Wintertime photochemistry in Beijing: observations of RO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; radical concentrations in the North China Plain during the BEST-ONE campaign

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Abstract

Abstract. The first wintertime in situ measurements of hydroxyl (OH), hydroperoxy (HO2) and organic peroxy (RO2) radicals (ROx<math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>=</mo><mrow class="chem"><mi mathvariant="normal">OH</mi></mrow><mo>+</mo><mrow class="chem"><msub><mi mathvariant="normal">HO</mi><mn mathvariant="normal">2</mn></msub></mrow><mo>+</mo><mrow class="chem"><msub><mi mathvariant="normal">RO</mi><mn mathvariant="normal">2</mn></msub></mrow><mo>)</mo></mrow></math><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="97pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="944a75217af8ae66ca74a397a5a4af05"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-18-12391-2018-ie00001.svg" width="97pt" height="13pt" src="acp-18-12391-2018-ie00001.png"/></svg:svg> in combination with observations of total reactivity of OH radicals, kOH 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 <math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">2.4</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mn mathvariant="normal">6</mn></msup><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">cm</mi><mrow><mo>-</mo><mn mathvariant="normal">3</mn></mrow></msup></mrow></math><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="72pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="1b574e08d1d294c1bbb82d0d06b5aabd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-18-12391-2018-ie00002.svg" width="72pt" height="14pt" src="acp-18-12391-2018-ie00002.png"/></svg:svg> in severely polluted air (<math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>k</mi><mi mathvariant="normal">OH</mi></msub><mo>∼</mo><mn mathvariant="normal">27</mn><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">s</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup><mo>)</mo></mrow></math><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="66pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="cfee02d4bafcdb1e32d7dfe42cc19925"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-18-12391-2018-ie00003.svg" width="66pt" height="16pt" src="acp-18-12391-2018-ie00003.png"/></svg:svg> to <math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">3.6</mn><mo>×</mo><msup><mn mathvariant="normal">10</mn><mn mathvariant="normal">6</mn></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">cm</mi><mrow><mo>-</mo><mn mathvariant="normal">3</mn></mrow></msup></mrow></math><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="72pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="670a4d1ee3f84b49e1a35e15ac70ca80"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-18-12391-2018-ie00004.svg" width="72pt" height="14pt" src="acp-18-12391-2018-ie00004.png"/></svg:svg> in relatively clean air (<math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>k</mi><mi mathvariant="normal">OH</mi></msub><mo>∼</mo><mn mathvariant="normal">5</mn><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">s</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup><mo>)</mo></mrow></math><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="60pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="c3f9fa9c822b7983def705c86faf425e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-18-12391-2018-ie00005.svg" width="60pt" height="16pt" src="acp-18-12391-2018-ie00005.png"/></svg:svg>. 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 ROx radicals was dominated by the photolysis of nitrous acid accounting for 46% of the identified primary production pathways for ROx 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 NOx 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.

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As the world's second largest economy, China has experienced severe haze pollution, with fine particulate matter (PM) recently reaching unprecedentedly high levels across many cities, and an understanding of the PM formation mechanism is critical in the development of efficient mediation policies to minimize its regional to global impacts. We demonstrate a periodic cycle of PM episodes in Beijing that is governed by meteorological conditions and characterized by two distinct aerosol formation processes of nucleation and growth, but with a small contribution from primary emissions and regional transport of particles. Nucleation consistently precedes a polluted period, producing a high number concentration of nano-sized particles under clean conditions. Accumulation of the particle mass concentration exceeding several hundred micrograms per cubic meter is accompanied by a continuous size growth from the nucleation-mode particles over multiple days to yield numerous larger particles, distinctive from the aerosol formation typically observed in other regions worldwide. The particle compositions in Beijing, on the other hand, exhibit a similarity to those commonly measured in many global areas, consistent with the chemical constituents dominated by secondary aerosol formation. Our results highlight that regulatory controls of gaseous emissions for volatile organic compounds and nitrogen oxides from local transportation and sulfur dioxide from regional industrial sources represent the key steps to reduce the urban PM level in China.

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Gehui Wang, Renyi Zhang, Mario E. Gómez … (2016)

Sulfate aerosols exert profound impacts on human and ecosystem health, weather, and climate, but their formation mechanism remains uncertain. Atmospheric models consistently underpredict sulfate levels under diverse environmental conditions. From atmospheric measurements in two Chinese megacities and complementary laboratory experiments, we show that the aqueous oxidation of SO2 by NO2 is key to efficient sulfate formation but is only feasible under two atmospheric conditions: on fine aerosols with high relative humidity and NH3 neutralization or under cloud conditions. Under polluted environments, this SO2 oxidation process leads to large sulfate production rates and promotes formation of nitrate and organic matter on aqueous particles, exacerbating severe haze development. Effective haze mitigation is achievable by intervening in the sulfate formation process with enforced NH3 and NO2 control measures. In addition to explaining the polluted episodes currently occurring in China and during the 1952 London Fog, this sulfate production mechanism is widespread, and our results suggest a way to tackle this growing problem in China and much of the developing world.