Volume 16, No. 5, May 2016, Pages 1152-1164 PDF(3.03 MB)
Supplementary MaterialPDF (1016 KB)
Continuous Measurement of Ambient Aerosol Liquid Water Content in Beijing
Oscar A. Fajardo1, Jingkun Jiang1,2, Jiming Hao1,2
1 State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, 100084, China
2 State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, Beijing 100084, China
- An instrument was developed for measuring aerosol water content.
- This instrument was deployed to study PM pollution in Beijing.
- Efflorescence of ambient aerosol in Beijing was detected at ~ 50% RH.
- Aerosol inorganic species accounts for water uptake at high PM concentration.
- The contribution of organics becomes predominant at low PM concentration.
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 organic-associated water is comparable with that associated with inorganic compositions during the clean periods at low ambient RH conditions.
Urban aerosol; Aerosol water content; Beijing pollution; Aerosol acidity.