I like to have the second itemize list to appear at the top of the frame, i.e. at the same place as the previous itemize list. At this frame I have two columns where each item on the left column are associated with several items which I like to be appeared on the right column after each item on the left column. Then, item list associate with first item on right should disappear and again the item list associate to second item on the left should start from the top of the frame.
\documentclass{beamer}
\mode<presentation> {
\usetheme{Hannover}
\usecolortheme{seagull}
}
\usepackage{graphicx}
\usepackage{booktabs}
\usepackage{siunitx}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{document}
\begin{frame}
\frametitle{ applications}
\begin{columns}
\begin{column}{0.4\textwidth}
\begin{itemize}
\setbeamercovered{transparent}
\item <1-5> {\small Combustion}
\item<6-> {\small Airborne }
\item<10->{\small Environmental}
\end{itemize}
\end{column}
\begin{column}{7cm}
\begin{itemize}[label={}]
\item[] <2-5>{\tiny PN$\mathrm{_{ d< 1\,\si{um}}}$ $\gg$ PN$\mathrm{_{ d> 1\,\si{um}}}$ }
\item[] <2-5>{ \tiny A correlation }
\item[] <3-5>{\tiny No universal }
\item[] <4-5> {\tiny limited}
\end{itemize}
\begin{itemize}[label={}]
\item[] <7-7>{\tiny PN$\mathrm{_{ d< 1\,\si{um}}}$ $\gg$ PN$\mathrm{_{ d> 1\,\si{um}}}$ }
\end{itemize}
\end{column}
\end{columns}
\end{frame}
\end{document}
{}
to format code blocks -- or simply indent them by 4 spaces. It is easier to help you if you add a minimal working example that takes the form\documentclass{...}\usepackage{....}\begin{document}...\end{document}
. If possible, it should compile and have the minimum amount of code needed to illustrate your problem. This makes it much easier for people to help you - and much more likely that they will!