Journal article

Effect of humidity on the composition of isoprene photooxidation secondary organic aerosol

Nguyen T, Roach P, Laskin J, Laskin A, Nizkorodov S ...see all

Atmospheric Chemistry and Physics, vol. 11, issue 14 (2011) pp. 6931-6944

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The effect of relative humidity (RH) on the composition and concentrations
of gas-phase products and secondary organic aerosol (SOA) generated
from the photooxidation of isoprene under high-NOx conditions was
investigated. Experiments were performed with hydrogen peroxide as
the OH precursor and in the absence of seed aerosol. The relative
yields of most gas-phase products were the same regardless of initial
water vapor concentration with exception of hydroxyacetone and glycolaldehyde,
which were considerably affected by RH. A significant change was
observed in the SOA composition, with many unique condensed-phase
products formed under humid (90 % RH) vs. dry (without any detectable effect on the rate and extent of the SOA mass
growth. There is a 40 % reduction in the number and relative abundance
of distinct particle-phase nitrogen-containing organic compounds
(NOC) detected by high resolution mass spectrometry. The suppression
of condensation reactions, which produce water as a product, is the
most important chemical effect of the increased RH. For example,
the total signal from oligomeric esters of 2-methylglyceric acid
was reduced by about 60 % under humid conditions and the maximum
oligomer chain lengths were reduced by 7–11 carbons. Oligomers formed
by addition mechanisms, without direct involvement of water, also
decreased at elevated RH but to a much smaller extent. The observed
reduction in the extent of condensation-type oligomerization at high
RH may have substantial impact on the phase characteristics and hygroscopicity
of the isoprene aerosol. The reduction in the amount of organic nitrates
in the particle phase has implications for understanding the budget
of NOC compounds.

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