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Lower Atmosphere
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Night-time conditions and chemistry
The chemistry taking place in the atmosphere does not only depend on the emitted compounds, but also on the conditions in the atmosphere. The latter, however, are defined by the seasons, by the change of day and night, by the temperature profile, by the humidity, ... |
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1. Dissolving temperature inversion in a morning of July (measured in the Isar Valley / Germany)
by Elmar Uherek adapted from: Schirmer - Wetter und Klima - Wie funktioniert das?
Please click to enlarge (50 K)!
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Diurnal cycle
In the last section we learned about the diurnal cycle of OH, which depends on the sunlight. But not only the chemicals themselves have diurnal cycles, also the conditions in the atmosphere as e.g. the temperature profile. Conditions close to the ground in the so called planetary boundary layer vary and do not always follow the general rules (e.g. of decreasing temperature with increasing altitude) because of interactions with the Earth's surface. A very typical example is the night-time inversion layer which breaks open in the morning.
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Night-time inversion in the planetary boundary layer
Air masses below the free troposphere (D) belong to the planetary boundary layer. Some changes occur during a day (from left to right): At noon the air is mixed (light blue). After sunset a stable nocturnal layer forms (A) and residual air remains above (B). Air from the surface layer (below the dotted brown line) cannot easily go up to high altitudes during the night. The driving energy is missing. This energy comes back with sunrise. The ground warms, air starts ascending (red arrow), the stable layer from the night breaks open. A so called entrainment zone is raising from the ground to the top of the boundary layer (dark blue) and leaves the air mixed during the day (C).
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2. Model of the planetary boundary layer. The thickness of the planetery boundary layer may vary.
by Elmar Uherek, adapted from Stull 1988
Please click to enlarge! (70 K)
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In winter times it can happen, that the sun is not able to dissolve the inversion layer for the whole day or several days. In such cases pollution accumulates over the cities and leads to smog. There are also very often special conditions in mountain valleys, as shown on the photo on the right. The inversion remains below a cloud layer.
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3. Inversion layer in winter in the mountains
© Inst. for geographical education, Univ. of Erlangen-Nürnberg
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4. Important nitrate reactions at night (daytime photolysis in the yellow box)
Scheme by Elmar Uherek
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Nitrate chemistry
Due to the different conditions in the night, also the chemistry is different. There are fewer sources for OH and nitrate radicals undergo comparable reactions with organic species than OH does at daytime. Nitrate radicals NO3 are formed from the reaction of ozone O3 with nitrogen dioxide NO2 (Basics Unit 3). It reacts back with NO. This is why NO and NO3 cannot coexist at high concentrations. The reaction of NO3 and NO2 is the only way to form N2O5 in the atmosphere. It can either hydrolyse in water or decompose back to NO2 and NO3.
Comparable to OH, nitrate radicals abstract an H-atom from alkanes. Nitric acid HNO3 and alkyl radicals R are formed which react with the oxygen of the air and form peroxy radicals RO2. |
NO3 adds also to the double bonds of unsaturated organic compounds and forms peroxy nitrates after addition of O2.
It is of some interest that the lifetime of NO3 / N2O5 decreases drastically with increasing water vapour. HNO3 is formed from the reaction with water, in particular on liquid films on surfaces.
All peroxy species, coming from OH, NO3 or ozone reactions undergo rather complicated and numerous further reactions in the atmosphere with O2, HO2, RO2, NO or NO2 leading e.g. to alcohols, aldehydes, nitrates and carboxylic acids. It would go by far beyond the scope of this introduction to give details of all those reactions, which have only partially been investigated.
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5. Nitrate reaction with alkanes and alkenes
Please click to enlarge (30 K)!
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6. Nitrate absorption spectrum in the visible range with maxima in the red part of the spectrum (600-700 nm)
adapted from Sander (1986), measured at 230 K
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NO3 absorbs light in the red part of the visible spectrum. As soon as daylight arrives, it is photolysed mainly to NO2 and O and OH-chemistry dominates again. Ozone, the third important oxidant in the atmosphere, is not able to react with alkanes but can compete for alkenes if OH concentrations decrease under relatively dark conditions (winter or evening). A special chapter will focus on ozone reactions.
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About this page:
author: Dr. Elmar Uherek - Max Planck Institute for Chemistry, Mainz
scientific reviewer: Dr. Mark Lawrence - Max Planck Institute for Chemistry, Mainz 2004-05-05
educational proofreading: Michael Seesing - Uni Duisburg - 2003-07-02
revised and last published: 2004-05-07
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