Supermicron modes of ammonium ions related to fog in rural atmosphere 1Key Lab of Marine Environmental Science and Ecology, Ministry of Education, Ocean University of China, Qingdao 266100, China
26 Nov 2012
2Air Quality Research Division, Science and Technology Branch, Environment Canada, 4905 Dufferin Street, Toronto, Ontario, M3H 5T4, Canada
Received: 26 January 2012 – Published in Atmos. Chem. Phys. Discuss.: 21 February 2012 Abstract. Fog-processed aerosols were identified and analyzed in
detail from a large-sized database in which size-segregated atmospheric
particles and gases were simultaneously measured at eight Canadian rural
sites. In ten samples collected during or following fog events, at least one
supermicron mode of particulate NH4+ was observed. The supermicron
modes were likely associated with fog events since they were absent on
non-fog days. The supermicron mode of NH4+ in the 5–10 μm
size range probably reflected the direct contribution from fog droplets.
Based on detailed analysis of the chemical compositions and the extent of
neutralization, the supermicron mode of NH4+ in the 1–4 μm
size range was believed to be caused by fog-processing of ammonium salt
aerosols. These aerosol particles consisted of incompletely neutralized
sulfuric acid aerosols in NH3-poor conditions or a mixture of ammonium
nitrate and ammonium sulfate aerosols in NH3-rich conditions.
Interstitial aerosols and fog droplets presented during fog events likely
yielded a minor direct contribution to the measured NH4+. The mass
median aerodynamic diameter (MMAD) of the 1–4 μm mode of
NH4+ strongly depended on ambient temperature (T) and can be
grouped into two regimes. In one regime, the MMAD was between 1.1 and 1.7 μm
in four samples, when fog occurred at T > 0 °C,
and in two samples, at T > −3 °C.
The MMAD of NH4+ in this size range was
also observed in various atmospheric environments, as discussed in the
literature. In the other regime, the MMAD was between 2.8 and 3.4 μm
in four samples when fog occurred at T < −4 °C, a
phenomenon that was first observed in this study. The MMAD was not related
to chemical composition and concentration of ammonium salts. Further
investigations are needed in order to fully understand the cause of the
MMAD. The larger supermicron mode of ammonium salts aerosol observed at
T < −4 °C has added new knowledge on the size
distributions and chemical compositions of fog-processed aerosols under
various ambient conditions.
Revised: 16 November 2012 – Accepted: 19 November 2012 – Published: 26 November 2012
Citation: Yao, X. H. and Zhang, L.: Supermicron modes of ammonium ions related to fog in rural atmosphere, Atmos. Chem. Phys., 12, 11165-11178, doi:10.5194/acp-12-11165-2012, 2012.