% Theory

Probably the oldest scientific theory about precipitation manipulation by humans is the inducement of convection. Referring to this \citet{ashworth1929} writes: ``In a confined manufacturing area, such as the town of Rochdale, with a large number of factories burning quantities of coal of the order of 500 to 10,000 tons a day, it is not unlikely that the volume of heated gases which rises is sufficient to give that uplift to the atmosphere which is required to provoke an increase of rain.'' At that time it was already known that  a slight uplift to the air---such as caused by the passage of the wind over a small elevation of the land in a flat country---augments the precipitation. In this context \citet{ashworth1929} states: ``This effect on an uplift to the air may be looked for over a collection of mill chimneys from which a considerable upward current of hot gases issues for a third to a half of the 24 hours.''  He also assumes that there might be another possible effect which triggers precipitation: ``There is also the probability that the fine flue dust ejected by the draught up the chimney may supply an abundance of the nuclei which promote the formation of rain [\dots].''

In cloud physics, the hypothesis---by supplying additional cloud condensation and ice nuclei, pollution downwind from urban centres would increase precipitation occurrence, precipitation amount, or both---has been supported for a long time. Research since the late 1980s, however, suggests that anthropogenic aerosols may decrease precipitation occurrence and amount because pollution particles cause the same amount of cloud water to be distributed among more droplets, hence the droplets are smaller and less likely to grow to precipitation-sized particles. The hypothesised result is that precipitation is less likely to occur \cite{schultz2007}.
\citet{baeumer2007feb} conclude: ``Since also cloud amount and precipitation are modified in the course of a week, we suppose that the indirect aerosol effects on cloud properties and precipitation play an important role. The prevalent ideas about coherences between an increase in aerosol particle number, an increase in cloud droplets number but a decrease of their radii, and a following decrease of precipitation but longer cloud lifetimes, is not reflected by our results.''

\citet{gong2007} hypothesised that the changes in the atmospheric circulation may be triggered by the accumulation of \PM{10} through diabatic heating of the lower troposphere. During the early part of a week the anthropogenic aerosols are gradually accumulated in the lower troposphere. Around midweek, the accumulated aerosols could induce radiative heating, likely destabilising the middle to lower troposphere and generating anomalously vertical air motion and thus resulting in stronger winds. The resulting circulation could promote ventilation to reduce aerosol concentrations in the boundary layer during the later part of the week. Corresponding to this cycle in anthropogenic aerosols, the frequency of precipitation, particularly the light rain events, tends to be suppressed around midweek days through indirect aerosol effects. This is consistent with the observed anthropogenic weather cycles, e.\,g. more (less) solar radiation near surface, higher (lower) maximum temperature, larger (smaller) diurnal temperature range, and fewer (more) precipitation events in midweek days (on weekends) \cite{gong2007}.

Not only the amount of aerosols influence the meteorology. It also depends on whether these aerosols can act as \ac{CCN} and on their size distribution: \label{chap:theory:ccn_size}
\begin{itemize}
 \item \citet{dusek2006} detected that aerosol size distribution, particularly that of fine aerosols, plays a significant role in the nucleation of cloud particles which is one of the primary mechanisms of indirect aerosol effects. Furthermore they found that the size matters more than chemistry for the cloud-nucleating ability of aerosol particles.
\item An experimental study by \citet{yin2000} investigated the effect of giant \ac{CCN} on the development of precipitation in mixed-phase convective clouds. Their results showed that the strongest effects of introducing giant \ac{CCN}  occur when the background concentration of small nuclei is high, as it is in continental clouds. Under these conditions, the coalescence between water drops is enhanced due to the inclusion of giant \ac{CCN}. This leads to an early development of large drops in the lower parts of the clouds. In maritime clouds, where the background concentration of small nuclei is low, the effect of the giant \ac{CCN} is smaller and the development of precipitation is dominated by the droplets formed on large nuclei.
\end{itemize}

These results show that it is not trivial to determine the important parameters that have to be analysed in the following. Due to the fact that only \ac{PM} has been analysed systematically over a longer time period and a certain range, the data used for the computations in \nameref{chap:results} Chapter, Section \ref{results:sec:pm} \nameref{results:sec:pm}, on page \pageref{results:sec:pm} are \PM{10} and \PM{1}.