Condensation Particle Counters Combined with a Low-Pressure Impactor for Fast Measurement of Mode-Segregated Aerosol Number Concentration

Figures & data

FIG. 1 Schematic diagram of LPI-CPCs (MFM: mass flow meter; T: temperature sensor; P: pressure sensor). The pressure sensors incorporated in the 3771 and 3772 (not explicitly shown) are used to measure the pressure downstream and upstream of the LPI, respectively. The additional pressure sensor downstream of the LPI is used for backup and checking consistency.

TABLE 1 Dimensions of the low-pressure impactor (LPI)

Stage	N	W, mm	S, mm	T, mm	S/W	T/W
2	20	1.1	4	2	3.6	1.8
1	30	0.25	1.2	0.8	4.8	3.2
N: Number of nozzles; W: nozzle diameter; S: separation distance between the nozzle and plate; T: nozzle throat length.

FIG. 2 Transmission efficiency of the LPI as a function of the square root of Stk for inlet pressures (P _in) of 980 and 475 hPa. The solid and dashed lines represent the fitting results for 980 and 475 hPa, respectively. Corresponding aerodynamic diameters at 980 hPa are shown on the top axis.

TABLE 2 Laboratory evaluation of the LPI stage 1^a

Parameter	P in = 980 hPa	P in = 475 hPa
U, m s^−1 b	1.2 × 10^2	61
Reynolds number^b	1.9 × 10^3	4.7 × 10^2
Pressure ratio^c	0.89	0.96
Experimentalda 50, μm^d	0.19	0.22
Experimental slope^e	1.4	1.3
Experimental (Stk50)^1/2	0.445	0.454
^aValues were determined based on laboratory experiment [1].
^bJet velocity (U) and Reynolds number were calculated using the mass flow rate through the LPI and upstream stagnant pressure.
^cPressure ratio across stage 1 was calculated assuming that the pressure ratio across stage 2 can be approximated as unity.
^dThe difference in the experimental d a50 values between 980 and 475 hPa is simply due to difference in the d a50,set values.
^eThe slope was calculated as the ratio of diameters corresponding to the transmission efficiencies of 0.3 and 0.7, respectively.

FIG. 3 Dependence of the transmission efficiency of the LPI on P _in for particles with a mobility diameter (d_m ) of 0.05 μm obtained before the aircraft measurement campaign (laboratory experiment [1]). The solid line represents the fitting result. The calculated cutoff diameter (d_{a 50,calc}) and volumetric flow rate through the LPI ranged from 0.23 to 0.19 μm and from 4 to 9 L min⁻¹, respectively, at inlet pressures ranging from 380 to 980 hPa.

TABLE 3 Possible errors (1σ)^a

Error sources		Total CN	PM0.17 CN	(Total − PM0.17) CN
Correction for flow rate^b		±1%	±1%	±1%
Correction for asymptotic detection efficiency^b		±1%	±1%	±1%
Pressure dependence of minimum detectable	1000 hPa	—	—	—
diameter^b,c	700 hPa	+1%	+1% of total CN	—
	350 hPa	+26%	+26% of total CN	—
Pressure dependence of LPI cutoff diameter^c,d	1000 hPa	—	−4% of total CN	+4% of total CN
	700 hPa	—	—	—
	350 hPa	—	+1% of total CN	−1% of total CN
Pressure dependence of LPI transmission	1000 hPa	—	+4%	−4% of PM0.17 CN
efficiency at dm = 50 nm^e	700 hPa	—	+5%	−5% of PM0.17 CN
	350 hPa	—	+15%	−15% of PM0.17 CN
^aPositive (negative) errors mean that the values may be underestimated (overestimated).
^bErrors are estimated based on Takegawa and Sakurai (2011).
^cWe assume typical number size distributions of ambient aerosol particles based on Takegawa and Sakurai (2011) (originally quoted from Seinfeld and Pandis [2006]): “remote continental air” for 1000 and 700 hPa and “urban air” for 350 hPa. The use of “urban air” instead of “free tropospheric air” for 350 hPa is simply because it appears to be more appropriate considering the size distribution measured by the SP2 (not shown).
^dErrors are estimated assuming ρp = 1.5 g cm^−3.
^eErrors are estimated based on Equation (2).

FIG. 4 Time series of some key variables of the LPI, including (a) P _in, jet velocity (U), and pressure ratio (r); (b) Reynolds number (Re), calculated cutoff diameter (d_{a 50,calc}); and (c) total CN concentration, and PM_0.17 CN concentration during flight 15 on 12 April 2009. The data are 1-s averages.

FIG. 5 Vertical profiles of (a) total CN concentration and (b) the ratio of PM_0.17 to the total CN concentration (PM_0.17/total ratio) observed over the Yellow Sea during flight 15. Open and solid circles represent the aircraft descent and ascent data, respectively. The data are 10-s averages. Error bars for some selected data points (at ∼1000, 700, 350 hPa) are shown. See Section 3.2 for details of the error estimate.

Takegawa , N. and Sakurai , H. 2011 . Laboratory Evaluation of a TSI Condensation Particle Counter (Model 3771) Under Airborne Measurement Conditions . Aerosol Sci. Technol. , 45 : 272 – 283 .

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Seinfeld , J. H. and Pandis , S. N. 2006 . Atmospheric Chemistry and Physics, , 2nd ed. , New York : John Wiley and Sons .

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