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Where personal-care siloxanes go in US air

CMAQ puts cyclic siloxanes highest in cities, with oxidized products peaking downwind at far lower concentrations than the parents.

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What chemicals are being modeled?

Cyclic volatile methyl siloxanes D4, D5, D6 from personal-care products.

What is the typical atmospheric lifetime?

About 5–10 days at typical OH, so long-range transport occurs.

Which model and domain were used?

CMAQ 4.7.1 over the contiguous US, northern Mexico, and southern Canada.

How was seasonality sampled?

Four 1-month runs: January, April, July, and October.

How large can urban mean D5 get versus oxidized D5?

D5 up to 432 ng m−3; o-D5 up to 9 ng m−3.

Where are oxidized siloxanes highest?

Downwind of major urban centers.

How do urban versus rural parent concentrations behave?

Urban air is highest and little affected by seasonal OH; rural air tracks transport and OH.

Why are D4 and D6 worse than D5?

Greater uncertainty in their emission estimates.

What D5 emission basis is used?

Country-specific antiperspirant market share plus 10% for other sources.

What OH rate constants were used?

1.01, 1.55, and 1.92 × 10−12 cm3 molecule−1 s−1 for D4, D5, D6.

What novelty do the authors claim for oxidized species?

First estimated atmospheric loadings and spatial distribution of the oxidized siloxanes.