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Total OH reactivity was measured during the California Research at the Nexus of Air Quality and Climate Change field campaign at the Pasadena ground site using a turbulent flow tube reactor with laser‐induced fluorescence detection of the OH radical. Collocated measurements of volatile organic compounds (VOCs), inorganic species, and meteorological parameters were made and used to calculate the total OH reactivity, which was then compared to the measured values. An analysis of the OH reactivity measurements finds that although the measured reactivity correlated well with the calculated reactivity, the measurements were consistently greater than the calculations for all times during the day, with an average missing OH reactivity of 8–10 s⁻¹, accounting for approximately 40% of the measured total OH reactivity. An analysis of correlations with both anthropogenic tracers of combustion and oxygenated VOCs as well as air trajectories during the campaign suggest that the missing OH reactivity was likely due to a combination of both unmeasured local emissions and unmeasured oxidation products transported to the site. Approximately 50% of the missing OH reactivity may have been due to emissions of unmeasured volatile chemical products, such as pesticides, cleaning agents, and personal care products.

 

Abstract. Reactions of the hydroxyl (OH) and peroxy (HO2 and RO2) radicals playa central role in the chemistry of the atmosphere. In addition to controlling the lifetimes ofmany trace gases important to issues of global climate change, OH radical reactionsinitiate the oxidation of volatile organic compounds (VOCs) which can lead to the production ofozone and secondary organic aerosols in the atmosphere. Previous measurements of these radicalsin forest environments characterized by high mixing ratios of isoprene and low mixing ratios ofnitrogen oxides (NOx) (typically less than 1–2 ppb) have shown seriousdiscrepancies with modeled concentrations. These results bring into question our understanding ofthe atmospheric chemistry of isoprene and other biogenic VOCs under low NOxconditions. During the summer of 2015, OH and HO2 radical concentrations, as well as totalOH reactivity, were measured using laser-induced fluorescence–fluorescence assay by gasexpansion (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 ofapproximately 4 ppb at 28 ∘C) and low mixing ratios of NO (diurnal averageof approximately 170 ppt). Supporting measurements of photolysis rates, VOCs,NOx, and other species were used to constrain a zero-dimensional box model basedon 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 thepresence of an interference with the LIF-FAGE measurements of OH that increased with bothambient concentrations of ozone and temperature with an average daytime maximum equivalentOH concentration of approximately 5×106 cm−3. Subtraction of theinterference resulted in measured OH concentrations of approximately4×106 cm−3 (average daytime maximum) that were in better agreement with modelpredictions although the models underestimated the measurements in the evening. The addition ofversions of the LIM1 mechanism increased the base RACM2 and MCM 3.2 modeled OH concentrationsby approximately 20 % and 13 %, respectively, with the RACM2-LIM1 mechanism providing thebest agreement with the measured concentrations, predicting maximum daily OH concentrationsto within 30 % of the measured concentrations. Measurements of HO2 concentrationsduring 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 modelpredictions to within 10 %–30 %. On average, the measured reactivity was consistent with thatcalculated from measured OH sinks to within 20 %, with modeled oxidation productsaccounting for the missing reactivity, however significant missing reactivity (approximately40 % of the total measured reactivity) was observed on some days.