Formation of Active Chlorine Oxidants in Saline-Oxone Aerosol

Figures & data

FIG. 1 Possible reaction mechanisms in the internally mixed saline Oxone aerosol.

FIG. 2 UV-Visible spectroscopy to monitor the oxidation capability of active chlorine and aerosol acidity in both internally and externally mixed aerosols. For externally mixed aerosol, Oxone particles were collected on the dyed filter containing the preexisting saline or saline-buffer particles. For the internally mixed aerosol, the saline-Oxone aerosol is sampled on the dyed filter using the different sampling systems (F1-F2-F3 and F1-D-F2).

TABLE 1 Experimental conditions to study aerosol reactivity and acidity using UV-Vis spectroscopy for the aerosol sample collected on the dyed filter with metanil yellow or thymol blue ^a

Dyed filter and	Aerosol composition^b	Sampling	Aerosol	Aerosol
aerosol system	(weight ratio)	time (min)	Mass^c (μg)	vol.conc. (nL/m^3)	(μg)	(ng/m^3)
Metanil yellow external mixture	saline	2.5	9.00	538	18.64	N.A.^f
	saline:buffer = 1:2	2.0	17.00	636	17.77	N.A.
	saline:buffer = 1:4	2.0	16.00	635	17.35
	Oxone	1.5	—	500	10.60	N.A.
Metanil yellow internal mixture	saline:Oxone = 1:2 (F1-D-F2)	2.0	9.97	440	—	N.A.
		2.17	11.00	408	—	N.A.
		3.0	12.07	355	—	N.A.
	saline:Oxone = 1:2 (F1-F2-F3)	2.50	10.15	321	—	N.A.
		3.0	10.57	317	—	N.A.
	saline:Oxone = 1:2 (F1-F2)^g	2.0	13.83	569	—	N.A.
		2.17	10.92	402	—	N.A.
		2.67	10.53	342	—	N.A.
Thymol blue external mixture	saline	6.0	11.00	284	21.47	46.86
	saline:buffer = 1:2	10.4	14.50	197	28.46	41.53
	Oxone	2.5	—	624	20.42	97.57
^aThe temperature and RH inside the Teflon chamber ranged from 22.8–27.4°C and 60.8–66.3%.
^bThe aerosol composition is described based on the aerosol mass deposited on the filter. The original aqueous solution concentrations are 1% (W/V: weight/volume of water) for saline; 2% for Oxone solution; 2% for buffer (sodium bicarbonate or phosphate buffer).
^cAerosol mass deposited on a dyed filter was measured using an analytical balance at the given laboratory RH. For internal mixture, aerosol mass was calculated from SMPS data (sampling flow rate ranged 10.1–11.5 L/min) and density of aerosol (1.1 g/cm^3).
^dSampled Oxone aerosol mass from Teflon chamber.
^eProton mass was calculated by Equation (7).
^fN.A.: Not applicable.
^gThe experiments using the F1–F2 sampling system were carried out to confirm the oxidation ability of either aerosols or the active chlorine gases (e.g., HOCl and Cl2).

FIG. 3 FTIR experimental setup to measure the water amount in oxidant particles. The small flow chamber equipped with silicon and ZnSe FTIR windows was placed in the FTIR optical beam path in the transmission mode.

TABLE 2 Experimental conditions to monitor the water content in saline, Oxone, and saline-Oxone particles using FTIR and the estimated water amount in various particles in the two different RH levels at 20% and 70%

						M dry−particle at two different RH^c (μg)		Mwater at two different RH (μg)
Solution^a (weight ratio) and composition	%RH (lab)	Temp (°C)	Particle mass^b (μg)	Possible major aerosol constituents	Phase transition (Figure 7)	20%	70%	20%	70%
NaCl	52.0	25.5	55	NaCl	ERH:55-57 DRH:83-87	37.6	37.6	0	88.8
saline salt mole%: Na^+ (41.94), Cl^− (48.84), SO^2− 4 (2.53), Mg^2+ (4.86), Ca^2+ (0.94), K^+ (0.89)	26.5	23.8	68	Na^+, Cl^−, SO^2− 4, Mg^2+	ERH:52-59 DRH:76-84	34.4	34.4	19.4	71.0
Oxone (2KHSO5 KHSO4 K2SO4)	19.7	19.4	51	K^+ salts of SO^2− 5, SO^2− 4, HSO^− 5, or HSO^− 4	No phase transition	23.6	23.6	10.5	27.2
Phosphate buffer (Na2HPO4)	49.3	26.0	65	Na2HPO4	No phase transition	23.3	23.3	10.0	30.9
Saline: Oxone = 1: 2 (no buffer)	38.0	22.5	53	Cl^−; HOCl; Mg^2+, K^+ or Na^+ salts of SO^2− 5, SO^2− 4, HSO^− 5, or HSO^− 4	No phase transition	25.8	25.8	16.7	56.8
Saline: Oxone: phosphate buffer (Na2HPO4)= 1: 2: 2.4	47.5	25.1	156	Cl^−; HPO^2− 4; HOCl; Mg^2+, K^+ or Na^+ salts of SO^2− 5, SO^2− 4, HSO^− 5, or HSO^− 4	ERH:12-19 DRH:N.A.	75.9	75.9	19.8	103.4
Saline: Oxone: bicarbonate buffer (NaHCO3)= 1: 2: 2.4	53.6	25.0	58	Cl^−; HOCl; Mg^2+, K^+ or Na^+ salts of SO^2− 5, SO^2− 4, HSO^− 5, or HSO^− 4; CO2	—	—	—	—	—
^aThe aerosol atomized from the aqueous solution of interest was impacted onto FTIR. The original aqueous solution concentrations are 1% (W/V: weight/volume of water) for saline; 2.5% for NaCl; 2% for Oxone solution; 2.4% for buffer (sodium bicarbonate or phosphate buffer).
^bParticle mass deposited on a FTIR silicon window at the laboratory humidity condition.
^cSolute mass: Actual dry particle mass impacted on a silicon window corrected by relative humidity.
N.A.: Not applicable.

FIG. 4 The ΔA’_420,oxi (Equation (2)) obtained using different sampling systems (F1-F2-F3 and F1-D-F2) for the internally mixed saline-Oxone aerosol after the Oxone was mixed with saline in the aqueous solution. Error bars in F1 filters are estimated from uncertainties in the SMPS data (6%), and UV-Visible absorbance (4%) and sampling flow rate (1%). Error bars in F2 and F3 filters which evaluate oxidation effects of gas-phase oxidants are estimated from uncertainties in the sampling flow rate and UV-Visible absorbance.

FIG. 5 Time profile of ΔA’_420,oxi (Equation (2)) of the external mixture of Oxone, saline-Oxone, saline-Oxone with phosphate buffer aerosol. Error bars were estimated from uncertainties in the sampling device, balance, SMPS data, and UV-Visible absorbance.

FIG. 6 FTIR spectra of the tested buffers (sodium bicarbonate and phosphate buffer) and the subtracted FTIR spectra ([spectrum of Saline/Oxone/Buffer] − [spectrum of Saline/Oxone])

FIG. 7 The M′ _water (Equations (5) and (6), μg/μg) of NaCl, saline, Oxone, and saline-Oxone aerosols as a function of decreasing humidity. Error bars were estimated from uncertainties in the FTIR absorbance at O-H band and balance.