Chemical feedbacks weaken the wintertime response of particulate sulfate and nitrate to emissions reductions over the eastern United States

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Abstract

Exposure to fine particulate matter is a leading cause of premature deaths and illnesses globally. In the eastern United States, substantial cuts in sulfur dioxide and nitrogen oxides emissions have considerably lowered particulate sulfate and nitrate concentrations for all seasons except winter. Simulations that reproduce detailed airborne observations of wintertime atmospheric chemistry over the eastern United States indicate that particulate sulfate and nitrate formation is limited by the availability of oxidants and by the acidity of fine particles, respectively. These limitations relax at lower ambient concentrations, forming particulate matter more efficiently, and weaken the effect of emission reductions. These results imply that larger emission reductions, especially during winter, are necessary for substantial improvements in wintertime air quality in the eastern United States. Sulfate ( <math id="i1" overflow="scroll"> <msubsup> <mrow> <mi mathvariant="normal">S</mi> <mi mathvariant="normal">O</mi> </mrow> <mrow> <mn>4</mn> </mrow> <mrow> <mn>2</mn> <mo>-</mo> </mrow> </msubsup> </math> ) and nitrate ( <math id="i2" overflow="scroll"> <msubsup> <mrow> <mi mathvariant="normal">N</mi> <mi mathvariant="normal">O</mi> </mrow> <mrow> <mn>3</mn> </mrow> <mrow> <mo>-</mo> </mrow> </msubsup> </math> ) account for half of the fine particulate matter mass over the eastern United States. Their wintertime concentrations have changed little in the past decade despite considerable precursor emissions reductions. The reasons for this have remained unclear because detailed observations to constrain the wintertime gas–particle chemical system have been lacking. We use extensive airborne observations over the eastern United States from the 2015 Wintertime Investigation of Transport, Emissions, and Reactivity (WINTER) campaign; ground-based observations; and the GEOS-Chem chemical transport model to determine the controls on winter <math id="i3" overflow="scroll"> <msubsup> <mrow> <mi mathvariant="normal">S</mi> <mi mathvariant="normal">O</mi> </mrow> <mrow> <mn>4</mn> </mrow> <mrow> <mn>2</mn> <mo>-</mo> </mrow> </msubsup> </math> and <math id="i4" overflow="scroll"> <msubsup> <mrow> <mi mathvariant="normal">N</mi> <mi mathvariant="normal">O</mi> </mrow> <mrow> <mn>3</mn> </mrow> <mrow> <mo>-</mo> </mrow> </msubsup> </math> . GEOS-Chem reproduces observed <math id="i5" overflow="scroll"> <msubsup> <mrow> <mi mathvariant="normal">S</mi> <mi mathvariant="normal">O</mi> </mrow> <mrow> <mn>4</mn> </mrow> <mrow> <mn>2</mn> <mo>-</mo> </mrow> </msubsup> </math> – <math id="i6" overflow="scroll"> <msubsup> <mrow> <mi mathvariant="normal">N</mi> <mi mathvariant="normal">O</mi> </mrow> <mrow> <mn>3</mn> </mrow> <mrow> <mo>-</mo> </mrow> </msubsup> </math> – <math id="i7" overflow="scroll"> <msubsup> <mrow> <mi mathvariant="normal">N</mi> <mi mathvariant="normal">H</mi> </mrow> <mrow> <mn>4</mn> </mrow> <mrow> <mo>+</mo> </mrow> </msubsup> </math> particulate concentrations (2.45 μg <math id="i8" overflow="scroll"> <msup> <mrow> <mi mathvariant="normal">s</mi> <mi mathvariant="normal">m</mi> </mrow> <mrow> <mo>-</mo> <mn>3</mn> </mrow> </msup> </math> ) and composition ( <math id="i9" overflow="scroll"> <msubsup> <mrow> <mi mathvariant="normal">S</mi> <mi mathvariant="normal">O</mi> </mrow> <mrow> <mn>4</mn> </mrow> <mrow> <mn>2</mn> <mo>-</mo> </mrow> </msubsup> </math> : 47%; <math id="i10" overflow="scroll"> <msubsup> <mrow> <mi mathvariant="normal">N</mi> <mi mathvariant="normal">O</mi> </mrow> <mrow> <mn>3</mn> </mrow> <mrow> <mo>-</mo> </mrow> </msubsup> </math> : 32%; <math id="i11" overflow="scroll"> <msubsup> <mrow> <mi mathvariant="normal">N</mi> <mi mathvariant="normal">H</mi> </mrow> <mrow> <mn>4</mn> </mrow> <mrow> <mo>+</mo> </mrow> </msubsup> </math> : 21%) during WINTER. Only 18% of <math id="i12" overflow="scroll"> <msub> <mrow> <mi mathvariant="normal">S</mi> <mi mathvariant="normal">O</mi> </mrow> <mrow> <mn>2</mn> </mrow> </msub> </math> emissions were regionally oxidized to <math id="i13" overflow="scroll"> <msubsup> <mrow> <mi mathvariant="normal">S</mi> <mi mathvariant="normal">O</mi> </mrow> <mrow> <mn>4</mn> </mrow> <mrow> <mn>2</mn> <mo>-</mo> </mrow> </msubsup> </math> during WINTER, limited by low [H 2O 2] and [OH]. Relatively acidic fine particulates (pH <math id="i14" overflow="scroll"> <mo>∼</mo> </math> 1.3) allow 45% of nitrate to partition to the particle phase. Using GEOS-Chem, we examine the impact of the 58% decrease in winter <math id="i15" overflow="scroll"> <msub> <mrow> <mi mathvariant="normal">S</mi> <mi mathvariant="normal">O</mi> </mrow> <mrow> <mn>2</mn> </mrow> </msub> </math> emissions from 2007 to 2015 and find that the H 2O 2 limitation on <math id="i16" overflow="scroll"> <msub> <mrow> <mi mathvariant="normal">S</mi> <mi mathvariant="normal">O</mi> </mrow> <mrow> <mn>2</mn> </mrow> </msub> </math> oxidation weakened, which increased the fraction of <math id="i17" overflow="scroll"> <msub> <mrow> <mi mathvariant="normal">S</mi> <mi mathvariant="normal">O</mi> </mrow> <mrow> <mn>2</mn> </mrow> </msub> </math> emissions oxidizing to <math id="i18" overflow="scroll"> <msubsup> <mrow> <mi mathvariant="normal">S</mi> <mi mathvariant="normal">O</mi> </mrow> <mrow> <mn>4</mn> </mrow> <mrow> <mn>2</mn> <mo>-</mo> </mrow> </msubsup> </math> . Simultaneously, NOx emissions decreased by 35%, but the modeled <math id="i19" overflow="scroll"> <msubsup> <mrow> <mi mathvariant="normal">N</mi> <mi mathvariant="normal">O</mi> </mrow> <mrow> <mn>3</mn> </mrow> <mrow> <mo>-</mo> </mrow> </msubsup> </math> particle fraction increased as fine particle acidity decreased. These feedbacks resulted in a 40% decrease of modeled [ <math id="i20" overflow="scroll"> <msubsup> <mrow> <mi mathvariant="normal">S</mi> <mi mathvariant="normal">O</mi> </mrow> <mrow> <mn>4</mn> </mrow> <mrow> <mn>2</mn> <mo>-</mo> </mrow> </msubsup> </math> ] and no change in [ <math id="i21" overflow="scroll"> <msubsup> <mrow> <mi mathvariant="normal">N</mi> <mi mathvariant="normal">O</mi> </mrow> <mrow> <mn>3</mn> </mrow> <mrow> <mo>-</mo> </mrow> </msubsup> </math> ], as observed. Wintertime [ <math id="i22" overflow="scroll"> <msubsup> <mrow> <mi mathvariant="normal">S</mi> <mi mathvariant="normal">O</mi> </mrow> <mrow> <mn>4</mn> </mrow> <mrow> <mn>2</mn> <mo>-</mo> </mrow> </msubsup> </math> ] and [ <math id="i23" overflow="scroll"> <msubsup> <mrow> <mi mathvariant="normal">N</mi> <mi mathvariant="normal">O</mi> </mrow> <mrow> <mn>3</mn> </mrow> <mrow> <mo>-</mo> </mrow> </msubsup> </math> ] are expected to change slowly between 2015 and 2023, unless <math id="i24" overflow="scroll"> <msub> <mrow> <mi mathvariant="normal">S</mi> <mi mathvariant="normal">O</mi> </mrow> <mrow> <mn>2</mn> </mrow> </msub> </math> and NOx emissions decrease faster in the future than in the recent past.

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Chemical and microphysical characterization of ambient aerosols with the aerodyne aerosol mass spectrometer.

M.R. Canagaratna, J.T. Jayne, J.L. Jiménez … (2007)

The application of mass spectrometric techniques to the real-time measurement and characterization of aerosols represents a significant advance in the field of atmospheric science. This review focuses on the aerosol mass spectrometer (AMS), an instrument designed and developed at Aerodyne Research, Inc. (ARI) that is the most widely used thermal vaporization AMS. The AMS uses aerodynamic lens inlet technology together with thermal vaporization and electron-impact mass spectrometry to measure the real-time non-refractory (NR) chemical speciation and mass loading as a function of particle size of fine aerosol particles with aerodynamic diameters between approximately 50 and 1,000 nm. The original AMS utilizes a quadrupole mass spectrometer (Q) with electron impact (EI) ionization and produces ensemble average data of particle properties. Later versions employ time-of-flight (ToF) mass spectrometers and can produce full mass spectral data for single particles. This manuscript presents a detailed discussion of the strengths and limitations of the AMS measurement approach and reviews how the measurements are used to characterize particle properties. Results from selected laboratory experiments and field measurement campaigns are also presented to highlight the different applications of this instrument. Recent instrumental developments, such as the incorporation of softer ionization techniques (vacuum ultraviolet (VUV) photo-ionization, Li+ ion, and electron attachment) and high-resolution ToF mass spectrometers, that yield more detailed information about the organic aerosol component are also described. (c) 2007 Wiley Periodicals, Inc.