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Photochemical processes in ambient air were studied using the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich, Germany. Ambient air was continuously drawn into the chamber through a 50 m high inlet line and passed through the chamber for 1 month in each season throughout 2019. The residence time of the air inside the chamber was about 1 h. As the research center is surrounded by a mixed deciduous forest and is located close to the city Jülich, the sampled air was influenced by both anthropogenic and biogenic emissions. Measurements of hydroxyl (OH), hydroperoxyl (HO 2), and organic peroxy (RO 2) radicals were achieved by a laser-induced fluorescence instrument. The radical measurements together with measurements of OH reactivity (kOH , the inverse of the OH lifetime) and a comprehensive set of trace gas concentrations and aerosol properties allowed for the investigation of the seasonal and diurnal variation of radical production and destruction pathways. In spring and summer periods, median OH concentrations reached 6 × 10 6 cm -3 at noon, and median concentrations of both HO 2 and RO 2 radicals were 3 × 10 8 cm -3. The measured OH reactivity was between 4 and 18 s -1 in both seasons. The total reaction rate of peroxy radicals with NO was found to be consistent with production rates of odd oxygen (O x= NO 2+ O 3) determined from NO 2 and O 3 concentration measurements. The chemical budgets of radicals were analyzed for the spring and summer seasons, when peroxy radical concentrations were above the detection limit. For most conditions, the concentrations of radicals were mainly sustained by the regeneration of OH via reactions of HO 2 and RO 2 radicals with nitric oxide (NO). The median diurnal profiles of the total radical production and destruction rates showed maxima between 3 and 6 ppbv h -1 for OH, HO 2 , and RO 2. Total RO X (OH, HO 2 , and RO 2) initiation and termination rates were below 3 ppbv h -1. The highest OH radical turnover rate of 13 ppbv h -1 was observed during a high-temperature (max. 40 ∘ C) period in August. In this period, the highest HO 2 , RO 2 , and RO X turnover rates were around 11, 10, and 4 ppbv h -1 , respectively. When NO mixing ratios were between 1 and 3 ppbv, OH and HO 2 production and destruction rates were balanced, but unexplained RO 2 and RO X production reactions with median rates of 2 and 0.4 ppbv h -1 , respectively, were required to balance their destruction. For NO mixing ratios above 3 ppbv, the peroxy radical reaction rates with NO were highly uncertain due to the low peroxy radical concentrations close to the limit of NO interferences in the HO 2 and RO 2 measurements. For NO mixing ratios below 1 ppbv, a missing source for OH and a missing sink for HO 2 were found with maximum rates of 3.0 and 2.0 ppbv h -1 , respectively. The missing OH source likely consisted of a combination of a missing inter-radical HO 2 to OH conversion reaction (up to 2 ppbv h -1) and a missing primary radical source (0.5–1.4 ppbv h -1). The dataset collected in this campaign allowed analyzing the potential impact of OH regeneration from RO 2 isomerization reactions from isoprene, HO 2 uptake on aerosol, and RO 2 production from chlorine chemistry on radical production and destruction rates. These processes were negligible for the chemical conditions encountered in this study. [ABSTRACT FROM AUTHOR]

Ambient measurements of nitryl chloride (ClNO 2) were performed at a rural site in Germany, covering three periods in winter, summer, and autumn 2019, as part of the JULIAC campaign (Jülich Atmospheric Chemistry Project) that aimed to understand the photochemical processes in air masses typical of midwestern Europe. Measurements were conducted at 50 m aboveground, which was mainly located in the nocturnal boundary layer and thus uncoupled from local surface emissions. ClNO 2 is produced at night by the heterogeneous reaction of dinitrogen pentoxide (N 2 O 5) on chloride (Cl -) that contains aerosol. Its photolysis during the day is of general interest, as it produces chlorine (Cl) atoms that react with different atmospheric trace gases to form radicals. The highest-observed ClNO 2 mixing ratio was 1.6 ppbv (parts per billion by volume; 15 min average) during the night of 20 September. Air masses reaching the measurement site either originated from long-range transport from the southwest and had an oceanic influence or circulated in the nearby region and were influenced by anthropogenic activities. Nocturnal maximum ClNO 2 mixing ratios were around 0.2 ppbv if originating from long-range transport in nearly all seasons, while the values were higher, ranging from 0.4 to 0.6 ppbv for regionally influenced air. The chemical composition of long-range transported air was similar in all investigated seasons, while the regional air exhibited larger differences between the seasons. The N 2 O 5 necessary for ClNO 2 formation comes from the reaction of nitrate radicals (NO 3) with nitrogen dioxide (NO 2), where NO 3 itself is formed by a reaction of NO 2 with ozone (O 3). Measured concentrations of ClNO 2 , NO 2 , and O 3 were used to quantify ClNO 2 production efficiencies, i.e., the yield of ClNO 2 formation per NO 3 radical formed, and a box model was used to examine the idealized dependence of ClNO 2 on the observed nocturnal O 3 and NO 2 concentrations. Results indicate that ClNO 2 production efficiency was most sensitive to the availability of NO 2 rather than that of O 3 and increased with decreasing temperature. The average ClNO 2 production efficiency was highest in February and September, with values of 18 %, and was lowest in December, with values of 3 %. The average ClNO 2 production efficiencies were in the range of 3 % and 6 % from August to November for air masses originating from long-range transportation. These numbers are at the high end of values reported in the literature, indicating the importance of ClNO 2 chemistry in rural environments in midwestern Europe. [ABSTRACT FROM AUTHOR]

Precise and accurate hydroxyl radical (OH) measurements are essential to investigate mechanisms for oxidation and transformation of trace gases and processes leading to the formation of secondary pollutants like ozone (O3) in the troposphere. Laser-induced fluorescence (LIF) is a widely used technique for the measurement of ambient OH radicals and was used for the majority of field campaigns and chamber experiments. Recently, most LIF instruments in use for atmospheric measurements of OH radicals introduced chemical modulation to separate the ambient OH radical concentration from possible interferences by chemically removing ambient OH radicals before they enter the detection cell (Mao et al., 2012; Novelli et al., 2014a). In this study, we describe the application and characterization of a chemical modulation reactor (CMR) applied to the Forschungszentrum Jülich LIF (FZJ-LIF) instrument in use at the atmospheric simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber). Besides dedicated experiments in synthetic air, the new technique was extensively tested during the year-round Jülich Atmospheric Chemistry Project (JULIAC) campaign, in which ambient air was continuously flowed into the SAPHIR chamber. It allowed for performing OH measurement comparisons with differential optical absorption spectroscopy (DOAS) and investigation of interferences in a large variety of chemical and meteorological conditions. Good agreement was obtained in the LIF–DOAS intercomparison within instrumental accuracies (18 % for LIF and 6.5 % for DOAS) which confirms that the new chemical modulation system of the FZJ-LIF instrument is suitable for measurement of interference-free OH concentrations under the conditions of the JULIAC campaign (rural environment). Known interferences from O3+H2O and the nitrate radical (NO3) were quantified with the CMR in synthetic air in the chamber and found to be 3.0×105 and 0.6×105 cm -3 , respectively, for typical ambient-air conditions (O3=50 ppbv, H2O = 1 % and NO3=10 pptv). The interferences measured in ambient air during the JULIAC campaign in the summer season showed a median diurnal variation with a median maximum value of 0.9×106 cm -3 during daytime and a median minimum value of 0.4×106 cm -3 at night. The highest interference of 2×106 cm -3 occurred in a heat wave from 22 to 29 August, when the air temperature and ozone increased to 40 ∘ C and 100 ppbv, respectively. All observed interferences could be fully explained by the known O3+H2O interference, which is routinely corrected in FZJ-LIF measurements when no chemical modulation is applied. No evidence for an unexplained interference was found during the JULIAC campaign. A chemical model of the CMR was developed and applied to estimate the possible perturbation of the OH transmission and scavenging efficiency by reactive atmospheric trace gases. These can remove OH by gas phase reactions in the CMR or produce OH by non-photolytic reactions, most importantly by the reaction of ambient HO2 with NO. The interfering processes become relevant at high atmospheric OH reactivities. For the conditions of the JULIAC campaign with OH reactivities below 20 s -1 , the influence on the determination of ambient OH concentrations was small (on average: 2 %). However, in environments with high OH reactivities, such as in a rain forest or megacity, the expected perturbation in the currently used chemical modulation reactor could be large (more than a factor of 2). Such perturbations need to be carefully investigated and corrected for the proper evaluation of OH concentrations when applying chemical scavenging. This implies that chemical modulation, which was developed to eliminate interferences in ambient OH measurements, itself can be subject to interferences that depend on ambient atmospheric conditions. [ABSTRACT FROM AUTHOR]

Precise and accurate hydroxyl radical (OH) measurements are essential to investigate how trace gases are oxidized and transformed in the troposphere and how secondary pollutants like ozone (O3) are formed. Laser induced fluorescence (LIF) is a widely used technique for the measurement of ambient OH radicals and was used for the majority of field campaigns and chamber experiments. Recently, most LIF instruments in use for atmospheric measurements of OH radicals introduced chemical modulation to separate the ambient OH radical concentration from possible interferences by chemically removing ambient OH radicals before they enter the detection cell. In this study, we describe the application, characterization, and validation of a chemical modulation reactor (CMR) applied to the Forschungszentrum Jülich LIF (FZJ-LIF) instrument in use at the atmospheric simulation chamber SAPHIR. Besides dedicated experiments in synthetic air, the new technique was extensively tested during the year- round Jülich Atmospheric Chemistry Project (JULIAC) campaign, in which ambient air was continuously flowed into the SAPHIR chamber. It allowed performing OH measurement comparisons with Differential Optical Absorption Spectroscopy (DOAS) and investigation of interferences in a large variety of chemical and meteorological conditions. A good agreement was obtained in the LIF DOAS intercomparison within instrumental accuracies (18 % for LIF, 6.5 % for DOAS) which confirms that the new chemical modulation system of the FZJ-LIF instrument is suitable for measurement of interference-free OH concentrations. Known interferences from O3 + H2O and the nitrate radical (NO3) were quantified with the CMR in synthetic air in the chamber and found to be 3.0 x 105 cm-3 and 0.6 x 105 cm-3, respectively, for typical ambient air condition (O3 = 50 ppbv, H2O = 1 %, NO3 = 10 pptv). The interferences measured in ambient air during the JULIAC campaign in summer season had the median diurnal variation of the interference with a maximum daytime value of 0.9 x 106 cm-3 and a minimum nighttime value of 0.4 x 106 cm-3. The highest interference of 2 x 106 cm-3 occurred in a heat wave from 22-29 August, when the air temperature and ozone increased to 40 °C and 100 ppbv, respectively. All observed interferences could be fully explained by the known O3 + H2O interference, which is routinely corrected in FZJ-LIF measurements when no chemical modulation is applied. No evidence for unexplained interference was found during the JULIAC campaign. A kinetic chemical model of the chemical modulation reactor was developed and applied to estimate the possible perturbation of the OH transmission and scavenging efficiency by reactive atmospheric trace gases. These can remove OH by gas phase reactions in the reactor, or produce OH by non-photolytical reactions, most importantly by the reaction of ambient HO2 with NO. The interfering processes become relevant at high atmospheric OH reactivities. For the conditions of the JULIAC campaign with OH reactivities below 20 s-1, the influence on the determination of ambient OH concentrations was small (on average: 2 %). However, in environments with high OH reactivities, such as in a rain forest or megacity, the expected perturbation in the currently used chemical modulation reactor could be large (more than a factor of 2) and would need careful analysis and correction. This implies that chemical modulation, which was developed to eliminate interferences in ambient OH measurements, itself can be subject to interferences that depend on ambient atmospheric conditions. [ABSTRACT FROM AUTHOR]

Hydroxyl (OH) and peroxy radicals (HO2 and RO2) were measured in the Pearl River Delta, which is one of the most polluted areas in China, in autumn 2014. The radical observations were complemented by measurements of OH reactivity (inverse OH lifetime) and a comprehensive set of trace gases including carbon monoxide (CO), nitrogen oxides (NOx=NO , NO2) and volatile organic compounds (VOCs). OH reactivity was in the range from 15 to 80 s-1 , of which about 50 % was unexplained by the measured OH reactants. In the 3 weeks of the campaign, maximum median radical concentrations were 4.5×106 cm-3 for OH at noon and 3×108 and 2.0×108 cm-3 for HO2 and RO2 , respectively, in the early afternoon. The completeness of the daytime radical measurements made it possible to carry out experimental budget analyses for all radicals (OH, HO2 , and RO2) and their sum (ROx). The maximum loss rates for OH, HO2 , and RO2 reached values between 10 and 15 ppbvh-1 during the daytime. The largest fraction of this can be attributed to radical interconversion reactions while the real loss rate of ROx remained below 3 ppbvh-1. Within experimental uncertainties, the destruction rates of HO2 and the sum of OH, HO2 , and RO2 are balanced by their respective production rates. In case of RO2 , the budget could be closed by attributing the missing OH reactivity to unmeasured VOCs. Thus, the presumption of the existence of unmeasured VOCs is supported by RO2 measurements. Although the closure of the RO2 budget is greatly improved by the additional unmeasured VOCs, a significant imbalance in the afternoon remains, indicating a missing RO2 sink. In case of OH, the destruction in the morning is compensated by the quantified OH sources from photolysis (HONO and O3), ozonolysis of alkenes, and OH recycling (HO2+NO). In the afternoon, however, the OH budget indicates a missing OH source of 4 to 6 ppbvh-1. The diurnal variation of the missing OH source shows a similar pattern to that of the missing RO2 sink so that both largely compensate each other in the ROx budget. These observations suggest the existence of a chemical mechanism that converts RO2 to OH without the involvement of NO, increasing the RO2 loss rate during the daytime from 5.3 to 7.4 ppbvh-1 on average. The photochemical net ozone production rate calculated from the reaction of HO2 and RO2 with NO yields a daily integrated amount of 102 ppbv ozone, with daily integrated ROx primary sources being 22 ppbv in this campaign. The produced ozone can be attributed to the oxidation of measured (18 %) and unmeasured (60 %) hydrocarbons, formaldehyde (14 %), and CO (8 %). An even larger integrated net ozone production of 140 ppbv would be calculated from the oxidation rate of VOCs with OH if HO2 and all RO2 radicals react with NO. However, the unknown RO2 loss (evident in the RO2 budget) causes 30 ppbv less ozone production than would be expected from the VOC oxidation rate. [ABSTRACT FROM AUTHOR]

The degradation of trace gases and pollutants in the troposphere is dominated by their reaction with hydroxyl radicals (OH). The importance of OH rests on its high reactivity, its ubiquitous photochemical production in the sunlit atmosphere, and most importantly on its regeneration in the oxidation chain of the trace gases. In the current understanding, the recycling of OH proceeds through HO2 reacting with NO, thereby forming ozone. A recent field campaign in the Pearl River Delta, China, quantified tropospheric OH and HO2 concentrations and turnover rates by direct measurements. We report that concentrations of OH were three to five times greater than expected, and we propose the existence of a pathway for the regeneration of OH independent of NO, which amplifies the degradation of pollutants without producing ozone. [ABSTRACT FROM AUTHOR]