A comprehensive field campaign was carried out in summer 2014 in Wangdu, located in the North China Plain. A month of continuous OH, HO₂ and RO₂ measurements was achieved. Observations of radicals by the laser-induced fluorescence (LIF) technique revealed daily maximum concentrations between (5–15) × 10⁶ cm⁻³, (3–14) × 10⁸ cm⁻³ and (3–15) × 10⁸ cm⁻³ for OH, HO₂ and RO₂, respectively. Measured OH reactivities (inverse OH lifetime) were 10 to 20 s⁻¹ during daytime. The chemical box model RACM 2, including the Leuven isoprene mechanism (LIM), was used to interpret the observed radical concentrations. As in previous field campaigns in China, modeled and measured OH concentrations agree for NO mixing ratios higher than 1 ppbv, but systematic discrepancies are observed in the afternoon for NO mixing ratios of less than 300 pptv (the model–measurement ratio is between 1.4 and 2 in this case). If additional OH recycling equivalent to 100 pptv NO is assumed, the model is capable of reproducing the observed OH, HO₂ and RO₂ concentrations for conditions of high volatile organic compound (VOC) and low NO_x concentrations. For HO₂, good agreement is found between modeled and observed concentrations during day and night. In the case of RO₂, the agreement between model calculations and measurements is good in the late afternoon when NO concentrations are below 0.3 ppbv. A significant model underprediction of RO₂ by a factor of 3 to 5 is found in the morning at NO concentrations higher than 1 ppbv, which can be explained by a missing RO₂ source of 2 ppbv h⁻¹. As a consequence, the model underpredicts the photochemical net ozone production by 20 ppbv per day, which is a significant portion of the daily integrated ozone production (110 ppbv) derived from the measured HO₂ and RO₂. The additional RO₂ production from the photolysis of ClNO₂ and missing reactivity can explain about 10 % and 20 % of the discrepancy, respectively. The underprediction of the photochemical ozone production at high NO_x found in this study is consistent with the results from other field campaigns in urban environments, which underlines the need for better understanding of the peroxy radical chemistry for high NO_x conditions.

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.

Atmospheric oxidation capacity is the basis for converting freshly emitted substances into secondary products and is dominated by reactions involving hydroxyl radicals (OH) during daytime. In this study, we present in situ measurements of RO_x radical (hydroxy OH, hydroperoxy HO₂, and organic peroxy RO₂) precursors and products; the measurements are carried out in four Chinese megacities (Beijing, Shanghai, Guangzhou, and Chongqing) during photochemically polluted seasons. The atmospheric oxidation capacity is evaluated using an observation-based model and radical chemistry precursor measurements as input. The radical budget analysis illustrates the importance of HONO and HCHO photolysis, which account for ∼50 % of the total primary radical sources. The radical propagation is efficient due to abundant NO in urban environments. Hence, the production rate of secondary pollutants, that is, ozone (and fine-particle precursors (H₂SO₄, HNO₃, and extremely low volatility organic compounds, ELVOCs) is rapid, resulting in secondary air pollution. The ozone budget demonstrates its high production in urban areas; also, its rapid transport to downwind areas results in rapid increase in local ozone concentrations. The O₃–NO_x–VOC (volatile organic compound) sensitivity tests show that ozone production is VOC-limited and that alkenes and aromatics should be mitigated first for ozone pollution control in the four studied megacities. In contrast, NO_x emission control (that is, a decrease in NO_x) leads to more severe ozone pollution. With respect to fine-particle pollution, the role of the HNO₃–NO₃ partitioning system is investigated using a thermal dynamic model (ISORROPIA 2). Under high relative humidity (RH) and ammonia-rich conditions, nitric acid converts into nitrates. This study highlights the efficient radical chemistry that maintains the atmospheric oxidation capacity in Chinese megacities and results in secondary pollution characterized by ozone and fine particles.

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 (NO_x=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 (RO_x). The maximum loss rates for OH, HO₂, and RO₂ reached values between 10 and 15 ppbv h⁻¹ during the daytime. The largest fraction of this can be attributed to radical interconversion reactions while the real loss rate of RO_x remained below 3 ppbv h⁻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 ppbv h⁻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 RO_x 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 ppbv h⁻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 RO_x 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.