Article pubs.acs.org/est
Vertically Resolved Measurements of Nighttime Radical Reservoirs in Los Angeles and Their Contribution to the Urban Radical Budget Cora J. Young,*,†,‡,○ Rebecca A. Washenfelder,†,‡ James M. Roberts,‡ Levi H. Mielke,§,◆ Hans D. Osthoff,§ Catalina Tsai,∥ Olga Pikelnaya,∥ Jochen Stutz,∥ Patrick R. Veres,†,‡,¶ Anthony K. Cochran,⊥,▼ Trevor C. VandenBoer,# James Flynn,∇ Nicole Grossberg,∇ Christine L. Haman,∇ Barry Lefer,∇ Harald Stark,†,‡,● Martin Graus,†,‡ Joost de Gouw,†,‡ Jessica B. Gilman,†,‡ William C. Kuster,‡ and Steven S. Brown‡ †
Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, United States Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80305, United States § Department of Chemistry, University of Calgary, Calgary, Alberta, Canada ∥ Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095, United States ⊥ North Carolina Agricultural and Technical State University, Greensboro, NC 27411, United States # Department of Chemistry, University of Toronto, Toronto, Ontario, Canada ∇ Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204, United States ‡
S Supporting Information *
ABSTRACT: Photolabile nighttime radical reservoirs, such as nitrous acid (HONO) and nitryl chloride (ClNO2), contribute to the oxidizing potential of the atmosphere, particularly in early morning. We present the first vertically resolved measurements of ClNO2, 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 ClNO2 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 ClNO2 source from aerosol uptake of N2O5 throughout the boundary layer and a HONO source from dry deposition of NO2 to the ground surface and subsequent chemical conversion. At ground level, daytime radical formation calculated from nighttime-accumulated HONO and ClNO2 was approximately equal. Incorporating the different vertical distributions by integrating through the boundary and residual layers demonstrated that nighttime-accumulated ClNO2 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.
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INTRODUCTION
reactions with organic compounds initiate the formation of tropospheric O3,3 limit the lifetime of volatile organic compounds (VOCs),4 and drive the formation of secondary aerosols, particularly through oxidation of organics.5 Heterogeneous reactions of gas phase nitrogen oxide species on surfaces lead to formation of both HONO and ClNO2. Specifically, conversion of nitrogen dioxide (NO2) on wet surfaces leads to formation of HONO6 with the following stoichiometry:
Nighttime radical reservoirs are formed through chemical reactions or direct emissions during darkness and provide a photolytic radical source during daylight hours. Photolabile nighttime radical reservoirs typically accumulate during the night and photolyze in the early morning, forming radicals before other major radical sources, including ozone (O3) photolysis, become active. The most-studied radical reservoir in the troposphere to date is nitrous acid (HONO). Though another nighttime radical reservoir, nitryl chloride (ClNO2), has recently been observed in the atmosphere,1,2 its importance as a radical source remains poorly understood. Understanding the relative importance of these early morning radical sources is necessary to our understanding of the atmosphere, as their © 2012 American Chemical Society
Received: Revised: Accepted: Published: 10965
June 1, 2012 September 7, 2012 September 10, 2012 September 26, 2012 dx.doi.org/10.1021/es302206a | Environ. Sci. Technol. 2012, 46, 10965−10973
Environmental Science & Technology
Article
Table 1. Summary of Key Measurements species ClNO2 ClNO2 HONO HONO HONO O3 O3 O3
uncertainty (1σ)a
technique chemical ionization mass spectrometry (CIMS) with iodide ionization CIMS with iodide ionization long path differential optical absorption spectroscopy (LPDOAS) negative ion proton-transfer CIMS (NI-PT-CIMS) incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS) cavity ring-down spectroscopy (CaRDS) LP-DOAS UV differential absorption
HCHO speciated VOCs
LP-DOAS in situ gas chromatography−mass spectrometry (GC-MS)
CHOCHO
IBBCEAS
a
frequency
location/height
ref
±(50 pptv +25%)
2s
P-3 aircraft
1
±30% ±20%
30 s variable
10 m four path heights
26 27
±(20 pptv +30%) ±(52 pptv +30%)
1 min 10 min
3m 10 m
28 29,30
±(28 pptv +2.2%) ±5% ±(400 pptv +4%)
1s variable 1 min
P-3 aircraft four path heights 10 m
±5% ±5−25% (hydrocarbons) ±20−35% (oxygenates)
variable 30 min
four path heights 10 m
25 27 Thermo Scientific, model 49c 27 32
10 min
10 m
30
For uncertainties given as ±(x pptv + y%), x represents the precision, and y represents the accuracy.
2NO2 + H 2O(l) → HONO + HNO3
Calculations of the importance of HONO to primary radical formation vary significantly between locations and measurements.21 Furthermore, no direct comparison exists between ClNO2 and HONO. Few published measurements of ClNO2 exist, and much about the chemistry and distribution remain unknown, including its vertical distribution in the nighttime boundary layer (NBL). As such, ClNO2 has yet to be included in a measurement-based assessment of primary radical formation. Numerous radical budgets have included the contribution of HONO, but few of these have attempted to distinguish between nighttime and daytime HONO.16,21 In order to effectively compare the importance of nighttime radical reservoirs, this study will focus mainly on nighttime accumulated HONO. The impact of daytime HONO will be addressed in a forthcoming publication. The importance of vertical gradients on radical formation has not been considered in previous radical budgets. While many radical precursors, such as O3 and formaldehyde (HCHO), can be considered well-mixed throughout the planetary boundary layer (BL), measurements of HONO show a vertical trend, with HONO concentrations highest near the ground in the nighttime stratified atmosphere.23,24 The sampling height of in situ HONO measurements used to construct primary radical budgets is inconsistent from study to study, which may bias the perceived importance of HONO as a radical source. Until recently, no measurements were available for the vertical profile of ClNO2. The objective of this paper is to describe the following: (i) the first vertical profiles of ClNO2; (ii) the relative importance of ClNO2 and HONO as nighttime radical reservoirs, with explicit comparison between their vertical distributions; and (iii) the importance of ClNO2 and HONO as radical sources relative to other radical sources that influence air quality in Los Angeles.
(1)
while reaction of dinitrogen pentoxide (N2O5) with condensed phase chloride is the source of ClNO2:7 N2O5 + HCl(aq) → ClNO2 + HNO3
(2)
The primary sink for both compounds is photolysis to yield a nitrogen oxide species and radical (eqs 38 and 49): HONO + hν → NO + OH ClNO2 + hν → Cl + NO2
(λ < 405 nm) (λ < 475nm)
(3) (4)
Although formation of HONO through heterogeneous reaction of NO2 can occur at any time of day, it leads to appreciable accumulation of HONO only at night when the photolysis sink is inactive.10 Recent observations of daytime HONO concentrations have indicated that additional daytime sources must exist with a rate of formation more rapid than the nighttime source, though the mechanism for these sources remains highly uncertain.11,12 In contrast, ClNO2 is formed and accumulated primarily at night, because of the chemistry of its precursor, N2O5, which exists in equilibrium with NO3 and NO2: NO2 + NO3 ⇄ N2O5
(5)
The extreme photolability of NO3, along with its rapid reactivity with NO, preclude formation of N2O5 and thus significant amounts of ClNO2 during the day.13,14 Many unknowns limit our understanding of the role that HONO and ClNO2 play in radical budgets. While nighttime radical reservoirs have been included in previous radical budgets, there has not been a comprehensive assessment of these radical sources.15−21 Both HONO and ClNO2 may be regarded as primary radical sources because they form through reactions of NO2 and O3 and not from recycling of other radicals. Although the distinction between primary and secondary (i.e., derived from recycling of other radical species) radicals is often blurred, for simplicity, we compare HONO and ClNO2 to the reaction of O(1D), derived from ozone photolysis, with water vapor.22 We consider the latter to be the major primary radical source, even though formation of ozone does itself require an input of radicals.
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METHODS
The CalNex 2010 campaign was undertaken from May to July 2010, including measurements made from a ship, four aircraft, and two ground sites. Relevant measurement techniques for this work are summarized in Table 1. 10966
dx.doi.org/10.1021/es302206a | Environ. Sci. Technol. 2012, 46, 10965−10973
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Aircraft Measurements. The NOAA P-3 aircraft was based at Ontario International Airport, Ontario, Canada (Supporting Information, Figure S1). The plane completed numerous flights over the Los Angeles basin, including four flights that sampled mainly during nighttime hours, on 30 and 31 May, 1 and 2 June 2010. The aircraft included a measurement of ClNO 2 by chemical ionization mass spectrometry (CIMS) with iodide ionization.1 Other measurements relevant to this analysis included photolysis rate constants14 and O3.25 Ground-Site Measurements. The Los Angeles ground site was located on the campus of the California Institute of Technology in Pasadena, CA (34.140582 N, 118.122455 W, 236 m above sea level, Supporting Information, Figure S1). The site operated from 15 May to 15 June 2010. Measurements of ClNO2 were made at the site using CIMS with iodide ionization26 at a height of 10 m (all measurement heights are reported relative to ground level). Three HONO measurements are included in this work: (1) long-path differential optical absorption spectroscopy (LP-DOAS);27 (2) negativeion proton-transfer CIMS (NI-PT-CIMS)28 with acetate ionization; and (3) incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS).29,30 The NI-PT-CIMS and IBBCEAS measured at heights of 3 and 10 m, respectively. Measurement of HONO by the NI-PT-CIMS required correction for NO2 concentrations, which was done using NO2 detected by chemiluminescence. The light source of the LP-DOAS was mounted on top of a 33 m building approximately 500 m from the main ground site. Retro reflectors were installed on the foothills of the San Gabriel Mountains at four locations northeast of the ground site. Mean interval heights above ground level and distances from the LPDOAS light source were as follows: low (55.5 m, 5.4 km); mid (99.5 m, 5.6 km); upper (188 m, 6.3 km); and highest (405.5 m, 7.0 km). Measurements from the entire LP-DOAS path length were adjusted for specific height intervals as described by Wong et al.27 Ozone and HCHO were also measured by LPDOAS using the same setup. Other relevant chemical measurements at the ground site included the following: O3,31 glyoxal (CHOCHO) by IBBCEAS,30 hydrochloric acid (HCl) by NI-PT-CIMS,28 and speciated volatile organic compounds, including acetaldehyde (CH3CHO) and numerous anthropogenic and biogenic alkenes by in situ gas chromatography coupled to mass spectrometry.32 Photolysis rate constants, including JClNO2, JHONO, JO3, JNO2, JHCHO, JCH3CHO, and JCHOCHO, were determined from measured solar actinic flux spectra.33 The BL height was measured using a ceilometer.34,35 Meteorological measurements, including relative humidity, temperature, and pressure were also made. All ground-site instruments, with the exception of NI-PT-CIMS and LPDOAS, sampled at a height of 10 m. Vertical Atmospheric Dynamics in the Los Angeles Basin. Diurnal changes in the atmospheric structure in Los Angeles are important to understanding the relative importance of radical reservoirs, as these species may or may not be vertically well-mixed. Following conventional description of diurnal boundary layer dynamics,36 we define three vertical structures: (i) BL refers to the turbulent, well-mixed layer that exists during most daylight hours; (ii) the residual layer (RL) is the portion of the atmosphere that represents the previous day’s BL at night and during morning hours as the developing convective boundary layer grows to its height from the previous
day; and (iii) the NBL is a shallow, poorly mixed layer occurring from the surface to, on average, 250−300 m at night and into the morning. At the Pasadena site, the BL was relatively shallow, reaching an average maximum height of approximately 800 m during clear, mid-day conditions. Ceilometer measurements were used to define the height of the BL until it reached its maximum at approximately 15:00 Pacific daylight time (PDT = UT − 7 h). From 15:00 PDT until sunset, the BL was assumed to be well-mixed to 800 m. The RL was assumed to extend from the top of the developing, convective BL to 800 m from sunrise until 15:00 PDT, when the BL also reached this height (Supporting Information, Figure S2). Vertical Profiles. Aircraft measurements were used to determine ClNO2 vertical profiles. During night flights, the P-3 measured vertical profiles during missed approaches to seven airports within the Los Angeles basin. Only inland airports were considered for analysis of ClNO2 profiles (Ontario, Brackett, El Monte, and Chino, Supporting Information, Figure S1). A total of 21 profiles, taken on three flights between 23:00 and 06:00 PDT, were examined for vertical distribution of ClNO2. Data from the 30 May 2010 flight was excluded because of unusually dry conditions (
