Effect of Aerosol Volatility on the Sizing Accuracy of Differential Mobility Analyzers

Figures & data

Figure 1 Trajectories of stable and volatile (evaporating) particles inside a DMA. Virtual entrance and exit slits for the aerosol and monodisperse flows are represented by short dotted lines near the top and the bottom of each graph.

Figure 2 Diagram of the TDMA setup.

Figure 3 A comparison of ratios of sampled size to reported nominal size calculated with the simplified and the trajectory models for a compound with C_s = 10 μg/m³. The trajectory model results for the scanning mode with 30 s and 300 s scan rates are also shown.

Figure 4 A compilation of size distributions of aerosol transmitted by the first DMA as measured by the second DMA at 25°C.

Table 1 Summary of measured midpoint sizes of ammonium sulfate (AS) and ammonium nitrate (AN) aerosol during different experiments. All sizes are in nm

Room air		NH3-scrubbed		Room air
25°C		25°C		17.5°C
AS	AN	AS	AN	AS	AN
51.3	33.0	51.3	31.0	49.2	40.9
				75.1	68.7
83.0*	39.8**	83.0*	38.0**
103.2	87.5	102.9	86.0	101.3	95.8
203.5	190.6	203.4	189.2	204.6	200.3
303.5	291.0	301.8	288.8	303.5	300.9
406.4	394.7	406.0	393.0

*This size was estimated for doubly charged particles that have the same electrical mobility as 51.3 nm singly charged particles.**The nominal size of the second peak detected when measuring ammonium nitrate particles at set nominal size of 51.3 nm.

Figure 5 A comparison of observations with the model. Markers indicate the ratio of the measured nominal midpoint size to the nominal size set by the first DMA.

Figure 6 Predicted ratio of nominal diameter (d_{p, n}) to sampled diameter (d_{p, s}) when measuring aerosols of different volatilities at sheath flow 3 L/min.

Figure 7 Predicted ratio of nominal diameter (d_{p, n}) to sampled diameter (d_{p, s}) at different sheath flow rates.

Figure 8 Predicted ratio of transferred diameter (d_{p, t}) to nominal diameter (d_{p, n}) at different sheath flow rates.

Figure 9 Predicted ratio of transmitted (V_{p, t}) to nominal aerosol volume (V_{p, n}) when measuring ammonium nitrate aerosol at different temperatures using TSI long DMA at 3 L/min sheath flow rate.

Table 2 Estimated ratio of the transmitted to set mass when selecting 300 nm ammonium nitrate aerosol using a single DMA

Sheath flow,
L/min	17.5°C	20°C	22.5°C	25°C
3	0.974	0.966	0.957	0.946
6	0.987	0.983	0.979	0.972
10	0.992	0.990	0.987	0.984

Figure 10 Predicted d_{eq, 1}/d_{p, n} ratio for ammonium sulfate aerosol assuming different values of water accommodation coefficients (α). Aerosol is initially in metastable equilibrium with air sample at 40% RH. Sheath air RH is 90%.

Figure 11 Predicted d_{eq, 1}/d_{p, t} ratio for ammonium sulfate aerosol assuming different values of water accommodation coefficients (α). Conditions were taken as in Figure 10.

Figure 12 Selection (Ω_s) and transfer (Ω_t) functions modeled for constant voltage mode of operation when sampling ammonium nitrate aerosol at 3 L/min sheath flow rate. Ω_s and Ω_t are given for different centroid d_{p, n} of ammonium nitrate, which are indicated above individual functions. The transfer function for a stable aerosol is also given for reference.

Figure 13 Selection (Ω_s) functions modeled for scanning mode of operation when sampling ammonium nitrate aerosol at 3 L/min sheath flow rate. Ω_s are given for different centroid d_{p, n} of ammonium nitrate, which are indicated above individual functions. The selection function for a stable aerosol is also given for reference.

Figure 14 Modeled approximation of concentration bias, R_c, when measuring an evaporating aerosol as a function of centroid nominal diameter . Calculations were made for a sheath airflow rate of 3 L/min and scan times of 30 s and 300 s scan times.