Fast photochemistry in wintertime haze: Consequences for pollution

Environmental Processes

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Fast photochemistry in wintertime haze: Consequences for pollution mitigation strategies. Keding Lu, Hendrik Fuchs, Andreas Hofzumahaus, Zhaofeng Tan, Haichao Wang, Lin Zhang, Sebastian Schmitt, Franz Rohrer, Birger Bohn, Sebastian Broch, Huabin Dong, Georgios Gkatzelis, Thorsten Hohaus, Frank Holland, Xin Li, Ying Liu, Yuhan Liu, Xuefei Ma, Anna Novelli, Patrick Schlag, Min Shao, Yusheng Wu, Zhijun Wu, Limin Zeng, Min Hu, Astrid Kiendler-Scharr, Andreas Wahner, and Yuanhang Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b02422 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 19, 2019

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Environmental Science & Technology

and Pollution Control, College of Environmental Sciences and Engineering, Peking University Wu, Zhijun; Peking University, Zeng, Limin; Peking University, State Joint Key Laboratory of Environmental Simulation and Pollution Control Hu, Min; State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering Kiendler-Scharr, Astrid; Forschungszentrum Julich, Wahner, Andreas; Forschungszentrum Julich, Inst. Chemie und Dynamik der Geosphaere Zhang, Yuanhang; Peking University

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Fast photochemistry in wintertime haze: Consequences for pollution

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mitigation strategies.

3

Keding Lu1*, Hendrik Fuchs2, Andreas Hofzumahaus2, Zhaofeng Tan1,a, Haichao Wang1, Lin

4

Zhang3, Sebastian H. Schmitt2, Franz Rohrer2, Birger Bohn2, Sebastian Broch2, Huabin Dong1,

5

Georgios I. Gkatzelis2, Thorsten Hohaus2, Frank Holland2, Xin Li1, Ying Liu1, Yuhan Liu1, Xuefei

6

Ma1, Anna Novelli2, Patrick Schlag2,b, Min Shao1, Yusheng Wu1,c, Zhijun Wu1, Limin Zeng1, Min

7

Hu1, Astrid Kiendler-Scharr2, Andreas Wahner2, & Yuanhang Zhang1,4,5 *

8

1

9

Environmental Sciences and Engineering, Peking University, Beijing, China.

10

2

11

3

12 13

IEK-8: Troposphere, Forschungszentrum Jülich, Jülich, Germany. Laboratory for Climate and Ocean-Atmosphere Studies, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing 100871, China.

4 CAS

14 15

State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of

Center for Excellence in Regional Atmospheric Environment, Chinese Academy of Science,

Xiamen, China 5

16

Beijing Innovation Center for Engineering Sciences and Advanced Technology, Peking University, Beijing, China

17

a

18

bnow

at: Institute of Physics, University Sao Paulo, Sao Paulo, Brazil.

19

c now

at: Department of Physics, University of Helsinki, Helsinki, Finland.

now at: IEK-8: Troposphere, Forschungszentrum Jülich, Jülich, Germany.

20

1 ACS Paragon Plus Environment

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ABSTRACT

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In contrast to summer smog, the contribution of photochemistry to the formation of winter haze in

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northern mid-to-high latitude is generally assumed to be minor due to reduced solar UV and water

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vapor concentrations. Our comprehensive observations of atmospheric radicals and relevant

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parameters during several haze events in winter 2016 Beijing, however, reveal surprisingly high

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hydroxyl radical (OH) oxidation rates up to 15 ppbv/h which is comparable to the high values

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reported in summer photochemical smog and is 2-3 times larger than those determined in previous

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observations during winter in Birmingham1, Tokyo2, and New York3. The active photochemistry

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facilitates the production of secondary pollutants. It is mainly initiated by the photolysis of nitrous

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acid, ozonolysis of olefins and maintained by an extremely efficiently radical cycling process

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driven by nitric oxide (NO). This boosted radical recycling generates fast photochemical ozone

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production rates that are again comparable to those during summer photochemical smog. The

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formation of ozone, however, is currently masked by its efficient chemical removal by nitrogen

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oxides contributing to the high level of wintertime particles. The future emission regulations, such

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as the reduction of nitrogen oxide emissions, therefore are facing the challenge of reducing haze

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and avoiding an increase in ozone pollution at the same time. Efficient control strategies to mitigate

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winter haze in Beijing may require measures similar as implemented to avoid photochemical smog

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in summer.

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INTRODUCTION

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Winter haze formation has been experienced since the industrial revolution when more and more

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people moved into urban areas especially in the cities at northern hemisphere mid-to-high latitude.

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Taking Beijing, the capital city of China, for example, the number of winter haze days increased

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dramatically in the last decade, e.g. the number of winter haze days in 2016 were about 20 days

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more than experienced 20104. This change, to some extent, is thought to be related to warmer

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temperature in winter due to global climate change, which leads to meteorological conditions that

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favors stagnant conditions in this area5, 6. On a local scale, the winter haze is considered to be

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primarily related to the intensified emissions from coal burning (energy and heating supply), traffic

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and industry, that can accumulate under stagnant wind conditions (small southerly winds of about

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1-2 m/s 7) and in a low boundary layer (height ca. 340 m 8). In addition, high moisture (RH up to



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70-80%7) is experienced caused by the semi-closed terrain9, the warm cover structure in the middle

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troposphere9, and even the heavy particle pollution itself10.

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Nevertheless, as the winter haze is actually dominated (>50%) by secondary aerosols11, the

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chemical reactions that are responsible for the conversion of high concentrations of primary air

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pollutants to secondary pollutants are then of central interest for understanding winter haze

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pollution. Due to the generally reduced solar UV and water vapor as well as the enhanced aerosol

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liquid water, the contribution of the photochemistry is thought to be minor in the haze and the

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heterogeneous reactions have been proposed to be most important for the formation of haze12, 13.

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One of the major ingredients for haze, SO2, is emitted by coal burning. The mechanism for its

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conversion to sulfate in the particles is under discussion and the opinions are quite controversial.

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It is now proposed that SO2 is most likely oxidized by heterogeneous reactions on aerosol surfaces

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or in the bulk of the deliquesced aerosols under neutralized conditions14, 15 while the aerosols are

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more likely to be acidic16. Nevertheless, the contribution of sulfate to PM2.5 is currently only 10 -

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20% due to the efficient reduction of SO2 emissions11, 17, 18.

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The particulate nitrate and organic matters are the dominant portions in fine particles during

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haze episodes in the recent years. As the particulate nitrate and organic compounds in aerosol are

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mainly the result of photochemical oxidation of NOx and volatile organic compounds (VOCs), the

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contribution of photochemistry to the formation of haze needs to be explored. It was reported for

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the first time that significant high levels of OH radicals were present in the winter urban

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atmosphere in a winter campaign with measurements of radicals in 2000 in Birmingham1. High

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OH concentrations in that study give strong indication that photochemical processes can be

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important for the formation of winter air pollution in mid-to-high latitude in the northern

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hemisphere where strong emissions take place. Direct HOx measurements were also performed in

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other mega-cities such as in Tokyo in January and February 20042, and in New York in January

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and February 20043. In these three winter studies in urban atmosphere, relatively fast OH turnover

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rates were deduced based on the measurements of OH and modeled or measured total OH

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reactivity giving values of 5-8 ppbv/h around noon time. These values are lower than values

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determined in summertime at the same locations1-3. In a study performed at a rural site in Colorado

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in February 2011, the OH turnover rate was determined to be about 2 ppbv/h around noon time

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from observed OH concentrations and calculated OH reactivity19, much lower than in the

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campaigns in the urban environments. In recent studies, active photochemistry was found to be the 3 ACS Paragon Plus Environment

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key for the formation of winter ozone pollution20 and haze21 in Utah due to the efficient

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photochemistry in a low and stagnant atmospheric boundary layer.

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Because radical precursors like nitrous acid was found to be abundant in winter in Beijing22,

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a critical point to understand the winter haze formation mechanism is the determination of the role

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of the photochemistry. In the present work, measurements were done with a comprehensive suite

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of instruments available for studying photochemistry. The role of photochemical reactions in the

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formation of fine particles and ozone in winter haze Beijing is determined.

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METHODS

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Field measurement campaign. The field campaign BEST-ONE (Beijing winter finE particulate

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STudy - Oxidation, Nucleation and light Extinctions) was carried out from January 1st to March

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5th 2016 on the campus of the University of the Chinese Academy of Sciences at Huairou (Figure

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S1, 40.42°N, 116.69°E), which is located in a rural environment in the north of Beijing in the

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North China Plain. The north and west of the site is surrounded by mountains. Without major

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industry nearby, the area was only occasionally affected by local emissions from coal burning in

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a village 1 km to the east. A large suite of atmospheric trace gas and radical concentrations, and

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meteorological parameters were measured 20 m above the ground on top of a building (Table 1).

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The local meteorological conditions were on average characterized by low temperatures (-3  6

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oC)

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haze events.

and low relative humidity, RH (36  18%). However, the RH increased to up to 80% during

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Experimental. OH and HO2 radical concentrations were measured with a home-built instrument

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utilizing pulsed laser-induced fluorescence (LIF) at 308 nm11. The instrument samples ambient air

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by gas expansion into a low-pressure volume. At low pressure, OH is detected directly by LIF,

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whereas HO2 is first chemically converted to OH by reaction with added NO. The measurements

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of OH and HO2 are calibrated with a photochemical radical source which produces known radical

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concentrations by photolysis of water vapour at 185 nm. Total OH reactivity (kOH), which is

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equivalent to the inverse chemical OH lifetime, was measured by laser-photolysis laser induced

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fluorescence (LP-LIF). For that purpose, OH radicals are produced in ambient air in a flow tube

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by flash photolysis of ozone at 266 nm. The following OH decay due to reactions with atmospheric

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reactants is monitored in real-time by LIF. The total OH reactivity is then obtained as a pseudo

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first-order rate coefficient from the observed OH decay. In addition to the radical measurements, 4 ACS Paragon Plus Environment


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a large suite of trace gases including O3, CO, NO, NO2, N2O5, HONO, HCHO, VOCs, etc were

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measured locally near the inlets of the LIF instrument. Photolysis frequencies of O3 and NO2 were

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determined from solar actinic flux spectra that were measured by spectroradiometry. Aerosol

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chemical composition (< PM1.0) was measured using an Aerodyne High Resolution Time-of-Flight

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Aerosol Mass Spectrometer (HR-ToF-AMS; short: AMS). The separation of primary and

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secondary sources to the measured organic aerosol fraction from AMS was achieved by positive

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matrix factorization. The ion balance of ammonium, nitrate and sulfate shows that sulfate and

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nitrate explain the observed amount of ammonia. Further details of measurements of radicals, trace

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gases and aerosols can be found in the Supporting information.

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Calculations for the projection of NOx and VOC emission reduction strategies. The projection

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of emission reduction strategies is achieved by calculating the production rates of particulate

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nitrate and ozone in a chemical box model, in which VOC and NOx concentrations were varied.

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The model is based on the Regional Atmospheric Chemical Mechanism version 2 (RACM-2)23

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with modifications described in previous work24, 25. The model calculations were constrained to

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chemical conditions as currently found in the haze event in this work using measurements of

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HONO, NO2, CO, CH4, C2−C12 VOC and water vapor concentrations, as well as measured

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photolysis frequencies, temperature and pressure. In order to realistically estimate the effect of

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emission reductions, radical production was adjusted to reproduce formation rates as

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experimentally found in this campaign.

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The photochemical O3 formation rate, P(O3), is calculated from the NO2 production rate due to the reactions of peroxy radicals with NO. P(O3) = kHO2+NO[HO2][NO] + i kRO2i+NO[RO2i][NO]

Eq. 1

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The rate constant kHO2+NO is taken from NASA JPL Publication 15-1026. For the reaction of RO2

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with NO, rate constants (kRO2i+NO) and NO2 yields (i) are taken from RACM-2.

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The potential for particulate nitrate formation, P(NO3-), is calculated from the partitioning between the gas and particle phase of HNO3 and the heterogeneous hydrolysis of N2O5.

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P(NO3-) = kOH+NO2[OH][NO2]  p + [N2O5]  0.25 𝐶N2O5 Sa  𝛾N2O5

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HNO3 in the gas phase is mainly produced by the reaction of NO2 with OH. The rate

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constant kOH+NO2 is taken from NASA JPL Publication 15-1026. Other gas-phase reactions such as

Eq. 2


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HCHO + NO3, HO2 + NO, N2O5 + H2O that can form HNO3 can be neglected for the conditions

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analyzed here27. The HNO3 uptake into the particle phase was implemented in the model in such

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a way that the partitioning of HNO3 into the particle phase was 100% (p = 1). This is justified by

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high partitioning ratio (> 0.96) of NO3- / (HNO3+NO3-) estimated from both the direct

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measurements of the gaseous HNO3 and NO3- by a wet denuder system, GAC-IC (Table 1) and

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the calculations from ISORRPIA-II. The NO3- in the particle phase must also be balanced by NH4+

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ions, which requires that sufficiently high ammonia (NH3) was available. This is supported by the

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high NH3 concentrations that were observed in the pollution episodes. In addition, high ammonium

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concentrations in the particle phase were measured by the analysis of the chemical composition of

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particles in the AMS. The ion balance between ammonium and nitrate and sulfate explains the

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high ammonium concentrations, so that both, nitrate and sulfate, are full neutralized. For the

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transfer of gaseous N2O5 into the particle phase an uptake coefficient (𝛾N2O5) of 0.003 was used,

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derived from an iterative box model method28. The 𝐶N2O5 denotes the mean molecular speed of

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N2O5 and Sa denotes the total surface area concentrations derived from the SMPS measurement

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(Table 1).

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Modelling of regional ozone production. The GEOS-Chem chemical transport model (v11-02;

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www.geos-chem.org) is used to simulate the spatial and temporal distributions of ozone pollution

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over North China. The model is driven by the NASA Goddard Earth Observing System (GEOS)

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GEOS-FP assimilated meteorological data at a horizontal resolution of 0.25° latitude by 0.3125°

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longitude. The simulation is conducted with the nested-grid GEOS-Chem model that has the native

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0.25°×0.3125° horizontal resolution over East Asia with 3-hourly boundary conditions archived

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from a global simulation at 2°×2.5° resolution. The model includes a detailed NOx–Ox–

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hydrocarbon–aerosol tropospheric chemical mechanism, and accounts for heterogeneous uptake

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of HO2, NO2, NO3, and N2O5 by aerosol surfaces29. Chinese Anthropogenic emissions are from

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the Multi-resolution Emission Inventory for China (MEIC, http://www.meicmodel.org) for the

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year 2015.

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RESULTS AND DISCUSSION

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Overview and Concept of Winter Photochemistry. Either the outflow of urban Beijing due to

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southerly winds conditions or continental background air originating from Siberia due to strong

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northerly winds were observed at the measurement site (Figure S2 - S5). In total, five haze 6 ACS Paragon Plus Environment


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pollution events were captured with the full suite of measurement parameters. For each case

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observed, the accumulations of the secondary pollutants were quite similar. The continental air

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masses from the north flushed the site before the start of a haze event. Both the concentrations of

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the primary air pollutants and PM2.5 were very small at the beginning. Under stagnant

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meteorological conditions with slow southerly winds from the Beijing area secondary pollutants

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increased quickly due to chemical reactions of primary air pollutants from regional emissions.

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Finally, the haze was flushed away with again strong northerly wind ending the haze event within

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a few hours. According to the measurement of the aerosol chemical compositions, the dominant

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components of the fine particles during the haze conditions were nitrate and organics (Figure 1A).

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Moreover, the co-enhancement of PM2.5 and the photochemistry indicators - peroxy acetyl nitrate

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(PAN) and the total oxidant (Ox = O3 + NO2 + NOz, NOz = NOy - NOx) were continuously

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observed (Figure S2). Both measurements strongly indicate that the particle pollution was

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associated with atmospheric oxidation processes driven by photochemical smog reactions. Since

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all haze events showed a similar characteristic (Figure 1 and Figure S6 - S9), results of the episode

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are discussed in the following text when the highest aerosol load of up to 350 µg/m3 (PM2.5) was

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experienced.

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Due to the efficient emission control of SO2, the majority of particle mass in Beijing’s haze

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events consists of organic compounds and nitrate11, 15, 17, 30 also found in the present study (Figure

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1A and Figure S6A - S9A). Oxidized volatile organic compounds (VOCs) and nitrogen oxides

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partitioning in the particulate phase are typically the result of photochemical activity. In wintertime,

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however, solar radiation is weak due to the low sun and additionally due to the strong attenuation

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by aerosol extinction in the haze event. Therefore, light-driven chemistry is generally assumed to

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play a minor role in wintertime haze and different oxidation mechanisms in the aerosol aqueous

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phase have been suggested to be responsible for the sulfate and nitrate observed in particles13-15.

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In contrast, direct measurements in this study show that there is a surprisingly efficient

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photochemical conversion of NOx and VOCs to compounds found in aerosol (Figure 2) during

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Beijing's winter haze despite reduced solar radiation.

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Fine Particle and the Total Oxidants. Starting on February 29th, the mass concentration of fine

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particles (PM1) increased from a few g/m3 to up to 250 g/m3 on March 4th with a large fraction

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of ammonium nitrate (> 50%, Figure 1A). The build-up of the particle mass took place

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simultaneously with a gradual increase of total oxidants to over 100 ppbv. The total oxidant is the 7 ACS Paragon Plus Environment

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sum of ozone, NO2 and the oxidized nitrogen species in the gas phase (NOz) derived from total

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reactive nitrogen (NOy) and NOx measurements (Figure 1B), which were eventually formed from

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emissions of nitric oxide (NO) (Figure 2). The nitrogen compounds included inorganic species like

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N2O5 and HNO3, as well as organic compounds like PAN and other oxidized nitrogen compounds

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(Figure S10). In contrast, the concentration of ozone was relatively small (< 40 ppbv), as it was a

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major oxidant consumed in the production of oxidized nitrogen species (Figure 2). Moreover, an

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analysis of the simultaneous measurements of PM2.5 and the total oxidants from the regional

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network by Beijing Municipal Environmental Monitoring Center showed that the buildup of winter

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haze accompanied by the accumulation of oxidants were also a regional phenomenon (Figure S11).

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Trace gas removal and photochemical ozone productions. The impact of light extinction in the

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accumulating haze is seen as a strong decline in the solar radiation and concentration of hydroxyl

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radicals (OH) by a factor of 10 (Figure 1C). OH is the major atmospheric oxidant that controls the

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chemical removal of atmospheric pollutants (e.g. CO, NO2, VOCs) thereby producing secondary

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pollutants. The decrease in the OH concentration seems plausible because tropospheric OH is

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generally formed by photolysis reactions. Therefore, low OH concentrations apparently support

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the assumption that photochemistry plays a minor role during haze events13-15. However, the OH

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concentration is also reduced due to an increasing rate of OH destruction by reactive gases that

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accumulate during the haze event. The corresponding loss rate coefficient for OH, (total OH

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reactivity, kOH), was directly measured in the campaign. Values increased from 5 s-1 in relative

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clean air to up to 80 s-1 under highly polluted conditions (Figure 1D). The photochemical activity

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of OH can be quantified by the product of the measured OH concentration and OH reactivity

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([OH]kOH) which is equivalent to the oxidation rate of pollutants by reaction with OH.

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Surprisingly, this rate increased from approximately 5 ppbv/h to 15 ppbv/h during the formation

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of the haze (Figure 1E). The measured 3-fold increase demonstrates that the photochemical

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conversion of emitted compounds into secondary pollutants was very active in the winter haze

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with oxidation rates similar to values observed in summer in the NCP24, 31.

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The oxidation by OH was so efficient because OH is quickly recycled in the reaction of

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hydroperoxy (HO2) radicals with abundant NO from anthropogenic emissions. HO2 is either

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directly produced from the reaction of OH with CO, or is produced in the reaction of NO with

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organic peroxy radicals (RO2) resulting from the reaction of VOCs with OH. In the haze event,



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OH recycling amplified the oxidation rate initiated by a weak primary OH production rate (0.6

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ppbv/h) by a factor of 20 to 40. The amplification factor of the OH turnover rates is calculated as

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the ratio of the secondary OH production rate due to the reaction of HO2 with NO relative to the

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primary production rate of OH by ozonolysis of alkenes and photolysis of HONO and ozone. The

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determined amplification factor (also known as chain length of the HOx cycle) is much longer than

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found in previous winter campaigns when values varied between 4 to 7 in the urban areas1-3 and

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were around 2 in a rural site19. The variation of the amplification factor is not proportional to the

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changes in the VOCs to NOx ratio as seen in the different winter campaigns1-3, 19. The larger

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amplification factor observed in this campaign compared to other winter studies indicates that the

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photochemistry was exceptionally efficient in winter Beijing. These findings could not be

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explained by chemical box models that use measured long-lived species (VOCs, and NOx, etc.) as

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constraints25. In the global atmosphere, the most important primary source of OH is the reaction

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of water vapor with excited state oxygen atoms from the UV-B photolysis of ozone32. In this winter

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campaign, however, OH formation from ozone photolysis played a negligible role (


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80 ppbv) along the Taihang Mountain south of Beijing and the Yanshan Mountain east of Beijing.

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The mean photochemical ozone production rates during the haze event were highest in the cities

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Beijing and Tianjian with values of up to 10 ppbv/h. In the southern part of Hebei, ozone

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production was elevated (4 to 6 ppbv/h) compared to the north. The photochemistry in this model

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is mainly driven by the production of radicals by the photolysis of HONO for wintertime

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conditions. However, the mean photochemical ozone production rates at the measurement site

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Huairou were a factor of two higher than modeled values likely due to missing radical sources25.

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The underestimation of the ozone production rates by the model is also reflected in an

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underestimation of the total oxidant concentration. The GEOS-Chem model indicates that the

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measurement site is representative for a wide area in the North China Plain suggesting that current

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chemical transport models do not well represent the fast-photochemical activity for wintertime

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haze conditions.

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As announced in the 13th Five-Year Plan, China's national air pollution control strategies

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aim primarily at reducing emissions of NOx rather than of VOCs. This study demonstrates the

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potential for an increase of ozone pollution in Beijing during wintertime in the future due to an

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active photochemistry. The concurrent reduction of emissions of both, nitrogen oxides and organic

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compounds, would not only improve air quality by reducing particle loads, but also avoid the risk

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of ozone pollution (Figure 3) in wintertime. In addition, these measures would apply the entire

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year and therefore reduce summertime ozone smog. As suggested by emission inventories41,42, the

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most efficient target for regulations of winter air pollution control could be the vehicle emissions

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which contribute the largest part of the nitrogen oxide and organic compound emissions in China.

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The missing radical source is of urgent importance to be clarified to enabling the detailed policy

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projection for the different geographic areas.

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As a result of a comprehensive field campaign performed in winter in Beijing, we measured

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unexpectedly fast wintertime ozone production which is comparably high as observed in

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summertime photochemical smog. This fast production, however, does not lead to high ozone

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concentrations, because it is chemically converted by nitrogen oxides and taken up by particles

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enhancing severe wintertime haze events in Beijing. In addition, precursors of radical species

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responsible for the ozone formation can be formed from heterogeneous chemistry on particle

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surfaces, so that this reaction scheme is a self-energizing process. Therefore, wintertime haze in 12 ACS Paragon Plus Environment


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Beijing could be regarded as a type of photochemical smog. Control strategy would need to be

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similar as those applied to avoid summertime ozone pollution.

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ASSOCIATED CONTENT

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Supporting Information

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Additional methods and thirteen additional figures (Figures S1 – S13).

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AUTHOR INFORMATION

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Corresponding Authors

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E-mail: [email protected]

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E-mail: [email protected]

361 362

ACKNOWLEDGMENTS

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We thank the BEST-ONE campaign team, especially the local host - Prof. Dr. Y. X. Zhang, and

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Mr. P. Huo for their technical help and full support at the field site. Discussions with Miss. L. Mao,

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Mr. X. R. Chen and Dr. M. J. Tang are also appreciated. This work was supported by the National

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Natural Science Foundation of China (Grants No. 91544225, 21522701, 21190052, 41375124),

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the National Science and Technology Support Program of China (No. 2014BAC21B01), the

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Strategic Priority Research Program of the Chinese Academy of Sciences (Grants No.

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XDB05010500).


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Figure 1. Atmospheric measurements during the haze event measured in Huairou (north of

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Beijing) starting from clean air. Aerosol (PM1) chemical composition (A), ozone and nitrogen

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oxide species resulting from ozone oxidation of NO emissions (NO2, higher gaseous oxidation

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products NOz, particulate nitrate NO3-) (B), OH concentrations and solar UV-A intensity

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represented as NO2 photolysis frequency (jNO2) (C), partitioning of the total OH reactivity (kOH) to

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contribution from CO, NOX and organics (D), OH removal rate (kOH[OH]) (E), ozone production

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rate (P(O3)) from the reaction of HO2 with NO that is equivalent to the OH recycling rate (F). The

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total ozone production will be approximately twice a high due to the additional production from

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the reaction of NO with RO2 whose concentration is typically as high as that of HO2 for high NO

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concentrations. Grey shaded areas indicate nighttime. 14 ACS Paragon Plus Environment


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Figure 2. Photochemical pathways mediating the transformation of emitted volatile organic compounds (VOC) and nitric oxide (NO) to particulate matter (PM). OH radicals produced by the photolysis of nitrous acid (HONO) and ozonolysis of alkenes react with VOCs and initiate the formation of oxygenated VOCs that contribute to the formation of organic aerosol (OA). NO2 is formed from the oxidation of NO in the reaction of ozone with peroxy radicals (HO2, RO2). During the day, NO2 reacts with OH to nitric acid (HNO3), which is efficiently converted into particulate nitrate (NO3-) with the participation of ammonia (NH3). Ozone produced by NO2 photolysis during the day is consumed by nightly oxidation of NO2 to N2O5, which adds to nitrate formation in the particle phase. 381


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382 383

Figure 3. Projection of the impact of emission reductions for NOx and VOCs on the

384

photochemical production rates of the secondary pollutants for the conditions of the haze

385

event. (A) The daily integrated photochemical production rates of particulate nitrate, P(NO3-), (B)

386

the daily integrated production rates of ozone, P(O3).



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387 388

Figure 4. Chemical transport model results for daytime averaged total concentrations of

389

ozone and oxidized nitrogen compounds (NO2 and NOz) (A) and the photochemical ozone

390

production rate (B) during the haze event (3rd and 4th March). The colored circles denote the

391

corresponding experimentally determined values at the Huairou site. The experimentally

392

determined ozone production rate is assumed to be twice as large as calculated from measured

393

HO2 and NO concentrations to account for ozone production from organic peroxy radicals which

394

concentration can be assumed to be similar as that of HO2.

395


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396

Table 1. Overview of parameters of instruments for the detection of gas phase trace gas

397

concentrations, particle properties and physical parameters used for the analysis in this study. Species

Time resolution

Limit of Detection

Method

Accuracy (1 )

OH

30 s

0.8×106 cm-3

LIF

± 14%

HO2

30 s

0.2×108 cm-3

LIF

± 17%

kOH

90 s

0.3 s-1

LP – LIF

± 5-20%±0.7 s-1

N2O5

60 s

2 pptv

CEAS

± 19%

HONO

300 s

10 pptv

LOPAP

± 20%

HNO3

0.5 h

65 pptv

GAC

± 30%

HCHO

120 s

25 pptv

Hantzsch

± 5%

PAN

300 s

50 pptv

GC-ECD

± 10%

NH3

0.5 h

30 pptv

GAC

± 30%

O3

60 s

0.5 ppbv

UV

± 10%

CO

60 s

40 ppbv

NDIR

± 10%

SO2

60 s

0.1 ppbv

UV-F

± 10%

NO

60 s

50 pptv

CL

± 10%

NO2

60 s

50 pptv

Photolytic Conv. + CL

± 10%

NOy

60 s

50 pptv

Mo Conv. + CL

± 1%

VOCs

1h

5-70 pptv

GC-MS/FID

± 10-15%

Aerosol surface concentrations

300 s

--

SMPS, APS

± 30%

PM2.5

60 s

0.1 μg/m3

TEOM

± 10%

PM1.0 component

300 s

0.005-0.424 μg/m3

HR-ToF-AMS

± 30%

NO3- of PM2.5

0.5 h

0.034 μg/m3

GAC

± 10%

J-values

60 s

--

SR

± 10%

Temperature

60 s

-50°C - 50°C

Met One 083E

±0.1 °C

Pressure

60 s

600 - 1100 hPa

Met One 092

±0.35 hPa

Relative humidity

60 s

0 - 100%

Met One 083E

±2.0%

Wind speed

60 s

0.45 - 60 m/s

Met One 014A

± 0.11 m/s

Wind direction

60 s

0°- 360° m/s)

Met One 024A

±5°

(>0.45

398 18 ACS Paragon Plus Environment


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399

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Fast photochemistry in wintertime haze: Consequences for pollution

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