Constraining annual and seasonal radon-222 flux density from the Southern Ocean using radon-222 concentrations in the boundary layer at Cape Grim

Figures & data

Fig. 1.  Annual composite angular radon concentration (mBq/SCM) distributions at Cape Grim for 2001–2008, characterised by 25th, 50th, and 75th percentiles: (a) all wind directions; (b) baseline sector only. Bin width is 5 deg.

Fig. 2.  2001–2008 back trajectory density functions for all baseline observations at Cape Grim. The density is defined by (a) the time spent by trajectories in 50×50 km grid cells; (b) the mean time required by air parcels moving along trajectories to reach Cape Grim (in units of radon half-lives, T_1/2=3.82 days).

Table 1. Radon concentration (mBq/SCM) distribution at Cape Grim in 2001–2008 for baseline (local wind direction between 190° and 280°) and non-baseline conditions

	Radon concentration in air (mBq/SCM)
Wind sector	Mean	No. of observations	5th	10th	25th	Median	75th	90th	95th
Baseline	176	28622	8	15	26	42	96	390	850
Non-baseline	976	38799	28	45	121	378	1164	2881	4101

Table 2. Radon concentration (mBq/SCM) distributions of baseline observations in composite year and season at Cape Grim in 2001–2008

Radon concentration in air (mBq/SCM)
Set	Season	Mean	No. of observations	5th	10th	25th	median	75th	90th	95th
	Summer	151	7792	4	9	20	34	75	375	808
	Autumn	241	6523	12	18	31	50	149	605	1273
0	Winter	170	6484	16	21	32	46	94	350	731
	Spring	151	7823	7	13	24	40	84	292	651
	All	176	28622	8	15	26	42	96	390	850
R1	All	229	19927	8	16	28	48	152	598	1121
	Summer	31	2401	4	8	16	26	37	54	71
	Autumn	62	2048	11	15	25	39	57	121	203
1	Winter	74	2075	14	21	31	43	61	167	291
	Spring	48	2171	7	11	20	31	53	92	133
	All	53	8695	8	13	22	34	52	91	173
R2	All	60	3416	7	12	21	34	55	114	221
	Summer	29	1473	5	8	16	24	35	52	67
	Autumn	54	1374	12	16	26	39	56	96	171
2	Winter	70	1224	17	23	33	43	59	124	245
	Spring	43	1208	8	12	20	31	51	77	111
	All	48	5279	9	13	22	34	51	79	135
R3	All	194	208	35	48	87	165	241	389	507
	Summer	29	1458	4	8	16	24	35	51	64
	Autumn	48	1299	11	16	25	38	53	71	121
3	Winter	53	1109	17	22	32	41	53	79	118
	Spring	43	1205	8	12	20	31	50	76	111
	All	42	5071	8	13	21	33	48	68	99
R4	All	61	426	4	10	17	33	70	155	201
	Summer	28	1341	5	9	16	25	35	49	61
	Autumn	45	1171	11	15	25	38	51	66	84
4	Winter	52	1047	18	23	32	41	53	73	106
	Spring	40	1086	8	12	20	30	50	72	96
	All	41	4645	9	13	22	33	47	65	85
R5	All	39	988	9	13	23	34	47	63	79
	Summer	29	1170	5	9	16	25	35	49	61
	Autumn	45	903	10	14	24	36	52	67	85
5	Winter	55	751	20	25	33	43	54	77	114
	Spring	41	833	8	12	20	30	50	74	102
	All	41	3657	9	13	21	33	47	66	88
R6	All	38	1267	9	13	20	32	46	65	86
	Summer	28	920	5	9	17	25	35	49	61
	Autumn	46	736	11	15	25	36	51	68	93
6	Winter	61	442	24	27	35	44	55	84	114
	Spring	49	292	7	14	23	37	58	77	117
	All	42	2390	8	13	22	33	48	66	88
R7	All	39	1490	7	12	21	32	47	65	77
	Summer	31	361	9	12	18	27	36	51	62
	Autumn	56	219	14	19	29	40	55	71	212
7	Winter	49	207	25	28	36	44	52	81	104
	Spring	53	113	7	13	22	32	53	100	163
	All	44	900	11	15	23	35	48	66	99

A sequence of concentration sets (from top to bottom) characterises the results of selection steps for obtaining the least perturbed radon concentration set. The sequence starts with the initial baseline observation set defined by the local wind direction only. This is followed by seven sets each of which is obtained by removing some observations from the previous set. Annual distributions of the removed observations (R1–R7) are also shown. See Section 3 for details; bold entries are depicted in Fig. 5.

Fig. 3.  Radon concentration (mBq/SCM) distribution characterised by 5th, 10th, 25th, 50th, 75th, 90th, and 95th percentiles in the baseline sector as a function of duration in the baseline sector.

Fig. 4.  Radon concentration (mBq/SCM) distribution characterised by 5th, 10th, 25th, 50th, 75th, 90th, and 95th percentiles in the baseline sector in consecutive 3-hour period before a change to a non-baseline sector.

Fig. 5.  Radon concentration (mBq/SCM) distributions for the seven selection steps to extract from the initial baseline observations (Set 0 in Table 2) a set of least perturbed radon concentrations in air parcels that are in equilibrium with their oceanic radon source (Set 7 in Table 2). Each step is represented by a graph contrasting a distribution of rejected concentrations (in red) with associated distributions before and after application of the step's selection criterion shown to its left (in black) and right (in green). The number of rejected concentrations expressed as a percentage of the number of events in the initial baseline set (Set 0 in Table 2) is shown in the top right corner. All distributions are identified by their numbers shown in Table 2. Note change of the radon concentration scale between different graphs.

Fig. 6.  2001–2008 back trajectory density functions for least perturbed baseline observations (Set 7 in Table 2) at Cape Grim. The density is defined by the time spent by trajectories in 50×50 km grid cells.

Fig. 7.  ERA-Interim mixing height (labelled model) at the closest oceanic grid point to Cape Grim compared with mini-lidar mixing height measurements at Cape Grim during 14 baseline events in 1998. An orthogonal distance regression fitting line (slope 1.03) shown on the scatter plot explains 85% of the variance.

Table 3. Comparison of oceanic radon flux densities (mBq m⁻² s⁻¹) obtained in this paper (top section of the table) followed by spot measurements (accumulator and profile techniques), modelled flux, and assumed in or inferred from general circulation models

			Radon-222 flux density (mBq m^−2 s^−1)			Wind speed (m s^−1)
Source	Method	Region	10th/50th/90th percentiles	Mean and standard deviation	No. of samples	Mean and standard deviation Range 10th/50th/90th percentiles
			Summer	0.190±0.122	361	12.2/15.2/19.9^a
			0.071/0.164/0.311
			Autumn	0.347±0.37	219	11.0/15.0/21.0^a
			0.109/0.273/0.513
This study	Radon concentrations in boundary layer air at costal site	Southern Ocean	Winter	0.303±0.17	207	12.6/16.5/22.0^a
			0.167/0.267/0.508
			Spring	0.318±0.39	113	11.1/15.3/18.8^a
			0.081/0.202/0.631
			All seasons	0.270±0.261	900	12.1/15.7/20.7^a
			0.092/0.218/0.429
Flux spot measurements
Wilkening and Clements, 1975	Accumulator	Tropical Pacific		0.16±0.02	Not reported	3
Duenas et al., 1983	Accumulator	West Mediterranean	0.05/0.11/0.30	0.14±0.09	36	0.5/2.6/5.6
Broecker and Peng, 1971	Radon profiles	North Atlantic trade wind belt	0.021/0.024/0.029	0.024±0.0031	45	5–7.5
Hoang and Servant, 1972	Radon profiles	Southern Ocean		0.24±0.15	12	6.6±4.3
Peng et al., 1974	Radon profiles	North Pacific		0.08±0.03	7	8.9±3.0
Peng et al., 1979	Radon profiles	South Atlantic^b		0.04±0.02	9	7.8±3.3
Smethie et al., 1985	Radon profiles	Tropical Atlantic	0.05/0.15/0.30	0.17±0.12	29	4.9/7.1/10.0
Kawabata et al., 2003	Radon profiles	West North Pacific		0.12±0.08	13	15.7±7.5
Flux model
Schery and Huang, 2004	Flux model	Southern Ocean^c	0.04/0.06/0.08	0.06±0.02	n/a	n/a
Flux assumed for or inferred from models
Jacob et al., 1997	Model	All oceans, 60°N–60°S	0.11			n/a
Mahowald et al., 1997	Model	Southern Ocean	0.06			n/a
Schery and Wasiolek, 1998	Model	All oceans and ice covered regions	0.14			n/a
Taguchi et al., 2002	Model	All oceans, 60°N–60°S	0.21			n/a
Law et al., 2008	Model	All oceans, 70°N–70°S	0.11			n/a

^aDistribution of trajectory averaged wind speeds weighted by radon decay.

^bNine samples from the Southern Atlantic – the study reports 100 measurements.

^cGlobal model, results shown in the table refer to the Southern Ocean only.

Fig. 8.  Oceanic radon flux density (mBq m⁻² s⁻¹) as a function of: (a) radon weighted trajectory mean wind speed (0.6 m s⁻¹ bins), and (b) significant wave height (0.15 m bins). Whiskers show the quartile range of the radon flux density with crosses indicating the bin median and dots the bin mean. Solid lines represent a quadratic fit a*x²+c. Coefficients (variance) of the fit are for (a) a=0.000580 (1.0293*10⁻⁸), c=0.0682 (0.000721), and for (b) a=0.017 (2.798*10⁻⁶), c=0.00679 (0.000534).

Fig. 9.  Oceanic radon flux density (mBq m⁻² s⁻¹) as a function of latitude (1° bins) for (a) summer and (b) non-summer months. Whiskers show the quartile range of the radon flux density with crosses indicating the bin median and dots the bin mean. Solid lines represent a linear fit. Coefficients of the fit are: (a) slope= −0.00667, intercept=0.135; correlation coefficient=−0.176, significance level=99.9%, and (b) slope= 0.00111, intercept= 0.314, correlation coefficient=0.034, significance level=55.3%.

Fig. 10.  Comparison of radon concentration (mBq/SCM; relative to the median) distributions for the least perturbed set in 2002–2003 obtained from experiment (E) and averaged model (M); the contributing models’ distributions are also shown (1=AM2.GFDL; 2=AM2t.GFDL; 3=CCAM.CSIRO; 4=CCSR_NIES1.FRCGC; 5=CCSR_NIES2.FRCGC; 6=PCTM.CSU; 7=TM3_vfg.BGC). For details regarding the models see Law et al., 2008, 2010.

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