You can use a filter node, set to block unless msg.topic changes.
Your function would have to send a message rather than null when temperature > 32.
That would result in an alert when the temperature dips and another when it rises again.
To prevent the rising notification use a switch node after the filter.
This is such a basic part of system control that you probably should code it up yourself just for the experience. On the other hand, you will learn a lot by looking at the descriptions of contributed nodes that do the job. Try searching the library for "hysteresis."
I don't like to use function nodes at all. But here they may be useful - as you can use a context of the node - means you can store within the function node, if an alert has been already sent. In other words - with a context you can store a state which can be retrieved each time a message is received.
However you must define a clear if - else block. Here I modified the code of your function node to remember if you already have been alerted. Nevertheless you should probably use a hysteresis as recommended by @drmibell. Otherwise you get severeal messages when temperature changes around the threshold.
Here the code for using context within a function node:
var alerted = context.get('alerted') || false;
if (msg.payload.Temperature <= 32) {
msg.topic = "Alert: Aspen temp below zero !!"
context.set('alerted',true);
if (!(alerted)) return msg;
} else
{
context.set('alerted',false);
return null;
}