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SUMMARY:Nocturnal Chemistry in the Troposphere: Laboratory and Field Studi
 es. - Dr Mingjin Tang\, University of Cambridge
DTSTART:20121022T131500Z
DTEND:20121022T143000Z
UID:TALK40512@talks.cam.ac.uk
CONTACT:Dr Alex Archibald
DESCRIPTION:The formation and loss of NO<sub>3</sub> radicals and N<sub>2<
 /sub>O<sub>5</sub> during the nighttime plays important roles in many aspe
 cts of tropospheric chemistry. It influences the NOx budget directly and t
 hus impacts the photochemical formation of ozone\, contributes significant
 ly to the degradation of\nvolatile organic carbons and the formation of or
 ganic nitrate and secondary organic aerosols\, and leads to the formation 
 of particulate nitrate and ClNO2 which is an important precursor of Cl ato
 ms in the troposphere. Field measurement and laboratory studies have been 
 combined to better understand the loss mechanism of NO<sub>3</sub> and N<s
 ub>2</sub>O<sub>5</sub> in the troposphere.\n\nThe two-channel Cavity Ring
 -Down instrument (CRD)\, with a detection limit of 1-3 pptv in 3 s\, was d
 eveloped to simultaneously detect NO<sub>3</sub> radicals and N<sub>2</sub
 >O<sub>5</sub>\, and has been successfully deployed in three large field c
 ampaigns between 2008 and 2011 at sites with large geographical difference
 s (a coastal site in south Spain\, a boreal forest in Finland\, and a rura
 l mountain site in Germany). Mixing ratios of NO<sub>3</sub> (from below d
 etection limit to >200 pptv) and N<sub>2</sub>O<sub>5</sub> (from below de
 tection limit to >2 ppbv)\, and NO<sub>3</sub> lifetimes (from seconds to 
 >1 h) were found to vary a lot and highly depend on the concentrations of 
 precursors (i.e. NO<sub>2</sub> and O<sub>3</sub>) and also the strengths 
 of different sinks. The contribution of different NO<sub>3</sub>/N<sub>2</
 sub>O<sub>5</sub> loss processes to the nighttime NOx removal will be pres
 ented for in a few interesting scenarios\, and the first measurement of Cl
 NO<sub>2</sub> over Europe will be introduced and its potential significan
 ce will be discussed.\n\nThe heterogeneous uptake N<sub>2</sub>O<sub>5</su
 b> to Saharan dust particles was investigated using aerosol flow tubes wit
 h detection of N<sub>2</sub>O<sub>5</sub> by cavity ring-down spectroscopy
 . The uptake coefficient was determined to be 0.02 ± 0.01 on airborne Sah
 aran dust particles\, independent of relative humidity (RH\, 0-67%) and in
 itial N<sub>2</sub>O<sub>5</sub> concentration (~ 20-1000 ppbv). The parti
 cles could be deactivated with respect to N<sub>2</sub>O<sub>5</sub> uptak
 e only when pre-treated with very high levels of HNO<sub>3</sub>. Analysis
  of gas- and particulate phase products suggests that N<sub>2</sub>O<sub>5
 </sub> undergoes heterogeneous hydrolysis forming particulate nitrate. The
  heterogeneous reaction of NO<sub>3</sub> radicals with Saharan dust parti
 cles was studied using a novel relative rate method. The uptake coefficien
 t ratio\, γ(NO<sub>3</sub>)/γ(N<sub>2</sub>O<sub>5</sub>)\, was determin
 ed to be 0.9±0.3 for Saharan dust\, independent of relative humidity and 
 exposure time\, though surface deactivation was observed for both species 
 even when mixing ratios were typically a few hundred pptv.\n\nA brief summ
 ary will be followed by discussing a few challenges in nocturnal chemistry
  and potential solutions.
LOCATION:Unilever Lecture Theatre\, Department of Chemistry
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