Introduction to Atmospheric Chemistry Measurements-II

John OrtegaNational Center for Atmospheric ResearchBoulder, CO, USA

National Center for Atmospheric ResearchBoulder, CO USA

United StatesNational Science Foundation

Outline

•Review•What do we want to measure?•Physical, chemical principles• Selectivity•Detection limits•Proxies

Some measurement types

• Spectroscopy (FTIR)• Mass spectrometry• UV absorption• IR absorption• Trap (focus) with chromatography• Filters

Some measurement types of MANY

• FTIR (Fourier Transform Infrared Spectroscopy)• Requires non-symmetric molecule or different atoms

• Stretching

• Bending

• What about CH4?• Output provides information on functional groups (OH, COOH,

double bonds) and some specific molecules (e.g. H2O).

O OCOC OC

O

OC

NNOO

Source

Path

Detector

Other IR absorption• H2O• CH4• H2O• Fast (10 Hz)• Couple with eddy covariance to get fluxes

Mass spectrometry

• Take in gases into an inlet• Reduced pressure• Charge molecule (EI or CI)• Direct ions using electric fields (lenses)• Detector

+

e-

Detector

Mass spectrometry

GC/MS

GC/MS

GC/MS Chromatogram

Element # massC 3 36H 8 8O 1 16Total Mass 60

NOx measurement

• NO + O3 -> NO2 + hn• Excess ozone used. Light of particular l is what is detected• # of photons detected is proportional to concentration

• Case 1: Measure total gas stream NO + NO2• Case 2: Pass gas through NO2 converter to convert NO2 to NO• Measure both (NO + NO2) = NOx• Obtain NO2 by subtracting NOx from total signal• Case 3: Pass gas through scrubber to remove all NOx• Get zero

NO2 converter• Headed Mo catalyst• Blue light• % conversion efficiency – note that when interpreting

instrumental data, converting NO2 to NO is not 100% efficient.

http://www.wmo.int/pages/prog/arep/gaw/documents/PollackNO2LEDsJAC.pdfPollack et al., Journal of Atmospheric Chemistry, DOI 10.1007/s10874-011-9184-3, 2011.