posted on 2020-03-09, 11:38authored byTanner Braman, Lydia Dolvin, Corey Thrasher, Hongmin Yu, Emma Q. Walhout, Rachel E. O’Brien
In
the atmosphere, brown carbon (BrC) molecules can contribute
to photochemistry and to the warming and evaporation of cloud droplets.
The lifetimes of BrC molecules in these cloud droplets are not well
constrained, especially considering the droplets can contain mixtures
of organic and inorganic molecules. Here we investigated the kinetics
of photodegradation for a representative brown carbon molecule, 4-nitrophenol
(4NP), in aqueous solutions at low pH. Three different solutions were
compared: one containing 4NP alone and the other two additionally
containing either fresh or photo-recalcitrant (pre-photobleached)
α-pinene secondary organic aerosol (SOA). Fresh SOA slightly
accelerates the photodegradation compared to that with 4NP alone.
In contrast, photo-recalcitrant SOA decelerates photodegradation,
increasing the estimated atmospheric lifetime of 4NP by 4 h (from
∼11 h for BrC alone to ∼14 h when it is mixed with photo-recalcitrant
SOA). Photo-recalcitrant SOA also shows minimal chemical changes after
irradiation for 20 h in aqueous solution. These results suggest that,
in contrast to fresh SOA, aqueous photo-recalcitrant SOA does not
serve as a source of OH radicals when irradiated but may instead act
as an OH radical scavenger. The presence of photo-recalcitrant SOA
could have important effects on atmospheric lifetimes of organics
in mixed aqueous systems.