What control the atmospheric lifetime of different metals in aerosols?

Question

Background

I'm analysing the metallic elements in $PM_{2.5}$ to identify their possible anthropogenic sources for the study area.

A total of 19 metallic elements were investigated including dust-derived elements such as $Al, Ca, Mg$ and pollution-derived elements such as $Pb, Cu, Cd, Zn$

To better investigate the characteristics of different elements, their temporal variation (daily samples, collecting once for 6 days at an industrial city in China) during the whole year was analyzed.

I found out the burning of fireworks for celebrating China Spring Festival presented significant contribution to related elements, such as:

• $Al, Mg$: they are utilized for the fuels of gunpowder.
• $K$: the chemical compounds of $K$$ (KClO_4, KClO_3, KNO_3$) are usually utilized for the oxidant of gunpowder.
• $Cu, Sr, Ba$ : used for generating lighting in different color.

Since the emission of firework burning were centered on the festival night, the influence should be short. We collect the aerosol samples and analyzed their species for the festival, one day after festival and two days after festival.

Observation results

• Most of related elements presented the significant levels on the festivity and decreased sharply in the next day (~ 100 times).

• However, I found three heavy metals with adverse health effects ($Pb, Cd, As$) had increases in concentration levels after the festival. Their concentration levels recovered to their normal level after 3 days.

My thought

• Most of the related elements recovered to their normal levels in one day, the accumulation of $Pb, Cd, As$ should not be blamed on meteorological conditions;
• There might not be other important sources leading to the continuous increases of $Pb, Cd, As$ in 3 days,

Therefore, I though the accumulation of high concentration levels in 3 days should be attributed to firework burning,

This finding would be valuable since a short-time emissions event resulted in a relative long-time exposure to high concentrations if my inference is correct.

My question

Does the atmospheric lifetime differs for specific elements? I assumed that the elements $Pb, Cd, As$ presented a long atmospheric lifetime with less deposition? I still felt confused by the phenomenon. Any advice or comments on my question would be hugely appreciated!

Answer 1

You could say that metals and aerosols reside in the atmosphere but I'm struggling to call that "atmospheric lifetime", which I've only heard in reference to gases. The residence time of particulate matter is mostly a function of meteorology (e.g. PBL height, wind speed) and deposition (e.g. raining, settling velocity). You've observed some metals that had a longer residence time. I can think of two reasons for that:

1. Unique reactivity with other atmospheric components will change the mass and deposition velocity of particles. Metals that do not react well in the atmosphere may not grow as quickly and therefore settle more slowly.

2. Specific metals may be used more prevalently in fireworks that obtain high altitude, such that the metal emissions are injected further up into the atmosphere and subjected to more atmospheric mixing and therefore longer settling times.

You might find this article by Licudine et al., (2012) of interest, which discusses monitoring of metals on New Years in Hawaii.