MACC Summer School – Working Group
Atmospheric Chemistry
1. The main sink for formaldehyde, CH2O, in the Arctic is photolysis, with a mean rate constant j = 1 × 10−5 s−1. Oxidation of CH4 is the only significant source of CH2O. The steady-­‐state concentration of CH2O is given by: [CH 2 O] =
k
[CH 4 ][OH] j
where k = 2.0 × 10−12 exp(−1700/T ) cm3 molecule−1 s−1 is the rate constant for oxidation of methane by OH. The Table below shows the atmospheric concentrations of CH2O and CH4 in Greenland for three historical periods: present, pre-­‐industrial (1600 AD) and last glaciation (18,000 BC). [CH4] / ppbv [CH2O] / ppbv T / K 1700 0.10 260 Preindustrial 740 0.050 260 Glacial 410 0.010 250 Present (a) What is the importance of OH in the atmosphere? (b) Calculate the OH concentration (in molecules cm−3) over Greenland for each (c) Interpret the difference in OH concentrations between pre-­‐industrial times (d) One possible explanation for the difference in OH concentrations between glacial and pre-­‐industrial times is that stratospheric ozone concentrations were higher in glacial times. Why would that be? How would that affect OH concentrations? (e) How has the atmospheric lifetime of methane changed over these three periods period. and today. and what is the importance of this? 1 2. Nitrous acid, HONO, has several potential loss processes in the troposphere. It may photodissociate it may react with the hydroxyl radical or it can be taken up onto aerosol particles in a first order heterogeneous loss process: Given typical atmospheric concentrations of [HONO] = 2.4 × 1012 molecule cm−3 and [OH] = 4 × 106 molecules cm−3, calculate the atmospheric lifetime of HONO. HONO + hν → OH + NO (see photochemical data given in the table below), OH + HONO → NO2 + H2O kOH = 4.5 × 10−12 cm3 molecule−1 s−1 HONO(g) → HONO(aerosol) khet = 5.0 × 10−4 s−1. Wavelength
range / nm
Solar intensity /
Photons cm−2 nm−1 s−1
Absorption cross section /
cm2 molecule−1
Quantum
yield
290-310
5.27 × 1012
9.02 × 10−21
1
310-330
4.47 × 1013
1.02 × 10−19
1
330-350
1.15 × 10
14
−20
1
350-370
1.45 × 1014
1.54 × 10−19
1
370-390
14
−20
1
1.71 × 10
2 9.57 × 10
8.17 × 10