09 Aug 2023
 | 09 Aug 2023
Status: this preprint is currently under review for the journal AMT.

SPARCLE 2: A new optical particle counter to measure particle size and refractive index

Moch Syarif Romadhon, Daniel Peters, and Roy Gordon Grainger

Abstract. In the last few decades, there has been an increasing need to improve in situ aerosol measurements to better understand the role of atmospheric aerosol on the Earth’s climate system and to assess the quality of ambient air. At the moment, in situ optically based aerosol photometers assume the refractive index of measured particles to give a size estimate. This assumption can result in large errors in estimated size. This study describes an instrument, called SPARCLE 2, that addresses this problem by simultaneously measuring the particle size and refractive index. SPARCLE 2 has two detectors to measure the pattern of aerosol-scattered light. The first is a detector with high sensitivity and high temporal resolution mainly to detect the presence of particles in the sensing volume. The second is a detector with high angular resolution to capture the pattern of scattered light. SPARCLE 2 is designed to measure particles whose diameters lie in the range 500 nm to 2,500 nm typical of the accumulation mode of troposphere aerosol. A theoretical lower particle size limit of 300 nm is determined by the optical and electronic noise. In practice, stray light limits the lower limit to a particle diameter of 800 nm. SPARCLE 2’s accuracy was tested using four monodisperse aerosols formed from non-absorbing polystyrene latex beads. The mean diameters of the test particles were 1,100 nm, 1,800 nm, 2,000 nm and 3,000 nm and their refractive index were 1.59. The standard deviation between SPARCLE 2’s measurement and the manufacturer’s stated size was 13 % for the 1,100 nm size particles and less than 3 % for the three larger sizes. The refractive index deviation was less than 1.25 % for all sizes. SPARCLE 2 was used to measure ambient Oxford (UK) city air. The size distributions measured by SPARCLE 2 were similar to those measured by a commercial optical particle counter. The refractive index distribution was consistent with the most abundant aerosol compositions around Oxford which are NO3, NH4+ and SO42−.

Moch Syarif Romadhon et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2023-140', Anonymous Referee #1, 12 Sep 2023
  • RC2: 'Comment on amt-2023-140', Anonymous Referee #2, 20 Sep 2023

Moch Syarif Romadhon et al.

Moch Syarif Romadhon et al.


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Short summary
The role of atmospheric aerosols on the Earth's climate and air quality is difficult to be determined quantitatively due to the drawback of available instruments. A widely used instrument to study the role is Optical Particle Counter (OPC). However, an assumption of particle refractive index is needed by OPCs to estimate particle size. This paper discusses SPARCLE 2: a new OPC that does not require such assumption. It was validated using standard particles and used to measure ambient air.