OH and HO2 radical chemistry in a midlatitude forest: measurements and model comparisons
Authors: Lew Michelle M. , Rickly Pamela S. , Bottorff Brandon P. , Reidy Emily , Sklaveniti Sofia , Léonardis Thierry , Locoge Nadine , Dusanter Sebastien , Kundu Shuvashish , Wood Ezra , Stevens Philip S. 
Published: August 2020

Abstract. Reactions of the hydroxyl (OH) and peroxy (HO2 and RO2) radicals play
a central role in the chemistry of the atmosphere. In addition to controlling the lifetimes of
many trace gases important to issues of global climate change, OH radical reactions
initiate the oxidation of volatile organic compounds (VOCs) which can lead to the production of
ozone and secondary organic aerosols in the atmosphere. Previous measurements of these radicals
in forest environments characterized by high mixing ratios of isoprene and low mixing ratios of
nitrogen oxides (NOx) (typically less than 1–2 ppb) have shown serious
discrepancies with modeled concentrations. These results bring into question our understanding of
the atmospheric chemistry of isoprene and other biogenic VOCs under low NOx
conditions. During the summer of 2015, OH and HO2 radical concentrations, as well as total
OH reactivity, were measured using laser-induced fluorescence–fluorescence assay by gas
expansion (LIF-FAGE) techniques as part of the Indiana Radical Reactivity and Ozone productioN InterComparison (IRRONIC). This campaign took place in a forested area near Indiana University's Bloomington campus which is characterized by high mixing ratios of isoprene (average daily maximum of
approximately 4 ppb at 28 ∘C) and low mixing ratios of NO (diurnal average
of approximately 170 ppt). Supporting measurements of photolysis rates, VOCs,
NOx, and other species were used to constrain a zero-dimensional box model based
on the Regional Atmospheric Chemistry Mechanism (RACM2) and the Master Chemical Mechanism (MCM 3.2),
including versions of the Leuven isoprene mechanism (LIM1) for HOx regeneration
(RACM2-LIM1 and MCM 3.3.1). Using an OH chemical scavenger technique, the study revealed the
presence of an interference with the LIF-FAGE measurements of OH that increased with both
ambient concentrations of ozone and temperature with an average daytime maximum equivalent
OH concentration of approximately 5×106 cm−3. Subtraction of the
interference resulted in measured OH concentrations of approximately
4×106 cm−3 (average daytime maximum) that were in better agreement with model
predictions although the models underestimated the measurements in the evening. The addition of
versions of the LIM1 mechanism increased the base RACM2 and MCM 3.2 modeled OH concentrations
by approximately 20 % and 13 %, respectively, with the RACM2-LIM1 mechanism providing the
best agreement with the measured concentrations, predicting maximum daily OH concentrations
to within 30 % of the measured concentrations. Measurements of HO2 concentrations
during the campaign (approximately a 1×109 cm−3 average daytime maximum)
included a fraction of isoprene-based peroxy radicals
(HO2*=HO2+αRO2) and were found to agree with model
predictions to within 10 %–30 %. On average, the measured reactivity was consistent with that
calculated from measured OH sinks to within 20 %, with modeled oxidation products
accounting for the missing reactivity, however significant missing reactivity (approximately
40 % of the total measured reactivity) was observed on some days.