Articles | Volume 9, issue 2
Research article
16 Feb 2016
Research article |  | 16 Feb 2016

Evaluation of the Sequential Spot Sampler (S3) for time-resolved measurement of PM2.5 sulfate and nitrate through lab and field measurements

A. Hecobian, A. Evanoski-Cole, A. Eiguren-Fernandez, A. P. Sullivan, G. S. Lewis, S. V. Hering, and J. L. Collett Jr.

Abstract. The Sequential Spot Sampler (S3), a newly developed instrument to collect aerosols for time-resolved chemical composition measurements, was evaluated in the laboratory and field for the measurement of particulate sulfate and nitrate. The S3 uses a multi-temperature condensation growth tube to grow individual aerosols to droplets which are then deposited as a ∼ 1 mm diameter dry spot at the end of the growth tube in a 100 µL well of a multi-well plate. The well plate advances automatically to provide a sequence of time-resolved samples. The collected aerosols are subsequently analyzed in the laboratory. The sample is concentrated during the collection process, and the laboratory extraction and analysis steps can be automated. The well plate, as received from the field, is placed onto a needle-based autosampler that adds liquid for sample extraction and injects sample extract from each well onto an ion chromatograph for analysis. Laboratory evaluation for sulfate and nitrate ions showed that poly ether ether ketone (PEEK) used as well plate material does not contribute any artifacts; a 60 min extraction procedure leads to the recovery of sulfate and nitrate from the dry spots at above 95 % extraction efficiency; and samples stored frozen and analyzed up to 23 months later show less than a 10 % change in sulfate and nitrate concentrations. The limit of detection was 0.5 µg m−3 for sulfate and 0.2 µg m−3 for nitrate for a 1 h sampling period. In a month-long field study conducted in southern California, two S3s were deployed alongside a URG denuder–filter-pack and a Particle-Into-Liquid Sampler combined with an Ion Chromatograph (PILS-IC). Collocated S3 sampler concentrations compared by linear regression show good agreement, with r2 =  0.99 and slope = 0.99 (±0.004) µg m−3 for sulfate and r2 = 0.99 and slope = 1.0 (±0.006) µg m−3 for nitrate. When compared to the URG denuder–filter-pack and the PILS-IC, the S3 sulfate and nitrate concentrations yielded correlations above 0.84 for the square of the correlation coefficient and regression slopes close to 1.

Short summary
A newly developed instrument, the Sequential Spot Sampler (S3) was evaluated in the laboratory and field for the hourly measurement of ambient PM2.5 nitrate and sulfate concentrations. The results from the comparison of two S3s and the S3s with other well-established methods show that this instrument is suitable for deployment; provides high-resolution aerosol nitrate and sulfate concentrations while requiring minimal operator involvement and low power input; and has a small footprint.