To what extent do needle trees contribute to fine dust pollution?

Question

Today I ran into an article concerning cars and their effect on the existence of particulate matter. Especially the last paragraph caught my attention where it is claimed that coniferous also contribute highly to the formation of particulate matter. After doing some research I ended up with three articles and I wonder how they fit together and if the claims are true.

The claims of the articles are (details below) 1. Claim in first article: Needle trees contribute to fine dust pollution. 2. Claim in second article: Needle trees emit parts, which in reaction with human made pollution results in fine dust pollution. 3. Claim in the third article: Needle trees reduce fine dust pollution.

So the following questions came to my mind:

1. To what extent do needle trees contribute to fine dust pollution, especially in contrast to fine dust pollution from vehicles?
2. If there wasn't any human made air pollution would there still be fine dust pollution from needle trees? Is there fine dust pollution in the Taiga?
3. There are articles claiming that needle trees contribute to fine dust pollution but also decrease fine dust pollution. Which effect is predominant?

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Details: I refer to three German articles ( my own translation of the most relevant parts is provided)

The paragraph in German, translation below:

Außerdem halte ich viel davon, die Dinge wissenschaftlich zu sehen. Eine der größten Quellen von Feinstaub ist der Wald. Jeder Nadelbaum gibt chemische Verbindungen in die Atmosphäre ab, die zu winzigen Aerosol-Partikeln kondensieren. Die Experten sprechen von "biogenen Emissionen".

Source: http://www.spiegel.de/politik/deutschland/dekarbonisierte-zukunft-wie-man-eine-wirtschaft-ruiniert-a-1140586.html

Own Translation :

Furthermore I think it's important to maintain a scientific point of view. One of the largest source for particulate matter is the forest. Every coniferous emits a chemical compound, condense to minute aerosol particles. Experts speak of "biogenic emissions".

(Note that the article is more about economics and politics than science driven. A source for the claim is not provided in the article)

This paragraph surprised me as I never heard of this before so I did some digging and found the following article:

Headline (translated):

Pines cause fine dust

At the end of the article it states:

Auch natürliche Aerosole wie Alpha-Pinene können zur Feinstaubbelastung in der Luft beitragen, dies zeigen die Experimente. Bäume setzen große Mengen dieser organischen Verbindungen frei. Aus diesen entstehen aber erst dann gesundheitsschädliche Partikel, wenn sie in der Atmosphäre mit bestimmten anderen Chemikalien reagieren.

Own translation:

Natural aerosols such as alpha-pinenes can also contribute to fine dust pollution in the air, as the experiments show. Trees emit large amounts of these organic compounds. However, particles which are harmful to health are only produced by these particles when they react with certain other chemicals in the atmosphere.

Source: http://www.pflanzenforschung.de/de/journal/journalbeitrage/kiefern-verursachen-feinstaub-1869/

And some further digging revealed:

London hat zwar schöne Parks, gilt aber nicht als grüne Stadt. Dennoch filtern die Bäume der Metropole jedes Jahr bis zu 2100 Tonnen Feinstaub aus der Großstadtluft.

...

Nadelbäume, so das überraschende Ergebnis der Modellrechnungen, sind die effektivsten Feinstaubfilter.

Source: http://www.spiegel.de/spiegel/print/d-80818273.html

Own translation:

London has beautiful parks, but is not considered a green city. Nevertheless, the trees of the metropolis filter up to 2100 tonnes of fine dust from the urban air every year.

...

Coniferous trees, according to the surprising result of the model calculations, are the most effective fine dust filters.

Answer 1

In coniferous regions, it is possible for conifers to be a major source of particulate emissions. A good study is Characterization of a large biogenic secondary organic aerosol event from eastern Canadian forests Atmos. Chem. Phys., 10, 2825–2845, (2010).

Here we present evidence for the formation of the most substantial biogenic organic aerosol formation event yet observed, from measurements at a rural location north of Toronto, Canada. The five day event is characterized by steadily increasing organic aerosol levels to a maximum of 15 μgm⁻³

...

The particle size distribution is represented in the CTM by 12 bins ranging in diameter from 0.01 to 40.96 μm, with the 8 lower bins corresponding to sizes below 2.5 μm. Particle composition is represented by nine species (sulfate, nitrate, ammonium, black carbon, primary organic aerosol, SOA, crustal material, sea salt, and particulate water)

...

As shown in Fig. 4, the organic composition during the biogenic event is characterized by increased OOA-2 relative to OOA-1 and very low BBOA and HOA. During the biogenic event, OOA-2 constitutes 65%±10% of the total OOA (i.e. OOA-1 + OOA-2), vs. 46%±5% during periods of urban outflow (e.g. 30 May to 3 June) (reported uncertainties are the standard deviation of the OOA-2/organic mass fraction). As discussed in the previous section, the correlation of OOA-2 with MACR+MVK during the biogenic period (R2 = 0.71) indicates that the organic aerosol during the biogenic period is relatively fresh and therefore not the result of highly processed anthropogenic or biomass burning emissions.

Overall, under certain warm weather conditions, conifers can be a major contributor to particulate emissions.