Observations and modeling of OH and HO2 radicals in Chengdu, China in summer 2019

This study reports on the first continuous measurements of ambient OH and HO₂ radicals at a suburban site in Chengdu, Southwest China, which were collected during 2019 as part of a comprehensive field campaign 'CompreHensive field experiment to explOre the photochemical Ozone formation mechaniSm in summEr - 2019 (CHOOSE-2019)'. The mean concentrations (11:00-15:00) of the observed OH and HO₂ radicals were 9.5 × 10⁶ and 9.0 × 10⁸ cm^-3, respectively. To investigate the state-of-the-art chemical mechanism of radical, closure experiments were conducted with a box model, in which the RACM2 mechanism updated with the latest isoprene chemistry (RACM2-LIM1) was used. In the base run, OH radicals were underestimated by the model for the low-NO regime, which was likely due to the missing OH recycling. However, good agreement between the observed and modeled OH concentrations was achieved when an additional species X (equivalent to 0.25 ppb of NO mixing ratio) from one new OH regeneration cycle (RO₂ + X → HO₂, HO₂ + X → OH) was added into the model. Additionally, in the base run, the model could reproduce the observed HO₂ concentrations. Discrepancies in the observed and modeled HO₂ concentrations were found in the sensitivity runs with HO₂ heterogeneous uptake, indicating that the impact of the uptake may be less significant in Chengdu because of the relatively low aerosol concentrations. The ROx (= OH + HO₂ + RO₂) primary source was dominated by photolysis reactions, in which HONO, O₃, and HCHO photolysis accounted for 34%, 19%, and 23% during the daytime, respectively. The efficiency of radical cycling was quantified by the radical chain length, which was determined by the NO to NO₂ ratio successfully. The parameterization of the radical chain length may be very useful for the further determinations of radical recycling.

Understanding the Spatial Heterogeneity of Indoor OH and HO2 due to Photolysis of HONO Using Computational Fluid Dynamics Simulation

Indoor photolysis of nitrous acid (HONO) generates hydroxyl radicals (OH), and since OH is fast reacting, it may be confined within the HONO-photolyzing indoor volume of light. This study investigated the HONO-photolysis-induced formation of indoor OH, the transformation of OH to hydroperoxy radicals (HO₂), and resulting spatial distributions of those radicals and their oxidation products. To do so, a computational fluid dynamics (CFD) model framework was established to simulate HONO photolysis in a room and subsequent reactions associated with OH and HO₂ under a typical range of indoor lighting and ventilation conditions. The results showed that OH and HO₂ were essentially confined in the volume of HONO-photolyzing light, but oxidation products were relatively well distributed throughout the room. As the light volume increased, more total in-room OH was produced, thereby increasing oxidation product concentrations. Spatial distributions of OH and HO₂ varied by the type of artificial light (e.g., fluorescent versus incandescent), due to differences in photon flux as a function of light source and the distance from the source. The HO₂ generation rate and air change rate made notable impacts on product concentrations.

The impact of biogenic VOC emissions on photochemical ozone formation during a high ozone pollution episode in the Iberian Peninsula in the 2003 summer season

Abstract. Throughout Europe the summer of 2003 was exceptionally warm, especially July and August. The European Environment Agency (EEA) reported several ozone episodes, mainly in the first half of August. These episodes were exceptionally long-lasting, spatially extensive, and associated to high temperatures. In this paper, the 10$ndash;15 August 2003 ozone pollution event has been analyzed using meteorological and regional air quality modelling. During this period the threshold values of the European Directive 2002/3/EC were exceeded in various areas of the Iberian Peninsula. The aim of this paper is to computationally understand and quantify the influence of biogenic volatile organic compound (BVOC) emissions in the formation of tropospheric ozone during this high ozone episode. Being able to differentiate how much ozone comes from biogenic emissions alone and how much comes from the interaction between anthropogenic and biogenic emissions would be helpful to develop a feasible and effective ozone control strategy. The impact on ozone formation was also studied in combination with various anthropogenic emission reduction strategies, i.e., when anthropogenic VOC emissions and/or NOx emissions are reduced. The results show a great dependency of the BVOC contribution to ozone formation on the antropoghenic reduction scenario. In rural areas, the impact due to a NOx and/or VOC reduction does not change the BVOC impact. Nevertheless, within big cities or industrial zones, a NOx reduction results in a decrease of the biogenic impact in ozone levels that can reach 85 μg/m3, whereas an Anthropogenic Volatile Organic Compound (AVOC) reduction results in a decrease of the BVOC contribution on ozone formation that varies from 0 to 30 μg/m3 with respect to the contribution at the same points in the 2003 base scenario. On the other hand, downwind of the big cities, a decrease in NOx produces a minor contribution of biogenic emissions and a decrease in AVOCs results in greater contributions of BVOCs to the formation of ozone.

A study of the trace gas columns of O3, NO2 and HCHO over Africa in September 1997

Retrievals of trace gas columns from the measurements of backscattered radiation by GOME (Global Ozone Monitoring Experiment) show that enhanced tropospheric columns of ozone (O3), nitrogen dioxide (NO2) and formaldehyde (HCHO), over the African continent occur frequently. This study focuses on the behaviour of trace gases over Africa in September 1997, a period impacted by the strongest known El Niño phase of the ENSO. It investigates our qualitative and quantitative understanding of the retrieved tropospheric trace gas column densities. The emissions of NOx and volatile organic compounds (VOC) from biomass burning, biogenic sources and lightning and their photochemical transformation have been investigated. By performing a trajectory analysis, the transport of air masses from the different emission regions was analysed and the potential atmospheric spatial distribution determined. BRemen's Atmospheric PHOtochemical model (BRAPHO) was applied to compute the chemistry along a large number of trajectories. From these results, tropospheric column amounts of O3, NO2 and HCHO were derived. Tropospheric trace gas columns retrieved from GOME measurements and those calculated are in reasonable agreement. Their general spatial extent was similar in the lower troposphere but the modeled trace gas columns in the upper troposphere were located south of the retrieved columns. We attribute this behaviour to uncertainties in the ERA-40 meteorological data in the upper troposphere. The significance of biomass burning and of biogenic emissions with respect to HCHO columns over Africa was investigated. The analysis reveals that the total amounts of HCHO generated over Africa during September 1997 as a result of biomass burning and biogenic emissions are similar. However the HCHO from biogenic sources has the highest specific columns and these are located close to their source. In comparison the HCHO from biomass burning is predicted to be produced and transported over a much wider area. Overall all the emission processes mix together to produce the plume of O3.