The atmospheric α-pinene oxidation leads to three
carboxylic
acids: norpinonic acid (NPA), pinic acid (PA), and 3-methyl-1,2,3-butanetricarboxylic
acid (MBTCA). In this study, the OH radical kinetics in the aqueous
phase of these carboxylic acids were investigated at different temperatures
and pH values of solutions. Activation parameters and the corresponding
atmospheric lifetimes of the acids in the troposphere were derived.
The overall second-order rate constants for the individual speciation
forms of the acids (AH and A– for NPA; AH2, AH– and A2– for PA; and AH3, AH2
–, AH2– and A3– for MBTCA) were determined. At 298 K,
the rate constants for reactions of protonated forms (AH
x
) of NPA, PA, and MBTCA with •OH,
were (1.5 ± 0.2) × 109 L mol–1 s–1, (2.4 ± 0.1) × 109 L
mol–1 s–1, and (4.1 ± 0.6)
× 108 L mol–1 s–1, respectively. For the fully deprotonated forms (A
x–) of studied acids, the second-order rate constants
were (2.2 ± 0.2) × 109 L mol–1 s–1, (2.8 ± 0.1) × 109 L
mol–1 s–1, and (10.2 ± 0.7)
× 108 L mol–1 s–1 at 298 K, respectively. It was found that the reactions of NPA and
PA with OH radicals are faster than with MBTCA. For MBTCA, the reaction
rate depends on pH more strongly at elevated temperatures (>298
K).
The atmospheric lifetimes of the acids considered due to their reactivity
with •OH were calculated for different model scenarios
at a temperature of 283 K and pH = 2 in the aqueous phase. For this
purpose, liquid water content (LWC) was used for aerosols and clouds
under storm conditions and at various aqueous-phase concentrations
of OH radicals. The lifetimes decreased with increasing LWC (from
10–12 m3 m–3 in aerosol
to 10–5 m3 m–3 in storms),
indicating that the acids undergo significant aqueous processing under
realistic atmospheric conditions. Besides, the aerosol systems appeared
less effective in removing PA and NPA, with lifetimes ranging from
hundreds of days to tens and hundreds of hours, respectively. Clouds
were more effective, with lifetimes ranging from tens of hours to
a single second or less. MBTCA, which dissolves better in water, was
effectively removed in all systems, with the longest lifetime of approximately
90 min.