# Is wind just mainly nitrogen particles moving in one direction?

The air is composed of mainly nitrogen.

Therefore, when you feel wind, is it mainly nitrogen particles hitting you that, on average, are moving in one direction?

In a nutshell, you're a little bit right. The Air pressure is essentially Nitrogen and Oxygen molecules hitting our bodies, but that's true with and without wind.

wind is more easily explained from the macro-scale not the micro-scale. It's not molecular behavior but governed by forces like high and low pressure systems and weather, or, fans if you're indoors.

The Brownian motion of molecules is pretty consistent like 3d billiard balls, though velocity of the molecules increases with temperature. This is kind of what air molecules movement looks like:

Wind is, in a nutshell, the entire box moving, not individual particles changing direction, but it could probably be looked at either way.

If all the air molecules, by some strange coincidence, all moved in the same direction 2 things would happen. One, the air would get very cold as temperature is essentially agitated molecules moving or vibrating against each other, and two, the wind speed would be over 1000 kph. Roughly the speed of sound. Fortunately this kind of thing is so statistically improbable it essentially never happens.

## Composition of gases forming air

The air around us consists of approximately 78% gaseous $\ce{N2}$ (gaseous nitrogen) by volume. When we assume that all atmospheric gases are ideal gases then 78 of 100 gaseous molecules, which hit us, are $\ce{N2}$ molecules.

## Distinguishing between particles and gases

However, we have gaseous and particulate compounds in the atmosphere. $\ce{N2}$, $\ce{O2}$, $\ce{NO}$, $\ce{NO2}$, and $\ce{SO2}$ are gaseous compounds in the atmosphere. We also have particles in the atmosphere. Such as pure ammonium sulfate ($\ce{\left(NH4^+\right)_2SO4^2-}$) particles, sea salt particles (consisting of dominantly $\ce{NaCl}$ and $\ce{Na2SO4}$), organic particles, or soot.

When we talk about the composition of the air, we usually mean the gaseous compounds and not the particulate compounds.

## Nitrogen at/in particles

We have gaseous $\ce{NH3}$ (ammonia) and $\ce{HNO3}$ (nitric acid) in the atmosphere. The first is primarily emitted from processed faeces. In Europe the intensive animal farming is a major source for $\ce{NH3}$ emissions. The latter is a reaction product of $\ce{NO2}$ (emitted by combustion processes) and $\ce{OH}$ ($\ce{OH}$ radicals). These two compounds condense on particles or form new particles as $\ce{NH4^+NO3^-}$, with sulfate ($\ce{\left(NH_4^+\right)_2SO_4^{2-}}$), or on sea salt particles as $\ce{NaNO3^-}$.

When we are in regions with high anthropogenic emissions by animal farming and industry/transportation, such as Northwestern Europe, Eastern China, or US Coast, we also have these nitrogen compounds in the air. They for new particles or condense on existing particles. Therefore, many particles in these regions are nitrogen-containing particles.

## Number of particle and molecules

How much particles do we have in a $m^3$ of air/aerosol?

For example, Penttinen et al. (2001) published a number of about 15000 particles per $cm^3$ (measurement in polluted air; commonly the number concentrations are lower). These are $1.5 \times 10^4\ cm^{-3}$ or $1.5\times 10^{10}\ m^{-3}$.

\begin{align} & 1\ m = 100\ cm \\ \Rightarrow & 1\ m^3 = \left(100\ cm\right)^3 = 1000000\ cm^3 = 10^6\ cm^3 \\ \Leftrightarrow & 1\ m^{-3} = 10^{-6}\ cm^{-3} \\ \Leftrightarrow & 10^6\ m^{-3} = 1\ cm^{-3} \\ \Rightarrow & 1.5\times 10^4\ cm^{-3} = 1.5\times 10^4\times 10^6\ m^{-3} = 1.5\times 10^{10}\ m^{-3} \end{align}

How much gaseous molescules? Assuming a standard pressure $p$ of 101325 Pa, a temperature $T$ of 293 K (20 degree C) we have:

$$\frac{n}{V} = \frac{p}{R\times T} = \frac{101325\ Pa}{8.314\ J\ K^{-1}\ mol^{-1} \times 293\ K} \approx 41.59\ \frac{mol}{m^3} \approx 2.5 \times 10^{25}\ \frac{molecules}{m^3}$$

$R$ is the gas constant, $n$ is the particle number in moles, and $V$ is the volume. The conversion from moles to number of molecules is performed via the Avogadro constant.

We have in the order of $10^{10}$ particles and in the order of $10^{25}$ molecules per $m^3$. Hence, we are are dominantly hit by gaseous molecules (assuming equal speed of particles and moolecules which is not necessarily correct).

• Could you explain how you converted $1.5 \times 10^4\ cm^{-1}$ to $1.5\times 10^{10}\ m^{-3}$? Also is cubic centimetre to the power of minus 1 ($cm^{-1}$) a typo? Thanks Jun 30 '16 at 16:04
• @G.Gip It is a typo and should be $cm^{-3}$. I corrected it. Thank you for the remark. Jun 30 '16 at 16:10
• My maths of volumes is a little poor, so I'm struggling to understand how you calculated 15 000 000 000 $m^{-3}$ from 15 000 $cm^{-3}$. Could you explain? Thanks Jun 30 '16 at 16:18