Abstract:
"In Earth's environment, the observed polar outflow rate for O+ ions, the main source of oxygen above gravitational escape energy, corresponds to the loss of ~18% of the present-day atmospheric oxygen over 3 billion years. However, part of this apparent loss can actually be returned to the atmosphere. Examining loss rates of four escape routes with high-altitude spacecraft observations, we show that the total oxygen loss rate inferred from current knowledge is about one order of magnitude smaller than the polar O+ outflow rate. This disagreement suggests that there may be a substantial return flux from the magnetosphere to the low-latitude ionosphere. Then the net oxygen loss over 3 billion years drops to ~2% of the current atmospheric oxygen content."
http://www.sciencemag.org/cgi/content/abstract/291/5510/1939

I would guess that the observation of Atmospheric gas leaving Earth can observed and measured by looking a the Earth's magnetosphere's tail. And one could argue that the earth's magnetosphere is part of the mechanism that allows atmospheric losses. Or the gas is accelerated by magnetic forces.
One could also ask whether there were other ways of accelerating the gases of an atmosphere other than by magnetic acceleration. Two other means that come to mind are, high speed charged particles and simply high temperature. {oh and also impactors].
Actually, impactors, solar wind, and high speed particles all seem similar as far as striping a atmosphere- they can remove the atmosphere but also can add to the atmosphere. In other words if an 100 meter diameter impactor hits earth, it will cause some of Earth atmosphere to leave earth and not only gases, but rocks could ejected and end up on Mars or the Moon. The type, size, where and how a impactor hits Earth could mean that more atmosphere is added than subtracted, or more atmosphere is lost [and the lost atmosphere may at later time be recovered].

I guess one could say that according to observation something like millions of tons of material from Space is deposited on Earth yearly- and of course some of this will be gaseous elements. Other than Oxygen being magnetically accelerated, what other measurable quantities are leaving earth?