Atmospheric nanoparticle observations in the low free troposphere during upward orographic flows at Izaña Mountain Observatory
This study investigates the processes and conditions favouring the formation of nanoparticles (diameter<10 nm) which are frequently observed on high mountains reaching the low free troposphere. This was done through an analysis of a data set collected at Izaña Global Atmospheric Watch Observatory...
| Main Authors: | , , , , , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2009-09-01
|
| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/9/6319/2009/acp-9-6319-2009.pdf |
| Summary: | This study investigates the processes and conditions favouring the formation
of nanoparticles (diameter<10 nm) which are frequently observed on high
mountains reaching the low free troposphere. This was done through an
analysis of a data set collected at Izaña Global Atmospheric Watch
Observatory (Canary Islands; 2367 m above sea level). This high
mountain supersite is located well above the stratocumulus layer
characteristic of the subtropical oceanic tropospheres. At night, when the
catabic flow regime is well established, free troposphere aerosols were
measured. The development of orographic buoyant upward flows during daylight
resulted in an increase of water vapour, SO<sub>2</sub> and NO<sub>y</sub>
concentrations. These ascending airflows perturbed the free troposphere and
resulted in high concentrations of 3–10 nm particles (N<sub>3–10</sub>) due to new
particle formation. An analysis of the 5-min average time series allowed
the identification of two main types of N<sub>3–10</sub> event. In Type I events a
linear relationship between N<sub>3–10</sub> and SO<sub>2</sub> was observed (<i>r</i><sup>2</sup>
coefficients 0.70–0.95 and a mean slope of 11 cm<sup>−3</sup> ppt<sup>−1</sup>
for 5-min averaged data; SO<sub>2</sub> concentrations from tens to hundreds of
ppt). These particles seem to be formed during upward transport (probably
within or after the outflows of clouds typically located below Izaña).
During Type II events, no correlation between SO<sub>2</sub> and N<sub>3–10</sub> was
observed and 3–10 nm particles were formed in-situ at noon and during the
afternoon due to the condensation of vapours linked to photochemistry. New
particle formation was observed almost every day owing to the favourable
conditions associated with the entry of boundary layer air in the low free
troposphere, even if SO<sub>2</sub> concentrations are rather low at Izaña
(tens to hundreds of ppt). The low surface area of pre-existing particles,
low temperature and high radiation intensity clearly favoured the formation
of nanoparticles. The low surface area of pre-existing particles in the
upward flows is furthered by in-cloud particles scavenging in the
stratocumulus layer typically located below Izaña. The higher
temperature and the presence of coarse Saharan dust particles decrease the
efficiency of the new particle formation mechanisms in summer. Thus, the
"N<sub>3–10</sub> versus SO<sub>2</sub>" slope (for <i>r</i><sup>2</sup>>0.7 cases) was higher in
autumn and winter (~15 cm<sup>−3</sup> ppt<sup>−1</sup> as average) than
in summer (2–8 cm<sup>−3</sup> ppt<sup>−1</sup>). These field observations
suggest that elevated mounts that reaches the free troposphere may act as
source regions for new particles. |
|---|---|
| ISSN: | 1680-7316 1680-7324 |