The first wintertime in situ measurements of hydroxyl (OH), hydroperoxy (HO₂) and organic peroxy (RO₂) radicals (RO_x = OH + HO₂ + RO₂) in combination with observations of total reactivity of OH radicals, k_OH in Beijing are presented. The field campaign Beijing winter finE particle STudy – Oxidation, Nucleation and light Extinctions (BEST-ONE) was conducted at the suburban site Huairou near Beijing from January to March 2016. It aimed to understand oxidative capacity during wintertime and to elucidate the secondary pollutants' formation mechanism in the North China Plain (NCP). OH radical concentrations at noontime ranged from 2.4×10⁶ cm⁻³ in severely polluted air (k_OH ∼ 27 s⁻¹) to 3.6×10⁶ cm⁻³ in relatively clean air (k_OH ∼ 5 s⁻¹). These values are nearly 2-fold larger than OH concentrations observed in previous winter campaigns in Birmingham, Tokyo, and New York City. During this campaign, the total primary production rate of RO_x radicals was dominated by the photolysis of nitrous acid accounting for 46 % of the identified primary production pathways for RO_x radicals. Other important radical sources were alkene ozonolysis (28 %) and photolysis of oxygenated organic compounds (24 %). A box model was used to simulate the OH, HO₂ and RO₂ concentrations based on the observations of their long-lived precursors. The model was capable of reproducing the observed diurnal variation of the OH and peroxy radicals during clean days with a factor of 1.5. However, it largely underestimated HO₂ and RO₂ concentrations by factors up to 5 during pollution episodes. The HO₂ and RO₂ observed-to-modeled ratios increased with increasing NO concentrations, indicating a deficit in our understanding of the gas-phase chemistry in the high NO_x regime. The OH concentrations observed in the presence of large OH reactivities indicate that atmospheric trace gas oxidation by photochemical processes can be highly effective even during wintertime, thereby facilitating the vigorous formation of secondary pollutants.

The reaction of Criegee intermediates (CI) with NO and RO₂ radicals is studied for the first time by theoretical methodologies; additionally, the reaction of CI with SO₂ molecules is re-examined. The reaction of CI with NO was found to be slow, with a distinct energy barrier. Their reaction with RO₂ radicals proceeds by the formation of a pre-reactive complex followed by addition of the RO₂ radical on the CI carbon over a submerged barrier, leading to a larger peroxy radical and opening the possibility for oligomer formation in agreement with experiment. The impact of singlet biradicals on the reaction of CI with SO₂ is examined, finding a different reaction mechanism compared to earlier work. For larger CI, the reaction with SO₂ at atmospheric pressures mainly yields thermalized sulfur-bearing secondary ozonides. The fate of the CI in the atmosphere is examined in detail, based on observed concentration of a multitude of coreactants in the atmosphere, and estimated rate coefficients available from literature data. The impact of SCI on tropospheric chemistry is discussed.

Stabilized Criegee intermediates (SCI) are reactive oxygenated species formed in the ozonolysis of hydrocarbons. Their chemistry could influence the oxidative capacity of the atmosphere by affecting the HO_x and NO_x cycles, or by the formation of low-volatility oxygenates enhancing atmospheric aerosols known to have an important impact on climate. The concentration of SCI in the atmosphere has hitherto not been determined reliably, and very little is known about their speciation. Here we show that the concentration of biogenic SCI is strongly limited by their unimolecular decay, based on extensive theory-based structure–activity relationships (SARs) for the reaction rates for decomposition. Reaction with water vapor, H₂O and (H₂O)₂ molecules, is the second most important loss process; SARs are also proposed for these reactions. For SCI derived from the most common biogenic VOCs, we find that unimolecular decay is responsible for just over half of the loss, with reaction with water vapor the main remaining loss process. Reactions with SO₂, NO₂, or acids have negligible impact on the atmospheric SCI concentration. The ambient SCI concentrations are further characterized by analysis of field data with speciated hydrocarbon information, and by implementation of the chemistry in a global chemistry model. The results show a highly complex SCI speciation, with an atmospheric peak SCI concentrations below 1 × 10⁵ molecule cm⁻³, and annual average SCI concentrations less than 7 × 10³ molecule cm⁻³. We find that SCI have only a negligible impact on the global gas phase H₂SO₄ formation or removal of oxygenates, though some contribution around the equatorial belt, and in select regions, cannot be excluded.

Anthropogenic emissions such as NO_x and SO₂ influence the biogenic secondary organic aerosol (SOA) formation, but detailed mechanisms and effects are still elusive. We studied the effects of NO_x and SO₂ on the SOA formation from the photooxidation of α-pinene and limonene at ambient relevant NO_x and SO₂ concentrations (NO_x: < 1to 20 ppb, SO₂: < 0.05 to 15 ppb). In these experiments, monoterpene oxidation was dominated by OH oxidation. We found that SO₂ induced nucleation and enhanced SOA mass formation. NO_x strongly suppressed not only new particle formation but also SOA mass yield. However, in the presence of SO₂ which induced a high number concentration of particles after oxidation to H₂SO₄, the suppression of the mass yield of SOA by NO_x was completely or partly compensated for. This indicates that the suppression of SOA yield by NO_x was largely due to the suppressed new particle formation, leading to a lack of particle surface for the organics to condense on and thus a significant influence of vapor wall loss on SOA mass yield. By compensating for the suppressing effect on nucleation of NO_x, SO₂ also compensated for the suppressing effect on SOA yield. Aerosol mass spectrometer data show that increasing NO_x enhanced nitrate formation. The majority of the nitrate was organic nitrate (57–77 %), even in low-NO_x conditions (< ∼ 1 ppb). Organic nitrate contributed 7–26 % of total organics assuming a molecular weight of 200 g mol⁻¹. SOA from α-pinene photooxidation at high NO_x had a generally lower hydrogen to carbon ratio (H ∕ C), compared to low NO_x. The NO_x dependence of the chemical composition can be attributed to the NO_x dependence of the branching ratio of the RO₂ loss reactions, leading to a lower fraction of organic hydroperoxides and higher fractions of organic nitrates at high NO_x. While NO_x suppressed new particle formation and SOA mass formation, SO₂ can compensate for such effects, and the combining effect of SO₂ and NO_x may have an important influence on SOA formation affected by interactions of biogenic volatile organic compounds (VOCs) with anthropogenic emissions.

Measurements of OH and HO2 radicals were conducted in a pine-dominated forest in southern Finland during the HUMPPA-COPEC-2010 (Hyytiälä United Measurements of Photochemistry and Particles in Air – Comprehensive Organic Precursor Emission and Concentration study) field campaign in summer 2010. Simultaneous side-by-side measurements of hydroxyl radicals were conducted with two instruments using chemical ionization mass spectrometry (CIMS) and laser-induced fluorescence (LIF), indicating small systematic disagreement, OHLIF / OHCIMS = (1.31 ± 0.14). Subsequently, the LIF instrument was moved to the top of a 20 m tower, just above the canopy, to investigate the radical chemistry at the ecosystem–atmosphere interface. Comprehensive measurements including observations of many volatile organic compounds (VOCs) and the total OH reactivity were conducted and analysed using steady-state calculations as well as an observationally constrained box model. Production rates of OH calculated from measured OH precursors are consistent with those derived from the steady-state assumption and measured total OH loss under conditions of moderate OH reactivity. The primary photolytic sources of OH contribute up to one-third to the total OH production. OH recycling, which occurs mainly by HO2 reacting with NO and O3, dominates the total hydroxyl radical production in this boreal forest. Box model simulations agree with measurements for hydroxyl radicals (OHmod. / OHobs. = 1.00 ± 0.16), while HO2 mixing ratios are significantly under-predicted (HO2mod. / HO2obs. = 0.3 ± 0.2), and simulated OH reactivity does not match the observed OH reactivity. The simultaneous under-prediction of HO2 and OH reactivity in periods in which OH concentrations were simulated realistically suggests that the missing OH reactivity is an unaccounted-for source of HO2. Detailed analysis of the HOx production, loss, and recycling pathways suggests that in periods of high total OH reactivity there are additional recycling processes forming OH directly, not via reaction of HO2 with NO or O3, or unaccounted-for primary HOx sources. Under conditions of moderate observed OH reactivity and high actinic flux, an additional RO2 source of approximately 1 × 106 molec cm−3 s−1 would be required to close the radical budget. Nevertheless, a major fraction of the OH recycling occurs via the reaction of HO2 with NO and O3 in this terpene-dominated environment.

The first direct observation of OH radicals from syn -CH ₃ CHOO Criegee intermediates decomposition suggests it affects tropospheric chemistry and ambient OH measurements.