Reply to Cao and Zhang: Tightening nonfossil emissions alone is inefficient for PM2.5 mitigation in China

Abstract

In the short commentary by Fang Cao and Yan-Lin Zhang, the authors suggest that tightening nonfossil emissions control may serve as a potential opportunity for fine particulate matter (PM with the particle size smaller than 2.5 μm or PM2.5) mitigation in China (1). Cao and Zhang state that “Guo et al.’s…important new finding fails to consider particles emitted by nonfossil sources (e.g., biomass burning, cooking, and biogenic emissions) either from primary emissions or secondary formations” (1), but provide no technical or scientific detail to invalidate the results and conclusions of the work by Guo et al. (2). Instead, Cao and Zhang argue that nonfossil PM sources represent “a critically important (sometimes dominant) contributor of fine particles observed in both Chinese megacities…and other urban areas around world” (1) simply by citing several previous studies (e.g., ref. 3). The dominant importance of the nonfossil emissions to the formation of severe urban PM episodes, as claimed by Cao and Zhang, is unsupported in our measurements of the aerosol properties and their gaseous precursors in Beijing (2). Our results reveal that the development of a pollution episode includes two distinct secondary aerosol formation processes: that is, nucleation to initially produce high number concentrations of nano-sized particles, and continuous growth from the nucleation mode particles. The particle compositions measured in Beijing during the pollution episodes are consistent with those dominated by secondary formation processes. For example, the mass fractions of organic, sulfate, and nitrate measured by an aerosol mass spectrometer are 43%, 17%, and 22%, respectively, averaged over our entire measurement period in Beijing (2). In addition, our hygroscopicity and density measurements also show a noticeable absence of the primary aerosol constituents during the polluted periods, including black carbon, primary organic aerosols, and mineral dusts. The formation of the particle-phase organics, sulfate, and nitrate are clearly attributable to emissions of anthropogenic volatile organic compounds (VOCs), sulfur dioxide (SO2), and nitrogen oxides (NOx = NO + NO2), respectively (2, 4), because our gaseous measurements show highly elevated concentrations of those species during the pollution episodes in Beijing. Hence, our results unambiguously illustrate the importance of aerosol gaseous precursor emissions from urban transportation and nearby industrial facilities in the formation of severe haze episodes in Beijing (2).