Articles | Volume 17, issue 23
Atmos. Chem. Phys., 17, 14747–14770, 2017
Atmos. Chem. Phys., 17, 14747–14770, 2017

Research article 12 Dec 2017

Research article | 12 Dec 2017

Observational assessment of the role of nocturnal residual-layer chemistry in determining daytime surface particulate nitrate concentrations

Gouri Prabhakar1, Caroline L. Parworth2, Xiaolu Zhang1, Hwajin Kim2,a, Dominique E. Young2,b, Andreas J. Beyersdorf3,c, Luke D. Ziemba3, John B. Nowak3, Timothy H. Bertram4, Ian C. Faloona5, Qi Zhang2, and Christopher D. Cappa1 Gouri Prabhakar et al.
  • 1Department of Civil and Environmental Engineering, University of California, Davis, Davis, CA, USA
  • 2Department of Environmental Toxicology, University of California, Davis, Davis, CA, USA
  • 3NASA Langley Research Center, Hampton, Virginia, USA
  • 4Department of Chemistry, University of Wisconsin, Madison, WI, USA
  • 5Department of Land, Air and Water Resources, University of California, Davis, Davis, CA, USA
  • anow at: Center for Environment, Health and Welfare Research, Korea Institute of Science and Technology, Seoul, South Korea
  • bnow at: Air Quality Research Center, University of California, Davis, Davis, California, USA
  • cnow at: Department of Chemistry, California State University, San Bernardino, San Bernardino, CA, USA

Abstract. This study discusses an analysis of combined airborne and ground observations of particulate nitrate (NO3(p)) concentrations made during the wintertime DISCOVER-AQ (Deriving Information on Surface Conditions from COlumn and VERtically resolved observations relevant to Air Quality) study at one of the most polluted cities in the United States – Fresno, CA – in the San Joaquin Valley (SJV) and focuses on developing an understanding of the various processes that impact surface nitrate concentrations during pollution events. The results provide an explicit case-study illustration of how nighttime chemistry can influence daytime surface-level NO3(p) concentrations, complementing previous studies in the SJV. The observations exemplify the critical role that nocturnal chemical production of NO3(p) aloft in the residual layer (RL) can play in determining daytime surface-level NO3(p) concentrations. Further, they indicate that nocturnal production of NO3(p) in the RL, along with daytime photochemical production, can contribute substantially to the buildup and sustaining of severe pollution episodes. The exceptionally shallow nocturnal boundary layer (NBL) heights characteristic of wintertime pollution events in the SJV intensify the importance of nocturnal production aloft in the residual layer to daytime surface concentrations. The observations also demonstrate that dynamics within the RL can influence the early-morning vertical distribution of NO3(p), despite low wintertime wind speeds. This overnight reshaping of the vertical distribution above the city plays an important role in determining the net impact of nocturnal chemical production on local and regional surface-level NO3(p) concentrations. Entrainment of clean free-tropospheric (FT) air into the boundary layer in the afternoon is identified as an important process that reduces surface-level NO3(p) and limits buildup during pollution episodes. The influence of dry deposition of HNO3 gas to the surface on daytime particulate nitrate concentrations is important but limited by an excess of ammonia in the region, which leads to only a small fraction of nitrate existing in the gas phase even during the warmer daytime. However, in the late afternoon, when diminishing solar heating leads to a rapid fall in the mixed boundary layer height (BLH), the impact of surface deposition is temporarily enhanced and can lead to a substantial decline in surface-level particulate nitrate concentrations; this enhanced deposition is quickly arrested by a decrease in surface temperature, which drops the gas-phase fraction to near zero. The overall importance of enhanced late-afternoon gas-phase loss to the multiday buildup of pollution events is limited by the very shallow nocturnal boundary layer. The case study here demonstrates that mixing down of NO3(p) from the RL can contribute a majority of the surface-level NO3(p) in the morning (here,  ∼  80 %), and a strong influence can persist into the afternoon even when photochemical production is maximum. The particular day-to-day contribution of aloft nocturnal NO3(p) production to surface concentrations will depend on prevailing chemical and meteorological conditions. Although specific to the SJV, the observations and conceptual framework further developed here provide general insights into the evolution of pollution episodes in wintertime environments.

Short summary
This work assesses the processes that control the ambient concentrations of particulate nitrate in the the wintertime San Joaquin Valley of California through a combination of aircraft and surface measurements made during the DISCOVER-AQ study. We provide an observational demonstration of how nocturnal production and advection in aloft layers combines with daytime production and loss from entrainment and deposition to give rise to a distinct diurnal profile in surface nitrate concentrations.
Final-revised paper