Vertical profiles of O<sub>3</sub> and NO<sub>x</sub> chemistry in the polluted nocturnal boundary layer in Phoenix, AZ: I. Field observations by long-path DOAS

Abstract

Abstract. Nocturnal chemistry in the atmospheric boundary layer plays a key role in determining the initial chemical conditions for photochemistry during the following morning as well as influencing the budgets of O 3 and NO 2 . Despite its importance, chemistry in the nocturnal boundary layer (NBL), especially in heavily polluted urban areas, has received little attention so far, which greatly limits the current understanding of the processes involved. In particular, the influence of vertical mixing on chemical processes gives rise to complex vertical profiles of various reactive trace gases and makes nocturnal chemistry altitude-dependent. The processing of pollutants is thus driven by a complicated, and not well quantified, interplay between chemistry and vertical mixing.In order to gain a better understanding of the altitudedependent nocturnal chemistry in the polluted urban environment, a field study was carried out in the downtown area of Phoenix, AZ, in summer 2001. Vertical profiles of reactive species, such as O 3 , NO 2 , and NO 3 , were observed in the lowest 140 m of the troposphere throughout the night. The disappearance of these trace gas vertical gradients during the morning coincided with the morning transition from a stable NBL to a well-mixed convective layer. The vertical gradients of trace gas levels were found to be dependent on both surface NO x emission strength and the vertical stability of the NBL. The vertical gradients of O x , the sum of O 3 and NO 2 , were found to be much smaller than those of O 3 and NO 2 , revealing the dominant role of NO emissions followed by the O 3 +NO reaction for the altitude-dependence of nocturnal chemistry in urban areas. Dry deposition, direct emissions, and other chemical production pathways of NO 2 also play a role for the O x distribution. Strong positive vertical gradients of NO 3 , that are predominantly determined by NO 3 loss near the ground, were observed. The vertical profiles of NO 3 and the calculated vertical profiles of its reserCorrespondence to: J. Stutz (jochen@atmos.ucla.edu) voir species (N 2 O 5 ) confirm earlier model results suggesting complex vertical distributions of atmospheric denoxification processes during the night.

Validation of farm-scale methane emissions using nocturnal boundary layer budgets

“…In addition, turbulence and wind speed measurements of our eddy covariance system suggested that intermittent turbulent processes affected the thermodynamic structure of the boundary layer. Although local meteorological conditions favoured the build-up of a stable and stratified NBL, turbulent processes within the NBL have also been reported by other studies (Parker and Raman, 1993;Holden et al, 2000;Mahrt and Vickers, 2002;Wang et al, 2006). In addition, some of our wind speed profiles indicated the presence of a low-level jet, which -besides cold air drainagehad a significant impact on vertical transport within the NBL (Mathieu et al, 2005;Pinto et al, 2006;Sun et al, 2007).…”

Diurnal variation of midlatitudinal NO<sub>3</sub> column abundance over table mountain facility, California

“…A number of measurements have confirmed the role of NO 3 -N 2 O 5 chemistry in the nocturnal boundary layer (Aldener et al, 2006;Allan et al, 2000;Ambrose et al, 2007;Ayers and Simpson, 2006;Brown et al, 2003Carslaw et al, 1997a;Geyer et al, 2001;Geyer and Platt, 2002;Li et al, 2008;Matsumoto et al, 2006;Mihelcic et al, 1993;Nakayama et al, 2008;Penkett et al, 2007;Platt et al, 1981;Smith et al, 1995;Stutz et al, 2004;Vrekoussis et al, 2007;Wang et al, 2006). Fewer have probed above the boundary layer, essentially those using LP-DOAS (Carslaw et al, 1997b), aircraft measurements (Brown et al, 2007a,b) and zenith sky measurements at sunrise Coe et al, 2002;von Friedeburg et al, 2002).…”

Four years of ground-based MAX-DOAS observations of HONO and NO<sub>2</sub> in the Beijing area

“…It should be noted that altitude-resolved measurements of trace gases are also possible with long-path DOAS F. Hendrick et al: MAX-DOAS observations of HONO and NO 2 in the Beijing area 767 or in-situ techniques, generally with a better vertical resolution and signal-to-noise ratio than MAX-DOAS, but these require more sophisticated instrumental set up like, for example, placing retro-reflectors or in-situ instruments on different floors of a building (e.g. Stutz et al, 2002;Wang et al, 2006;Wong et al, 2012;Villena et al, 2011).…”