Table 1. Volcanic gas compositions, expressed as molar ratios and molar proportions.

Reported uncertainties on molar ratios are 95% confidence bounds (1.96 × standard error of the regression). Uncertainties on flux measurements are asymmetrical and therefore quoted as ± [upper/lower bounds]. H₂S was not detected above the 13% cross-sensitivity of the sensor to SO₂. The SO₂ flux reported for each compositional measurement is the average of all flux measurements made on the same day by multiple techniques (see Table 2).

Date/Time (UTC)		30 October 2018/21:00	22 May 2019/06:30	23 May 2019/00:00	23 May 2019/00:45	26 May 2019*/00:00	Average (May 2019) ± propagated error
Vent sampled		Main Crater	Southern Crater	Southern Crater	Southern Crater	Southern Crater
Molar	CO2/SO2 (±2σ)	1.19 ± 0.13	1.03 ± 0.14	1.16 ± 0.09	1.12 ± 0.12	0.95 ± 0.10	1.07 ± 0.06
	r2	0.91	0.69	0.69	0.96	0.94
	H2O/SO2 (±2σ)	161 ± 18	18.7 ± 2.4	31.3 ± 3.1	–	–
	r2	0.94	0.7	0.58
	BrO/SO2 (×10−5 ± 2σ)	–	1.80 ± 0.03	1.20 ± 0.08	1.20 ± 0.08	1.20 ± 0.03	2.02 ± 0.04
	r2	–	0.87	0.74	0.74	0.8
Mass	CO2/SO2	0.82 ± 0.09	0.71 ± 0.10	0.80 ± 0.06	0.77 ± 0.08	0.65 ± 0.07
	H2O/SO2	45.3 ± 5.1	5.3 ± 0.7	8.8 ± 0.9	–	–
	BrO/SO2 (×10−5)		2.69 ± 0.04	1.80 ± 0.6	1.80 ± 0.6	1.80 ± 0.04
Molar composition	H2O (mol%)	98.7	90.2	93.5	–	–
	CO2 (mol%)	0.7	5	3.5	–	–
	SO2 (mol%)	0.6	4.8	3	–	–
	BrO (×10−7 mol%)	–	8.7	3.6	–	–
Mass flux	SO2 flux (tons day−1)	–	5825 ± [927/987]	4900 ± [346/1816]	4900 ± [346/1816]	4973 ± [841/1015]	5150 ± [336/733]
	CO2 flux (tons day−1)	–	4122 ± [863/896]	3905 ± [410/1479]	3770 ± [484/1455]	3245 ± [646/745]	3760 ± [313/595]
	H2O flux (×103 tons day−1)	–	30.6 ± [6.3/6.5]	43.1 ± [5.2/16.2]			36.9 ± [4.1/8.9]
	BrO flux (×10−2 tons day−1)	–	16 ± [0.3/0.3]	8.8 ± [0.9/3.3]	8.8 ± [0.9/3.3]	8.9 ± [1.5/1.8]	10 ± [0.8/1.4]

*This measurement was made using a different multi-gas instrument from the preceding dates. See Materials and Methods for full details.