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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 on 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 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. 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 one.

The acidity of aqueous atmospheric solutions is a key parameter driving both the partitioning of semi-volatile acidic and basic trace gases and their aqueous-phase chemistry. In addition, the acidity of atmospheric aqueous phases, e.g., deliquesced aerosol particles, cloud, and fog droplets, is also dictated by aqueous-phase chemistry. These feedbacks between acidity and chemistry have crucial implications for the tropospheric lifetime of air pollutants, atmospheric composition, deposition to terrestrial and oceanic ecosystems, visibility, climate, and human health. Atmospheric research has made substantial progress in understanding feedbacks between acidity and multiphase chemistry during recent decades. This paper reviews the current state of knowledge on these feedbacks with a focus on aerosol and cloud systems, which involve both inorganic and organic aqueousphase chemistry. Here, we describe the impacts of acidity on the phase partitioning of acidic and basic gases and buffering phenomena. Next, we review feedbacks of different acidity regimes on key chemical reaction mechanisms and kinetics, as well as uncertainties and chemical subsystems with incomplete information., Finally, we discuss atmospheric implications and highlight the need for future investigations, particularly with respect to reducing emissions of key acid precursors in a changing world, and the need for advancements in field and laboratory measurements and model tools.