title: Vertically resolved measurements of nighttime radical reservoirs in Los Angeles and their contribution to the urban radical budget.
abstract: Photolabile nighttime radical reservoirs, such as nitrous acid (HONO) and nitryl chloride (ClNO(2)), contribute to the oxidizing potential of the atmosphere, particularly in early morning. We present the first vertically resolved measurements of ClNO(2), together with vertically resolved measurements of HONO. These measurements were acquired during the California Nexus (CalNex) campaign in the Los Angeles basin in spring 2010. Average profiles of ClNO(2) exhibited no significant dependence on height within the boundary layer and residual layer, although individual vertical profiles did show variability. By contrast, nitrous acid was strongly enhanced near the ground surface with much smaller concentrations aloft. These observations are consistent with a ClNO(2) source from aerosol uptake of N(2)O(5) throughout the boundary layer and a HONO source from dry deposition of NO(2) to the ground surface and subsequent chemical conversion. At ground level, daytime radical formation calculated from nighttime-accumulated HONO and ClNO(2) was approximately equal. Incorporating the different vertical distributions by integrating through the boundary and residual layers demonstrated that nighttime-accumulated ClNO(2) produced nine times as many radicals as nighttime-accumulated HONO. A comprehensive radical budget at ground level demonstrated that nighttime radical reservoirs accounted for 8% of total radicals formed and that they were the dominant radical source between sunrise and 09:00 Pacific daylight time (PDT). These data show that vertical gradients of radical precursors should be taken into account in radical budgets, particularly with respect to HONO.
authors.affiliation: Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, USA. cora.young@mun.ca
authors.forename: Cora J
authors.initials: CJ
authors.lastname: Young
authors.forename: Rebecca A
authors.initials: RA
authors.lastname: Washenfelder
authors.forename: James M
authors.initials: JM
authors.lastname: Roberts
authors.forename: Levi H
authors.initials: LH
authors.lastname: Mielke
authors.forename: Hans D
authors.initials: HD
authors.lastname: Osthoff
authors.forename: Catalina
authors.initials: C
authors.lastname: Tsai
authors.forename: Olga
authors.initials: O
authors.lastname: Pikelnaya
authors.forename: Jochen
authors.initials: J
authors.lastname: Stutz
authors.forename: Patrick R
authors.initials: PR
authors.lastname: Veres
authors.forename: Anthony K
authors.initials: AK
authors.lastname: Cochran
authors.forename: Trevor C
authors.initials: TC
authors.lastname: VandenBoer
authors.forename: James
authors.initials: J
authors.lastname: Flynn
authors.forename: Nicole
authors.initials: N
authors.lastname: Grossberg
authors.forename: Christine L
authors.initials: CL
authors.lastname: Haman
authors.forename: Barry
authors.initials: B
authors.lastname: Lefer
authors.forename: Harald
authors.initials: H
authors.lastname: Stark
authors.forename: Martin
authors.initials: M
authors.lastname: Graus
authors.forename: Joost
authors.initials: J
authors.lastname: de Gouw
authors.forename: Jessica B
authors.initials: JB
authors.lastname: Gilman
authors.forename: William C
authors.initials: WC
authors.lastname: Kuster
authors.forename: Steven S
authors.initials: SS
authors.lastname: Brown
mesh_terms: D000393:Air Pollutants / Q000032:analysis*; D000397:Air Pollution / Q000706:statistics & numerical data; D001272:Atmosphere / Q000737:chemistry; D004784:Environmental Monitoring; D005609:Free Radicals / Q000032:analysis*; D015141:Los Angeles; D009573:Nitrites / Q000032:analysis; D009608:Nitrous Acid / Q000032:analysis
chemical_list: D000393:Air Pollutants; D005609:Free Radicals; D009573:Nitrites; D009608:Nitrous Acid; C445660:nitryl chloride