Chemical budgets of OH, HO2+RO2 and H2SO4 in a sub-urban temperate forest near Paris

Understanding the OH and peroxy radical chemistry in different environments is essential to predict atmospheric lifetimes and chemical transformations of compounds emitted to the atmosphere of both biogenic and anthropogenic origin.  The extent to which insight into radical chemistry can be gained by comparing simulated and measured radical concentrations has been found to depend on the environment. In particular, significant discrepancies between modeled and measured OH and peroxy radical concentrations have been observed in forested regions characterized by relatively high VOCs and low NO concentrations. The objective of this study was to assess the importance of different radical production and loss processes as well as the role of OH in the production of sulfuric acid, a major precursor of newly formed atmospheric particles, in a sub-urban temperate forest.
Measurements were performed as part of the ACROSS project (Atmospheric ChemistRy Of the Suburban foreSt) during June-July of 2022 at a forested site in Rambouillet located along the path of pollution plumes from Paris. OH radicals were measured in a forest clearing at ground-level (about 6 m). Co-located measurements of HO2+RO2 (i.e. the sum of hydroperoxy and organic peroxy radicals) and gas-phase H2SO4 were also made on top of a 40 m tower (~20 m above the forest canopy).  A budget analysis was performed using steady-state calculations for OH and H2SO4 using other available measurements on the ground and on the tower (photolysis rates, NOx, O3, VOCs, OH reactivity, aerosol particle size distribution, etc.). A detailed budget analysis for OH and peroxy radicals was performed with a box-model using a MCM derived mechanism. 
Calculated daytime and nighttime OH concentrations on the ground, using measured OH reactivity, showed good correlation with the measurements and reproduced the observed daytime maximum and nighttime levels of about 4×106 molecule cm-3 and (2-6)×105 molecule cm-3, respectively. The production of OH radicals in the clearing and above the canopy during the day was found to be dominated by its regeneration in reactions of HO2 and RO2 with nitric oxide. During the night, the ozonolysis of monoterpenes was a significant OH production pathway with its contribution depending on the nighttime NO concentrations. The box-model resulted in significant underestimation, up to a factor of two in daytime OH and an overestimation of OH reactivity. At the same time, the model sum of peroxy radicals was larger than measurements, especially during the night with lowest observed NO concentration. However, most of the time the model reproduced the observed peroxy radical temporal behavior on the ground and above the canopy, as well as their slightly lower concentrations over the canopy during the day.
The formation of H2SO4 was observed every day during the measurement period, with the median maximum H2SO4 concentration of 2.5×106 molecule cm-3 similar to that observed at some other forested sites. The calculated and measured daytime H2SO4 concentrations were highly correlated with formation of sulfuric acid via SO2+OH reaction accounting for (90±20)% of the observed H2SO4.