In situ aerosol acidity measurements using a UV–Visible micro-spectrometer and its application to the ambient air

Figures & data

Figure 1. Experimental setup of the flow tube used to calibrate aerosol acidity, using the C-RUV method under varying humidity and aerosol compositions (${\mathrm{N}\mathrm{H}}_4^{+}$-${\mathrm{S}\mathrm{O}}_4^{2-}$-${\mathrm{H}}_2\mathrm{O}$ or ${\mathrm{N}\mathrm{a}}^{+}$-${\mathrm{N}\mathrm{H}}_4^{+}$-${\mathrm{S}\mathrm{O}}_4^{2-}$-${\mathrm{H}}_2\mathrm{O}$).

Table 1. Inorganic aerosols produced in the flow tube for the calibration of aerosol acidity using the C-RUV method.

Composition^a	Sampling duration (min)	Flowrate (L/min)	Aerosol density^b (g/cm^3)	Volume concentration^c (nL/m^3)	Sampling mass^d(μg)	Dry mass^e (μg)
H2SO4	3.0	1.85	1.34	446	3.32	1.48
H2SO4	4.2	1.70	1.35	474	4.59	2.07
(NH4)0.31H0.69SO4	3.3	1.40	1.34	635	3.93	1.82
(NH4)0.53H1.47SO4	2.0	1.99	1.41	552	3.10	1.75
(NH4)0.79H1.21SO4	4.0	1.42	1.43	525	4.26	2.63
(NH4)0.98H1.02SO4	9.0	1.40	1.52	126	24.12	18.99
(NH4)1.53H0.47SO4	10.0	2.06	1.49	422	12.99	9.54
(NH4)1.88H0.12SO4	20.5	1.00	1.54	531	16.77	12.63
Na(NH4)H4(SO4)3	7.5	1.80	1.52	201	4.13	2.58
Na(NH4)H3(SO4)2.5	8.0	1.80	1.57	237	5.35	3.59
Na(NH4)H2(SO4)2	7.0	1.80	1.54	216	4.21	2.72
Na(NH4)H(SO4)1.5	10.0	1.80	1.63	206	4.40	6.04
(Na)2H4(SO4)3	3.0	2.20	1.34	296	2.61	1.10
(Na)2H3(SO4)2.5	7.0	1.80	1.66	247	5.18	3.37
(Na)2H2(SO4)2	8.0	1.80	1.77	246	6.24	4.48
(Na)2H(SO4)1.5	10.0	2.10	1.70	90.9	3.29	2.08

^aThe aerosol composition was determined using PILS-IC. The humidity of the flow tube reactor was higher than the efflorescence RH of inorganic aerosol during the generation of aerosol using an atomizer.

^bThe aerosol density was estimated from the E-AIM model at a given aerosol composition, temperature, and RH.

^cThe concentration of aerosol volume was measured using SMPS data.

^dThe sampling mass was estimated as the product of sampling duration, flowrate, density, and aerosol volume concentration.

^eThe aerosol dry mass was estimated by subtracting the water content (calculated using E-AIM model) from the total sampled aerosol mass.

Figure 2. Measured proton concentration ([H⁺]_C-RUV,E-AIM) in a (NH₄)_xH_ySO₄ aerosol system using the C-RUV calibration curve (estimated with E-AIM). [H⁺]_C-RUV,E-AIM is compared to the proton concentration predicted by E-AIM ([H⁺]_E-AIM) and ISORROPIA ([H⁺]_ISORROPIA) models.

Figure 3. [H⁺]_{C-RUV, ISORROPIA} in the (NH₄)_xH_ySO₄ aerosol system using the C-RUV calibration curve (estimated with ISORROPIA). [H⁺]_C-RUV is compared with both [H⁺]_E-AIM and [H⁺]_ISORROPIA.

Figure 4. Comparison of [H⁺]_C-RUV,E-AIM in the (NH₄)_xH_ySO₄ aerosol system with the predicted [H⁺]_E-AIM and [H⁺]_ISORROPIA under varying FS at a given RH.

Figure 5. Comparison of [H⁺]_{C-RUV,ISORROPIA} in the (NH₄)_xH_ySO₄ aerosol system with the predicted [H⁺]_E-AIM and [H⁺]_ISORROPIA under varying FS at a given RH.

Figure 6. Classification of the ambient aerosol collected at the sampling site located on campus at the University of Florida, Gainesville, Florida. Detailed information on ambient sampling is provided in Table 2.

Table 2. Ambient sampling information and meteorological conditions.

Class	Date	Temp.^a (°C)	RH^a (%)	Wind speed and direction^b	Average ion concentration^c (µmol/m^3)				Average gas concentration^d (ppb)		predicted [HNO3]^e (ppb)		predicted [NO3^-]^f (µmol/m^3)
					${\mathrm{N}\mathrm{H}}_4^{+}$	${\mathrm{N}\mathrm{a}}^{+}$	${\mathrm{S}\mathrm{O}}_4^{2-}$	${\mathrm{N}\mathrm{O}}_3^{-}$	${\mathrm{N}\mathrm{O}}_x$	${\mathrm{O}}_3$	$\left[\mathrm{H}{\mathrm{N}\mathrm{O}}_3\right]$lower	$\left[\mathrm{H}{\mathrm{N}\mathrm{O}}_3\right]$upper	${[\mathrm{N}\mathrm{O}}_3^{-}$]lower	${[\mathrm{N}\mathrm{O}}_3^{-}$]upper
1 (Na^+–NO3^−–SO4^2−) – (NH4^+–SO4^2−)	04/06/2018	10-29	34-95	0-12 mph, N-ESE-N	0.004	0.016	0.020	0.006	31.8	37.9	0.95	95.5	0.00026	0.00034
	04/13/2018	14-29	44-96	5-13 mph, E-ESE	0.010	0.009	0.015	0.005	7.0	40.1	0.21	20.9	0.00087	0.00103
	04/20/2018	16-23	60-72	6-18 mph, N-ENE	0.007	0.018	0.012	0.008	1.5	32.6	0.04	4.5	0.00184	0.00186
2 (Na^+–NH4^+–SO4^2−)	05/12/2018	18-33	43-77	0-13 mph, Various-SE	0.021	0.016	0.086	0	1.6	25.7	0.05	4.8	7E-7	7E-5
	07/12/2018	23-34	54-91	0-16 mph, SW	0.004	0.006	0.013	0	2.9	30.3	0.09	8.7	3E-7	3E-5
3 (NH4^+–SO4^2−–NO3^−)	11/28/2018	1-12	31-81	5-12 mph, WNW	0.009	0	0.002	0.006	5.7	23.9	0.17	16.9	0.005	0.014
	12/06/2018	2-17	29-84	0-7 mph, N- Calm	0.025	0	0.007	0.006	10.7	22.1	0.32	32.0	0.005	0.019
	12/11/2018	7-14	44-88	6-14 mph, WNW	0.011	0	0.003	0.004	1.9	22.0	0.06	5.7	0.002	0.012
	01/10/2019	1-14	26-66	0-13 mph, N	0.025	0	0.007	0.013	4.1	28.1	0.12	12.4	0.009	0.017

^aHygrometer was located on the roof of Black Hall at the University of Florida, Gainesville, Florida.

^bThe wind speeds and directions were obtained from https://www.wunderground.com/.

^cIon concentrations were determined using PILS-IC data. Aerosol samples were collected every hour. The data reported was averaged. The error associated with ion concentrations is 10%.

^dThe ozone concentration was averaged using 1-h intervals at the maximum concentration. The concentration of NO_x was averaged using the data obtained over the course of the experiment.

^eThe upper and lower boundaries of $[\mathrm{H}{\mathrm{NO}}_3$] were estimated using averaged ambient NO_x data and their factors, which are determined using the HNO₃/NO_x ratios reported in the literature. The lower boundary was set at 0.03[NO_x], and the upper boundary at 3[NO_x].

^f The upper and lower boundaries in $\left[{\mathrm{NO}}_3^{-}\right]$ were predicted using the E-AIM model, based on the upper and the lower boundaries of the estimated [HNO₃].