Continuous Measurement of Ambient Aerosol Liquid Water Content in Beijing

Abstract

The amount of liquid water present on ambient aerosol particles influences its radiative properties, the partitioning of gas-phase organic and inorganic compounds to the condensed phase, and the assessment of particle health effects upon inhalation. An improved home-made Dry-Ambient Aerosol Size Spectrometer (DAASS) was implemented and deployed at a sampling site in Beijing to measure water content and volumetric growth factor of fine ambient aerosol during clean and polluted days. Aerosol chemical composition was characterized by an aerosol chemical speciation monitor (ACSM). Meteorological conditions were also assessed to trace origin of the air masses arriving at the site. A thermodynamic model, ISORROPIA II, was used to predict aerosol water content based on inorganic compositions measured by ACSM. It was then compared with the aerosol water content measured by DAASS. During the two-week sampling period, an aerosol efflorescence behavior was observed around 50% relative humidity (RH). Aerosol at the sample location was predominantly alkaline, with an average acidity ratio of 1.3 during the polluted days but very variable during the clean days. This fact coupled with generally low ambient RH resulted in low amounts of liquid water detected during the campaign. When RH reached its maximum of 87% and aerosol appeared to be more acidic, the maximum amount of aerosol water content measured was 1.3 µg m -3 (44.9% wt of DAASS measured ambient aerosol). Measured aerosol water content and model predictions seem to show a better agreement during the more polluted days, but during the clean days aerosol organic fraction appears to have an important contribution in the water content. We argue that the organicassociated water is comparable with that associated with inorganic compositions during the clean periods at low ambient RH conditions.

Interactions between aerosol organic components and liquid water content during haze episodes in Beijing

“…In addition, higher uncertainties in NR-PM 1 measurements of low NR-PM 1 loadings and in ALW modelling at low RH may also contribute to the large variability. High ALW org /ALW in low aerosol mass concentrations is consistent with previous studies (Dick et al, 2000;Fajardo et al, 2016). Those studies showed that modelled ALW inorg was much lower than measured total ALW under low aerosol mass loadings in Beijing (Fajardo et al, 2016) and that ALW org was comparable to ALW inorg in low RH (Dick et al, 2000).…”

“…Long-term data are needed to evaluate the amount of ALW and its interactions with aerosol organic compositions. So far, short-term ALW data in Beijing (Bian et al, 2014;Fajardo et al, 2016) have been collected by directly measuring size-resolved aerosol hygroscopic volume growth factors (VGFs) and particle size distributions using a hygroscopicity tandem differential mobility analyser (H-TDMA) (Rader and McMurry, 1986) or dry-ambient aerosol size spectrometer (DAASS) (Engelhart et al, 2011;Stanier et al, 2004). However, long-term measurements of ALW are rare because of the challenge in maintaining these instruments.…”

“…Although this approximation provides reasonable ALW in many ambient conditions, it fails in some cases. Especially when organics contribute a dominant fraction to particle loading, large discrepancies arise between the modelled inorganic water and the actual ALW content (Fajardo et al, 2016). Therefore, it is important to take the organic contribution to ALW into consideration.…”

Interactions between aerosol organic components and liquid water content during haze episodes in Beijing

“…In addition, higher uncertainties in NR-PM1 measurements of low NR-PM1 loadings and in ALW modelling at low RH may also contribute to the large variability. High ALWorg/ALW in low aerosol mass concentrations is consistent with previous studies (Dick et al, 2000;Fajardo et al, 2016). Those studies showed that modelled ALWinorg was much lower than measured total ALW under low aerosol mass loadings in Beijing (Fajardo et al, 2016) and that ALWorg was 6 https://doi.org/10.5194/acp-2019-316 Preprint.…”

Physicochemical Characteristics of Individual Aerosol Particles during the 2015 China Victory Day Parade in Beijing

“…Uptake of water by atmospheric particles modifies the mass of the aerosol particle and increases the particle size [30,31]. Previous studies have shown that particle diameter increases with increasing RH [32].…”