Rainforest-like Atmospheric Chemistry in a Polluted Megacity

The impact of volatile organic compound (VOC) emissions to the atmosphere on the production of secondary pollutants, such as ozone and secondary organic aerosol (SOA), is mediated by the concentration of nitric oxide (NO). Polluted 30 urban atmospheres are typically considered to be “high-NO” environments, while remote regions such as rainforests, with minimal anthropogenic influences, are considered to be “low-NO”. Policy to reduce urban air pollution is typically developed assuming that the chemistry is controlled by the high-NO regime. However, our observations from central Beijing show that this simplistic separation of regimes is flawed. Despite being in one of the largest megacities in the world, we observe significant formation of gas and aerosol phase oxidation products associated with the low-NO ‘rainforest-like’ regime during 35 the afternoon. This is caused by a surprisingly low concentration of NO, coupled with high concentrations of VOCs and of the atmospheric oxidant hydroxyl (OH). Box model calculations suggest that during the morning high-NO chemistry predominates (95%) but in the afternoon low-NO chemistry plays a greater role (30%). With increasing global emphasis on reducing air pollution, the modelling tools used to develop urban air quality policy need to adequately represent both highand low-NO regimes if they are to have utility. 40 https://doi.org/10.5194/acp-2020-35 Preprint. Discussion started: 13 February 2020 c © Author(s) 2020. CC BY 4.0 License.


Introduction
The atmosphere in polluted urban areas has a markedly different chemical composition to that in remote regions (e.g. rainforests). This can lead to changes in the chemical oxidation pathways for volatile organic compounds (VOCs), giving rise to the formation of different secondary pollutants. Oxidation by hydroxyl radicals (OH) is the dominant daytime sink for VOCs, leading to the formation of highly reactive peroxy radicals (RO2). In atmospheres with high concentrations of nitric 45 oxide (NO), emitted by combustion sources such as vehicles, cooking, and energy generation, RO2 radicals react predominantly with NO (Orlando and Tyndall, 2012). This tends to break the initial VOC down to smaller, more oxidised VOCs, and can also produce organic nitrates (RONO2). This pathway also produces NO2, the photolysis of which leads to ozone production. In contrast, in low-NO atmospheres RO2 predominantly react with other RO2, including hydroperoxyl radicals (HO2), or can isomerize/auto-oxidise to form different multi-functionalized oxygenated RO2 (Crounse et al., 2013). These low NO pathways 50 tend to maintain the original carbon skeleton. The large highly oxidised molecules formed can efficiently partition to the aerosol phase to yield secondary organic aerosol (SOA) (Bianchi et al., 2019), which often comprises a large fraction of submicron atmospheric particulate matter (PM) in many regions (Jimenez et al., 2009).
Biogenic sources dominate global emissions of VOCs to the atmosphere, with the highly reactive VOC isoprene (2-methyl-55 1,3-butadiene) contributing ~70% by mass (Sindelarova et al., 2014). The gas and aerosol phase products of isoprene oxidation have been extensively characterized in the laboratory (Wennberg et al., 2018). In this work a range of isoprene oxidation products are used as tracers of the changing atmospheric chemical environment. For isoprene, the low-NO oxidation pathway leads to low volatility products, such as isoprene hydroperoxides (ISOPOOH), that can go on to form significant quantities of SOA via formation of isoprene epoxides (IEPOX) (Figure 1) (Paulot et al., 2009;Surratt et al., 2010;Lin et al., 2012). The 60 high-NO pathway can also form SOA via the formation of methacrolein (MACR), which can react further to form SOA constituents such as 2-methylglyceric acid (2-MGA) and corresponding oligomers (Kroll et al., 2006;Surratt et al., 2006;Nguyen et al., 2015) (Figure 1). Other significant contributors to isoprene-SOA formed via the high NO pathway include nitrates (e.g. ISOPONO2) and dinitrates (Schwantes et al., 2019). ppbv in the afternoon, that on many days NO concentrations fall to < 0.5 ppbv in the afternoon, and on some days to < 0.1 ppbv (see Supplementary Information).
The observed diurnal cycles of 'low-NO' and 'high-NO' isoprene oxidation products (Figure 1) in both the gas and aerosol phases can be explained by the observed diurnal cycle of NO (Figure 2a). The high-NO product isoprene nitrate (ISOPONO2) 75 ( Figure 2c) is produced through the morning from reaction of isoprene peroxy radicals (ISOPOO) with NO. During the afternoon, an increasing fraction of ISOPOO begins to react with HO2 or RO2 as the NO concentration drops. This leads to the observed decrease in ISOPONO2, and an increase in IEPOX + ISOPOOH through the afternoon (Figure 2b). The profile of the high-NO products MACR+MVK (Figure 2d) is very similar to that of ISOPONO2 until about 15:00, when they begin to increase, with a second peak observed at around 17:00. This latter peak may be an observational artefact as a result of the 80 conversion of ISOPOOH to MACR on metal surfaces within the inlet of the PTR instrument (Rivera-Rios et al., 2014). Isoprene oxidation products can also partition into the particle phase and undergo heterogeneous reactions to form organosulfates, with concentrations driven by a number of additional factors such as particulate sulfate and water vapour concentrations. Specific organosulfate tracers from the high-NO (2-MGA-OS, Figure 2e) and low-NO (2-methyltetrol-OS formed from IEPOX, Figure   2f) pathways were observed, with low concentrations overnight, increasing during the day to a peak around 15:00-16:00. 85 The observed temporal profiles of the isoprene tracer products suggest a chemical cycle switching from a high-NO to a low-NO chemical regime during the day in Beijing. First, isoprene nitrates, formed predominantly during the morning (Figure 2c), are characteristic of high-NO chemistry. Second, isoprene hydroperoxides (ISOPOOH) and epoxydiols (IEPOX) (Figure 2b), formed predominantly during the afternoon, are characteristic of low-NO chemistry, where the reaction of ISOPOO with HO2 90 dominates over reaction with NO. The formation of highly oxygenated molecules (HOMs), characteristic of RO2 isomerisation and auto-oxidation in low NO environments, has also been observed during the afternoon at this site (Brean et al., 2019). Third, the observation of large amounts of 2-methylglyceric acid (2-MGA-OS) (Figure 2e) in the aerosol is suggestive of both high and low NO chemistry having occurred. The precursor to 2-MGA, methacrolein (MACR), is formed predominantly via reactions of ISOPOO with NO (Figure 1), i.e. during the morning. In the afternoon, the RO2 formed from oxidation of MACR 95 reacts with NO2 (in preference to NO, because of the very low NO/NO2 ratio) and further oxidation leads to 2-MGA (Surratt et al., 2010;Chan et al., 2010;Nguyen et al., 2015).

Box Modeling
The chemical box model DSMACC (Emmerson and Evans, 2009), coupled with the near-explicit oxidation mechanism for isoprene from the Master Chemical Mechanism (MCM v3.3.1) (Jenkin et al., 1997;, was used to assess the sensitivity 100 of the fraction of ISOPOO reacting with NO (fNO) to varying NO concentrations and the OH reactivity (∑ #$%&#' [VOC]).
The model was run to steady state at a range of different fixed concentrations of [OH], [NO], and [isoprene], using fixed photolysis rates typical of Beijing daytime (see Supplementary Information). Figure 3 shows that, as expected, fNO increases https://doi.org/10.5194/acp-2020-35 Preprint. Discussion started: 13 February 2020 c Author(s) 2020. CC BY 4.0 License.
with increasing NO concentration. It also shows that fNO is not a fixed value for a given concentration of NO, but decreases with the increasing reactivity of the system (the x-axis in Figure 3). The reactivity varies as a function of the VOC mixing 105 ratios, the reactive mix of VOCs, and the OH concentration, i.e. [OH] x OH reactivity* (Equation E1). This term effectively defines the rate of production of RO2. Higher reactivity and higher OH concentrations both lead to a higher concentration of peroxy radicals ([HO2] + å[RO2]), reducing fNO. Average measurements of ([OH] ´ OH reactivity*) and [NO] for the afternoon (12:00 -20:00) from a range of different environments are shown in Figure 3 (see also Table S1). The RO2 chemistry in the rural southeastern US and the Borneo rainforest lies in the low NO regime (i.e. fNO < 0.5) for the whole afternoon. In the urban 110 areas of London and New York the chemistry remains in the high NO regime through the whole afternoon. However, in Beijing, the extreme suppression of NO concentrations in the afternoon drives the chemistry from a regime in which > 95 % of the RO2 is reacting with NO during the morning, to one in which less than 70 % is reacting with NO by mid-afternoon.

Discussion and Conclusions
The major driver of the very low afternoon NO mixing ratios is the high levels of ozone. However, model runs with the regional 115 chemical transport model GEOS-Chem show that the low NO levels cannot be explained solely through suppression by ozone using current understanding of atmospheric chemistry (Supplementary Information). One explanation may be additional NO sinks that recycle OH without producing O3 which have previously been proposed for the high VOC-low NO (< 1ppbv) conditions seen in Beijing and other cities (Hofzumahaus et al., 2009;Whalley et al., 2018;Tan et al., 2019). Another explanation may be the presence of high concentrations of other species that can convert NO to NO2 e.g. halogen oxides. The 120 failure of regional and global models to accurately replicate this chemistry has wider implications for the prediction of secondary pollutants and hence for determining policies to control air pollution episodes.
Our observations from Beijing challenge the commonly accepted view of polluted urban areas as high-NO atmospheric environments in two ways. First, very high ozone regularly reduces afternoon NO to < 1ppbv, and on some days to < 0.1 ppbv. 125 This leads to the formation of 'low-NO' products in the gas and aerosol phase. Second, the level of NO that is required for 'low-NO chemistry' to occur is not a fixed value, but is dependent on the concentration and reactivity of the VOCs present and the concentration of OH. Hence NO concentrations that represent 'low-NO' conditions in a tropical rainforest, for example, are different to those that represent 'low-NO' conditions in a highly polluted urban environment with elevated VOC/OH

reactivity. 130
Under the conditions observed in Beijing, the production of low-NO SOA and the associated increase in PM is shown to be closely linked to photochemical ozone production. Policies that reduce the afternoon ozone peak might also be expected to reduce the production of these aerosol-phase products. However, such policies must also take account of the complex interactions between NOx, VOCs, ozone, and PM. For example, reducing NOx emissions can counter-intuitively lead to increases in ozone, as has occurred in other major cities (Air Quality Expert Group, 2009), while reducing PM has also been shown to lead to increases in ozone (Jacob et al., 2019). With many existing and developing megacities being located in subtropical regions with high emissions of reactive biogenic VOCs, and with the continued and increasing use of fossil fuels for transport and power generation, such extreme chemical environments as that observed in Beijing can be expected to proliferate. In these environments, biogenic emissions of isoprene can dominate the OH reactivity making reduction of VOC 140 reactivity through emissions controls very difficult. Additionally, since many Asian megacities are situated within much larger densely populated regions, attempts to control air quality are likely to be ineffective unless implemented on a regional scale.
Identification of the best policy for a particular city-region will require detailed atmospheric chemical modelling with chemical mechanisms capable of simulating the appropriate chemical environments both for the present day and for future environments.

Methods
The site was located at the Institute of Atmospheric Physics, between the 3 rd and 4 th ring road. Measurements took place between 17/05/2017 and 24/06/2017. The site is typical of central Beijing, surrounded by residential and commercial properties and is near several busy roads. It is also close to several green spaces, including a tree-lined canal to the south and the Olympic forest park to the north-east. Isoprene mixing ratios were measured by dual channel gas chromatography (DC-GC-FID). 150 IEPOX/ISOPOOH were observed using iodide chemical ionisation mass spectrometry. The sum of MACR + MVK was measured using proton transfer mass spectrometry. Particle samples were collected onto filter papers at either 3 hourly or 1 hourly time periods, depending on pollution levels. Filters were extracted and analysed with a high throughput method using ultra high-pressure liquid chromatography coupled to a Q-Exactive Orbitrap mass spectrometer. Nitric oxide, NO, was measured by chemiluminescence with a Thermo Scientific Model 42i NOx analyser. Nitrogen dioxide, NO2, was measured 155 using a Teledyne Model T500U Cavity Attenuated Phase Shift (CAPS) spectrometer. Ozone, O3, was measured using a Thermo Scientific Model 49i UV photometer.

DC-GC-FID
Observations of VOCs were made using a dual-channel GC with flame ionisation detectors. Air was sampled at 30 L min -1 at 160 a height of 5m, through a stainless-steel manifold (½" internal diameter). 500 mL subsamples were taken, dried using a glass condensation finger held at -40 o C and then pre-concentrated using a Markes Unity2 pre-concentrator on a multi-bed Ozone Precursor adsorbent trap (Markes International Ltd). These samples were then transferred to the GC oven for analysis following methods described by Hopkins et al (2011).

CIMS
A time of fight chemical ionisation mass spectrometer (ToF-CIMS) (Lee et al., 2014;Priestley et al., 2018) using an iodide ionisation system was couple deployed here. Experimental set up of the University of Manchester ToF-CIMS has been previously described in Zhou et al. (2019). During the campaign, gas phase backgrounds were established through regularly overflowing the inlet with dry N2 for 5 continuous minutes every 45 minutes and were applied consecutively. The overflowing 170 of dry N2 will have a small effect on the sensitivity of the instrument to those compounds whose detection is water dependent.
Here we find that due to the very low instrumental background for C5H10O3 and C5H9NO4, the absolute error remains small from this effect (<10 ppt in both reported measurements).
Field calibrations were regularly carried out using known concentration formic acid gas mixtures made in a custom-made gas phase manifold. A range of other species were calibrated for after the campaign, and relative calibration factors were derived 175 using the measured formic acid sensitivity during these calibrations, as has been performed previously , Bannan et al. 2015. In addition to this, offline calibrations, prior to and after the field work project, of a wide range of organic acids, HNO3 and Cl2 were performed to assess possible large scale sensitivity changes over the measurement period. No significant changes were observed. Offline calibrations after the field work campaign were performed specific to the isoprene oxidation species observed here. IEPOX (C5H10O3) synthesized by the University of North Carolina, Department of 180 Environmental Sciences & Engineering was specifically calibrated for here. Aliquots of known concentrations of IEPOX (C5H10O3) were thermally desorbed into a known continuous flow of nitrogen. For C5H9NO4 there was no direct calibration source available and concentrations using the calibration factor of C5H10O3 are presented here.

PTR-MS 185
A Proton Transfer Reaction-Time of Flight-Mass Spectrometer (PTR-ToF-MS 2000, Ionicon Analytik GmBH, Innsbruck) was deployed at the base of the 325m meteorological tower at the IAP field site. This instrument has been described in detail by Jordan et al. (2009). The PTR-ToF-MS was operated at a measurement frequency of 5 Hz and an E/N ratio (where E represents the electric field strength and N the buffer gas density) in the drift tube of 130 Td. To enable accurate calibration of the mass scale trichlorobenzene was introduced by diffusion into the inlet stream. 190 The instrument was switched between two inlet systems in an hourly cycle. For the first 20 minutes of each hour, the PTR-MS sampled from a gradient switching manifold and for the next 40 minutes, the instrument subsampled a common flux inlet line running from the 102m platform on the tower to the container in which the PTR-ToF-MS was housed. Gradient measurements were made from 3, 15, 32, 64 and 102 m with air sampled down 0.25 inch O.D. PFA lines and split between a 3 L min -1 bypass and 300 ml min -1 sample drawn to a 10 L stainless steel container. During the gradient sampling period, the PTR-ToF-MS 195 subsampled for 2 minutes from each container giving an hourly average concentration at each height. In this work, only data from the 3m gradient height is discussed.
Zero air was generated using a platinum catalyst heated to 260 °C and was sampled hourly in the gradient switching cycle.
During the field campaign, the instrument was calibrated twice weekly using a 15 component 1 ppmv VOC standard (National Physical Laboratory, Teddington). The calibration gas flow was dynamically diluted into zero air to give a six-point calibration. 200 Data was analysed using PTR-MS Viewer 3. https://doi.org/10.5194/acp-2020-35 Preprint. Discussion started: 13 February 2020 c Author(s) 2020. CC BY 4.0 License.

PM2.5 filter sampling and analysis
PM2.5 filter samples were collected using an ECOTECH HiVol 3000 (Ecotech, Australia) high volume air sampler with a 205 selective PM2.5 inlet, with a flow rate of 1.33 m 3 min -1 . Filters were baked at 500 o C for five hours before use. After collection, samples were wrapped in foil, and then stored at -20 o C and shipped to the laboratory. Samples were collected at a height of 8 m, on top of a building in the IAP complex. Hourly samples were taken on 11th June between 08:00-17:00, with one further sample taken overnight. The extraction of the organic aerosol from the filter samples was based on the method of Hamilton et al. (2008). Initially, roughly an 8 th of the filter was cut up into 1 cm 2 pieces. 4 ml of LC-MS grade H2O was then added to the 210 sample and left for two hours. The samples were then sonicated for 30 minutes. Using a 2 ml syringe, the water extract is then pushed through a 0.22μm filter (Millipore) into another sample vial. An addition 1 mL of water was added to the filter sample, then extracted through the filter, to give a combined aqueous extract. This extract was then reduced to dryness using a vacuum solvent evaporator (Biotage, Sweden). The dry sample was then reconstituted in 1 mL 50:50 MeOH:H2O solution, ready for analysis. 215 The extracted filter samples and standards were analysed using UPLC-MS 2 , using an Ultimate 3000 UPLC (Thermo Scientific, USA) coupled to a Q-exactive Orbitrap MS (Thermo Fisher Scientific, USA) with a heated electrospray ionisation (HESI).
The UPLC method uses a reverse phase 5 μm, 4.6 x 100mm, Accucore column (Thermo scientifc, UK) held at 40 o C. The mobile phase consists of LC-MS grade water and 100 % MeOH (Fisher Chemical, USA). The water was acidified using 0.1 % formic acid to improve peak resolution. The injection volume was 2 μl. The solvent gradient was held for a minute at 90:10 220 H2O:MeOH, the gradient then changed linearly to 10:90 H2O:MeOH over 9 minutes, it was then held for 2 minutes at this gradient before returning to 90:10 H2O:MeOH over 2 minutes and then held at 90:10 for the remaining 2 minutes, with a flow rate of 300 μL min -1 . The mass spectrometer was operated in negative mode using full scan MS 2 . The electrospray voltage was 4.00 kV, with capillary and auxiliary gas temperatures of 320 o C. The scan range was set between 50 -750 m/z. Organosulfates were quantified using an authentic standard of 2-MGA-OS obtained from J. Surratt using the method in Bryant et al. (2019). 225

OH measurements
The OH radical measurements were made from the roof of the University of Leeds FAGE instrument container at the IAP field site. Two Fluorescence Assay by Gas Expansion (FAGE) detection cells where housed in a weather-proof enclosure at a sampling height of approximately 4 m. OH and HO2 radicals were detected sequentially in the first cell (the HOx cell), whilst 230 HO2 * and total RO2 radical observations were made using the second FAGE cell (the ROx cell) which was coupled with a flow reactor to facilitate RO2 detection . A Nd:YAG pumped Ti:Sapphire laser was used to generate 5 kHz pulsed tunable UV light at 308 nm and used to excite OH via the Q1(1) transition of the % , 0 = ← , " = band.
On-resonance fluorescence was detected using a gated micro-channel plate photomultiplier and photon counting. A background signal from laser and solar scatter and detector noise was determined by scanning the laser wavelength away from 235 the OH transition (OHWAVE-BKD). For the entire campaign, the HOx cell was equipped with an inlet pre injector (IPI) which chemically scavenged ambient OH by periodically by injecting propane into the air stream just above the FAGE inlet. The https://doi.org/10.5194/acp-2020-35 Preprint. Discussion started: 13 February 2020 c Author(s) 2020. CC BY 4.0 License. removal of ambient OH by chemical reaction provided an alternative means to determine the background signal (OHCHEM-BKD), without the need to tune the laser wavelength. By comparison with OHWAVE-BKD, OHCHEM-BKD was used to identify if any OH was generated internally within the FAGE cell, acting as an interference signal. In general, good agreement between OHCHEM-240 BKD and OHWAVE-BKD was observed, with a ratio of 1.07 for the whole campaign (Woodward-Massey, 2018). In this paper, the OHCHEM observations are used. The instrument was calibrated every few days by over-flowing the detection cell inlet with a turbulent flow of high purity humid air containing a known concentration of OH (and HO2) radicals generated by photolysing a known concentration of H2O vapour at 185 nm. The product of the photon flux at 185 nm and the time spent in the photolysis region was measured before and after the campaign using N2O actinometry (Commane et al., 2010). 245

OH reactivity measurements
OH reactivity measurements were made using a laser flash photolysis pump-probe technique (Stone et al., 2016). Ambient air, sampled from the roof of the FAGE container, was drawn into a reaction cell at a flow rate of 15 SLM. A 1 SLM flow of high purity, humidified air which had passed by a Hg lamp, generating ~50 ppbv of ozone, was mixed with the ambient air at the 250 entrance to the reaction cell. The ozone present was photolysed by 266 nm laser light at a pulse repetition frequency of 1 Hz along the central axis of the reaction cell, leading to the generation of a uniform profile of OH radicals following the reaction of O( 1 D) with H2O vapour. The decay in the OH radical concentration by reaction with species present in the ambient air was monitored by sampling a portion of the air into a FAGE cell positioned at the end of the reaction cell. A fraction of the 5 kHz, 308 nm radiation generated by the Ti:Sapphire laser, passed through the OH reactivity FAGE cell, perpendicular to the air 255 stream, electronically exciting the OH radicals and the subsequent laser-induced fluorescence signal was detected with a gated channel photomultiplier tube. The 1 Hz OH decay profiles were integrated for 5 minutes and fitted to a first-order rate equation to determine the observed loss rate of OH (kobs). The total OH reactivity, k(OH), was calculated by subtracting the rate coefficient associated with physical losses of OH (kphys) from kobs. kphys was determined by monitoring the decay of OH when the ambient air was replaced with a flow of 15 SLM high purity air. A small correction to account for dilution of the ambient 260 air by the 1 SLM flow of ozone-containing synthetic air was also applied.

Box Modelling
The box modelling that feeds into Figure 3 was performed using the Dynamically Simple Model of Atmospheric Chemical Complexity (DSMACC), zero-dimensional box model (Emmerson and Evans, 2009), together with the isoprene scheme, 265 together with the relevant inorganic chemistry, from the near explicit chemical mechanism the Master Chemical Mechanism (MCM) v3.3.1 (Jenkin et al., 1997;Jenkin et al., 2015). The complete isoprene degradation mechanism in MCM v3.3.1 consists of 1926 reactions of 602 closed shell and free radical species, which treat the chemistry initiated by reaction with OH radicals, NO3 radicals and ozone. It contains much of the isoprene HOX recycling chemistry identified as important in recent years under "low NO" conditions, including the peroxy radical 1,4 and 1,6 H-shift chemistry described in the LIM1 mechanism (Peeters et al., 2009;2014), as summarized in Wennberg et al. (2018). Model photolysis rates were calculated using the Tropospheric Ultraviolet and Visible Radiation Model (TUV v5.2) (Madronich, 1993).
[CH4] was fixed at 1.85 ppmv and [CO] at 110 ppbv, and T = 298 K. Entrainment loss rates for all model species were set to 1 × 10 -5 cm -3 s -1 . For the box model, a column average value for deposition velocity, 275 Vd, was calculated according to the functionalities of each species (Table S2). These terms prevent the build-up of secondary products. The values are based on reported deposition rates in Nguyen et al. (2015). A boundary layer height (BLH) of 1.5 km was assumed. Loss rates (Ld) for each species to dry deposition are then Ld = Vd/BLH. Photolysis rates were fixed to mean rates for the day time period 09:00-17:00 calculated for July 1. The model was then run to steady state for a range of fixed NO mixing ratios from 0 -16,000 pptv. 280
Specific data are available from the authors on request (jacqui.hamilton@york.ac.uk).

Author Contributions
JRH, RED, JFH, WJFA, CNH, BL and XW provided the VOC measurements. FAS, WSD and JDL provided the NOx and O3 285 measurements. TJB, AM, SDW, AB, CJP and HC collected and analysed the CIMS data. TQ and JDS provided the organosulfate standards. DB, WD and JFH provided the organo-sulfate aerosol measurements. LKW, DEH, EJS, RW-M and CY provided the OH and HO2 data. MN, PME and ARR provided the MCM box modelling. PDI and MJE provided the GEOS-Chem model run. ACL is the PI of the AIRPRO-Beijing project. MJN, JFH and ARR conceived and wrote the manuscript with input and discussion with all co-authors. 290

Competing Interests
The authors declare that they have no conflict of interest.
Ren, Qiaorong Xie, Wanyu Zhao, Linjie Li, Ping Li, Shengjie Hou, Qingqing Wang from IAP, Kebin He and Xiaoting Cheng from Tsinghua University, and James Allan from the University of Manchester for providing logistic and scientific support for the field campaigns. 300 Figure 1: Formation pathways of isoprene oxidation products used as tracers of high / low-NO chemistry in this work. Following reaction of the primary VOC, isoprene, with OH, a peroxy radical intermediate (ISOPOO) is formed. At low NO concentrations, ISOPOO reacts with HO2 (or other RO2), to yield hydroperoxide (ISOPOOH) isomers ((4,3)-ISOPOOH isomer is shown), which can be rapidly oxidized 500 to isoprene epoxydiol (IEPOX) isomers. At high NO concentrations, ISOPOO reacts with NO, a minor product of which is an isoprene nitrate (ISOPONO2). One of the major products of ISOPOO reaction with NO is methacrolein (MACR), the subsequent oxidation of which, in the presence of NO2, can lead to 2-methylglyceric acid (2-MGA) and its corresponding oligomers and organosulfates in the aerosol phase. Measurements of these products in the gas or aerosol phase can be used as tracers for the chemical environment in which they were formed.

Figure 2:
Mean diurnal variation of measured organic and inorganic species in the gas phase and aerosol during the Beijing summer observations. a. Mean diurnal cycle of the inorganic species NO and ozone (O3) and of the product precursor isoprene (C5H8). Shaded areas are at night; area between the dotted lines indicates where > 25% of the isoprene chemistry is driven by low NO pathways (Fig. 3). b&c. Mean diurnal cycle of the gas phase isoprene 'low NO' oxidation products, IEPOX + ISOPOOH (b), and 'high NO' oxidation product, isoprene nitrate (ISOPONO2) (c). d. Mean diurnal cycle of the gas phase isoprene oxidation products methacrolein (MACR) (precursor to 515 2-MGA) + methyl vinyl ketone (MVK). e&f. SOA components: 2-methyltetrol-organosulfate (2-MT-OS) and 2-methylglyceric acidorganosulfate (2-MGA-OS), both measured on the 11/12th June 2017, the last filter sample was taken from 17:30 11 June -08:30 12 June.

Figure 3:
Variation of the fraction of ISOPOO reacting with NO as a function of NO concentration and the reactivity of the system. The