Trends in particle-phase liquid water during the Southern Oxidant and Aerosol Study

Abstract

Abstract. We present in situ measurements of particle-phase liquid water. Measurements were conducted from 3 June to 15 July 2013 during the Southern Oxidant and Aerosol Study (SOAS) in the southeastern US. The region is photochemically active, humid, dominated by biogenic emissions, impacted by anthropogenic pollution, and known to contain high concentrations of organic aerosol mass. Measurements characterized mobility number size distributions of ambient atmospheric aerosols in three states: unperturbed, dry, and dry-humidified. Unperturbed measurements describe the aerosol distribution at ambient temperature and relative humidity. For the dry state, the sample was routed through a cold trap upstream of the inlet then reheated, while for the dry-humidified state the sample was rehumidified after drying. The total volume of water and semi-volatile compounds lost during drying was quantified by differencing dry and unperturbed volumes from the integrated size spectra, while semi-volatile volumes lost during drying were quantified differencing unperturbed and dry-humidified volumes. Results indicate that particle-phase liquid water was always present. Throughout the SOAS campaign, median water mass concentrations at the relative humidity (RH) encountered in the instrument typically ranged from 1 to 5 μg m−3 but were as high as 73 μg m−3. On non-raining days, morning time (06:00–09:00) median mass concentrations exceeded 15 μg m−3. Hygroscopic growth factors followed a diel cycle and exceed 2 from 07:00 to 09:00 local time. The hygroscopicity parameter kappa ranged from 0.14 to 0.46 and hygroscopicity increased with increasing particle size. An observed diel cycle in kappa is consistent with changes in aerosol inorganic content and a dependency of the hygroscopicity parameter on water content. Unperturbed and dry-humidified aerosol volumes did not result in statistically discernible differences, demonstrating that drying did not lead to large losses in dry particle volume. We anticipate that our results will help improve the representation of aerosol water content and aqueous-phase-mediated partitioning of atmospheric water-soluble gases in photochemical models.

Coalescence-based assessment of aerosol phase state using dimers prepared through a dual-differential mobility analyzer technique

“…The SMPS was configured to scan across a range of voltages corresponding to a range of diameters that included the expected coagulated peak. Details on the configuration of the column and SMPS configuration are described elsewhere (Suda and Petters 2013;Nguyen et al 2014). All sheath flows were controlled using critical orifices on the outlet and dry, compressed, particle free zero air on the inlet.…”

Measuring Mass-Based Hygroscopicity of Atmospheric Particles through in Situ Imaging

“…2,3 Frequently, in field measurements, hygroscopic tandem differential mobility 58 analysis (HTDMA) is used to measure particle hygroscopicity. [4][5][6] With HTDMA, hygroscopic 59 growth is quantified from changes in particle size as a function of relative humidity.…”

Understanding aerosol-cloud interactions in the development of orographic cumulus congestus during IPHEx

“…This is consistent with the finding from an 10 earlier study in the Amazon rainforest showing that accumulation mode particles are more hygroscopic than Aiken mode particles (Gunthe et al, 2009). Note that the average κ values at each supersaturation level are comparable to subsaturated κ (0.14-0.46) measured in the southeastern United States (Nguyen et al, 2014) and the approximate global average (κ ~ 0.3) for continental aerosols . At this surface site, the average NCCN (± one standard deviation) is 569±208 cm -3 at S = 0.19%, 1,022±387 cm -3 at S = 0.38%, 1,210±505 cm -3 at S = 0.51% (see Fig.…”

“…On average 178 measurement cycles were collected daily during the IPHEx IOP, except for occasional interruptions due to instrument maintenance. CN and CCN distributions were inverted as described previously (Nguyen et al, 2014;Petters and Petters, 2016). Supersaturation was calibrated using dried ammonium sulfate and a water activity model (Christensen and Petters, 2012;Petters and Petters, 2016).…”