledgements-The authors would like to express their gratitude to H. Drythout for her technical assistance in making the profile measurements.

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