16 Jun 2022
16 Jun 2022
Status: a revised version of this preprint is currently under review for the journal ACP.

Different physicochemical behaviors of nitrate and ammonium during transport: a case study on Mt. Hua, China

Can Wu1, Cong Cao2,a, Jianjun Li2, Shaojun Lv1, Jin Li2,b, Xiaodi Liu1, Si Zhang1, Shijie Liu1, Fan Zhang1, Jingjing Meng4, and Gehui Wang1,3 Can Wu et al.
  • 1Key Lab of Geographic Information Science of the Ministry of Education, School of Geographic Sciences, East China Normal University, Shanghai 200062, China
  • 2State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710061, China
  • 3Institute of Eco-Chongming, Chenjia Zhen, Chongming, Shanghai 202162, China
  • 4School of Environment and Planning, Liaocheng University, Liaocheng 252000, China
  • anow at: The State University of New York at Stony Brook
  • bnow at: Institute for Environmental and Climate Research, Jinan University

Abstract. To understand the chemical evolution of aerosols in the transport process, the chemistry of PM2.5 and nitrogen isotope compositions on the mountainside of Mt. Hua (~1120 m a.s.l.) in inland China during the 2016 summertime were investigated and compared with parallel observations collected at surface sampling site (~400 m a.s.l.). PM2.5 exhibited a high level at the surface (aver. 76.0±44.1 μg/m3) and could be transported aloft by anabatic valley winds, leading to the gradual accumulation of daytime PM2.5 with a noon peak at the mountainside sampling site. As the predominant ion species, sulfate exhibited nearly identical mass concentrations in both sites, but its PM2.5 mass fraction was moderately enhanced by ~4 % at the higher elevation. The ammonium variations were similar to the sulfate variations, the chemical forms of both of which mainly existed as ammonium bisulfate (NH4HSO4) and ammonium sulfate ((NH4)2SO4) at the lower and higher elevations, respectively. Unlike sulfate and ammonium, nitrate mainly existed as ammonium nitrate (NH4NO3) in fine particles and exhibited decreasing mass concentration and proportion trends with increasing elevation. This finding was ascribed to NH4NO3 volatilization, in which gaseous HNO3 from semi-volatile NH4NO3 subsequently reacted with dust particles to form nonvolatile salts, resulting in significant nitrate shifts from fine particles into coarse particles. Such scavenging of fine-particle nitrate led to an enrichment in the daytime 15N of nitrate at the mountainside site compared with to the lower-elevation site. In contrast to nitrate, at the higher elevation, the 15N in ammonium depleted during the daytime. Considering the lack of any significant change in ammonia sources during the vertical transport process, this 15N depletion in ammonium was mainly the result of unidirectional reactions, indicating that additional ammonia would partition into particulate phases and further neutralize HSO4- to form SO42-. This process would reduce the aerosol acidity, with a higher pH (3.4±2.2) at MS site and lower ones (2.9±2.0) at MF site. Our work provides more insight into physicochemical behaviors of semi-volatile nitrate and ammonium, which will facilitate the improvement in model for a better simulation of aerosol composition and properties.

Can Wu et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'RC on acp-2022-342', Anonymous Referee #2, 27 Jul 2022
    • AC1: 'Reply on RC1', Gehui Wang, 30 Sep 2022
  • RC2: 'Comment on acp-2022-342', Anonymous Referee #1, 05 Aug 2022
    • AC2: 'Reply on RC2', Gehui Wang, 30 Sep 2022
  • RC3: 'Comment on acp-2022-342', Anonymous Referee #3, 20 Aug 2022
    • AC3: 'Reply on RC3', Gehui Wang, 30 Sep 2022
    • AC4: 'Reply on RC3', Gehui Wang, 30 Sep 2022
    • AC1: 'Reply on RC1', Gehui Wang, 30 Sep 2022

Can Wu et al.


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Short summary
Over the past decade, the relative abundance of NH4NO3 in the aerosol has been enhanced in most urban areas of China, which profoundly affects the PM2.5 pollution episodes. Our work finds that nitrate and ammonium of fine particles exhibited distinct-different physicochemical behaviors in aerosol aging process.