Measuring Ambient Acidic Ultrafine Particles Using Iron Nanofilm Detectors: Method Development

Figures & data

FIG. 1 The schematic of acid-coated nanoparticles generation and collection system. Note: A is the ultrasonic nebulizer; B is the quartz tube furnace; C are two conical flasks with purified water for cooling gas and removing large-size particles preliminarily; D is the silicone gel dryer to absorb the water mist; E is the filter to remove the particles with a size larger than 1 μm; F is a pump (0.5 L/min flow rate) to draw the carbon nanoparticles to G; G is the conical flask with highly pure H₂SO₄ for the generation of sulfuric acid vapor on a heater; H is the water-cooled condenser; J is SMPS + ESP; and K is CPC.

FIG. 2 Two-dimensional (2D) and three-dimensional (3D) images of a reaction spot caused by a 75.4 nm sulfuric acid droplet generated from the H₂SO₄ solution with a concentration of 0.009 g/cm³.

FIG. 3 Correlation between horizontal distance (width) of reaction spot scanned by AFM and the diameter of generated acidic droplets. The primary vertical axis represents the width of reaction spots; the secondary vertical axis represents the ratio of the width of reaction spots to the diameter of generated acidic droplets. The vertical error bars represent the deviation of the measurements.

FIG. 4 Correlation between vertical distance (height) of reaction spot scanned by AFM and the diameter of generated acidic droplets. The primary vertical axis represents the height of reaction spots; the secondary vertical axis represents the ratio of the height of reaction spots to the diameter of generated acidic droplets. The vertical error bars represent the deviation of the measurements.

FIG. 5 Three-dimensional images of sulfuric acid-coated particle reaction spot on the 25-nm iron film; (a) A 32.5 nm acid-coated particle reaction spot; (b) A 51.5 nm sulfuric acid coated particle reaction spot. Note: The height of the reaction site above the iron-film surface was expressed as 0-2 vertical distance, and the width of the reaction site was taken as 2-2 horizontal distance i.e. maximum width of the reaction site. The central bump of the reaction site was taken as the width at half maximum value, shown as 1-1 horizontal distance.

FIG. 6 Correlation between the dimension (width and height) of the reaction spots of acid-coated particles scanned by the AFM and the mobility diameter (D_m) of the acid-coated particles. The vertical error bars represent the deviation of the replicate measurements.

FIG. 7 AFM image of 32.5 nm acid-coated particles on Fe–Ti–MS detector; (a) AFM image before exposure to 90% RH and 20°C; (b) AFM image after 3-month exposure to 90% RH and 20°C.

TABLE 1 Acidic and nonacidic particles collected during the field measurement at TMS

		Number of particles (mean ± SD)		Size of particles (GMD ± SD)*
Dates	Sampling time	Nonacidic particles per 100 μm^2	Acidic particles per 100 μm^2	Nonacidic particles size range and the diameter at peak per 100 μm^2 (nm)	Acidic particles size range and the diameter at peak per 100 μm^2 (nm)
22-Nov	15:00– 22:00	236.7 ± 85.3	64.3 ± 28.4	8.5–375	7.5–248
				56.7 ± 11.9	47.7 ± 8.4
23-Nov	15:00– 22:00	147.2 ± 64.4	62.2 ± 32.5	9.4–386	10.9–298
				62.8 ± 14.7	43.6 ± 18.4
24-Nov	15:00– 22:00	157.8 ± 68.5	58.2 ± 29.6	8.7–358	7.4–312
				65.4 ± 21.8	50.4 ± 16.7
25-Nov	15:00– 22:00	194.6 ± 93.5	56.9 ± 32.4	7.5–316	9.5–307
				49.6 ± 18.7	40.5 ± 11.2
*The size of nonacidic particles was verified using the results of AFM x and z calibrations; the diameter of acidic particle or acidic droplet was verified according to the ratios between the size of reaction spot and the acidic droplet or acidic particle (Section 3.1).

FIG. 8 Ambient acidic particles collected on the iron nanofilm detectors at TMS:(a) an AFM image with a 2.9 μm × 2.9 μm scanning area on an Fe–MS nanofilm detector; (b) a 2.8 μm × 2.8 μm scanning area on an Fe–VE nanofilm detector.

TABLE 2 Comparison of the number concentration of ambient particles calculated by AFM with the values derived from the SMPS + CPC in the field measurement

		Number concentration of particles (mean ± SD 10^3/cm^3)
Date	Sampling time	Nonacidic particles	Acidic particles	Total particles	SMPS + CPC data*
22-Nov	15:00–22:00	4.60 ± 0.86	2.10 ± 0.45	6.70 ± 0.41	7.74 ± 1.56
23-Nov		2.86 ±0.65	1.61 ± 0.42	4.48 ± 0.23	5.66± 1.10
24-Nov		3.07 ± 0.68	1.51 ± 0.35	4.58 ± 0.33	5.10± 1.03
25-Nov		3.78 ±0.71	1.48 ± 0.36	5.26 ± 0.35	6.29 ± 1.39
* For SMPS + CPC, the average hourly particle number concentration for the sampling period was calculated according to the trend of diurnal variation and the hourly averages for the nonsampling period. Two-tail t-tests between the ambient particle number concentration measured by the AFM and the SMPS + CPC at 15:00–22:00 on these four days indicated that the p value was larger than 0.05.