Why is there more pollution in the air when it's humid?

[Cowhide]

Smog and humidity seem to go hand and hand.
Why it is that when there's lots of humidity in the air, the ozone level is much higher than when the RH is very low?
Sorry for the rookie question. But I want to know.

[Aslkahuna]

Actually Ozone levels are independent of humidity since both Las Vegas and Phoenix get high ozone levels when they have very low humidity. In fact, they often have higher Ozone levels under low humidity conditions because the patterns that support low RH's (upper High overhead or stagnant trapped air in the Winter) also support higher pollution due to strong sunlight. What makes the smog more evident in high humidity is the formation of haze and aerosols.

Steve

[Wthrman13]

The higher the humidity, the more likely water vapor will condense on various types of aerosols. I'm not an expert in this area, but different types of aerosols generally have different thresholds of relative humidity where water vapor will condense on them. The RH does not have to be 100% for this to happen, because some aerosols will actually "attract" water molecules to them. Such aerosols are called "hygroscopic", which means "water seeking". They form haze particles at certain levels of RH, which reduces visibility and gives the air that "polluted" look, which in a sense, it really is.

There is a recent article by an SPC forecaster (Corfidi) that goes into the phenomena of haze in depth. I haven't read it yet myself, but folks tell me that it's a pretty good primer on haze and how it has become more prevalent in modern times:

http://www.spc.noaa.gov/publications/corfidi/haze.html

Apologies if this isn't what you were looking for, but it might be interesting to you nevertheless.

[Cowhide]

I guess it's not ozone then, it's Particulate matter or (PM) or higher concentration levels of PM when it's humid.

[Aslkahuna]

Particulate matter consists of dust, smoke, and soot for the most part and are also different from the aerosols that can form the haze. There are more than one component to smog-you have chemicals formed by photochemical reactions such as Ozone, Carbon Monoxide, various Nitrogen oxides, SO2, etc which form independently from humidity. Under humid conditions some of these plus other nuclei combine to form haze. Then you have particulates which come from combustion or other sources such as the above mentioned smoke, dust and soot which can originate elsewhere (such as the particulates from China that make it to the West Coast). The difference between aerosols are often a matter of size and Pollen can be both an aerosol or a particulate. Near the ocean, salt can aid in the formation of haze. In the Los Angeles area and along the CA coast in general salt haze has been in existence for millenia as has been fog which can also react with air pollution. Smog is a soupy combination of many things and can exist in both wet and dry forms.

Steve