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.

Evidence for a nitrous acid (HONO) reservoir at the ground surface in Bakersfield, CA, during CalNex 2010

“…Thus, the impact of the daytime HONO (g) source may be greatest near the ground surface and less important aloft. This is consistent with the results of Young et al [2012], which found that radicals generated from nocturnal HONO (g) sources may have greatest impact on oxidation chemistry near the surface during the first half of the sunlit day. This is further indicated by the aircraft measurements of Zhang et al [2009] showing that there is a volume HONO source required to explain observations at higher altitudes (>100 m agl), which cannot be derived from surface production and transport.…”

“…In some instances, HONO (g) has been shown to be a comparable OH source to the reaction of O( 1 D) with water vapor (R2) in summer months in Los Angeles [Young et al, 2012], Mexico City [Li et al, 2010a;Volkamer et al, 2010], and Santiago, Chile . Improvements in the accuracy and precision of field instrumentation have generated a consensus that HONO (g) is present throughout the day at mixing ratios on the order of tens to hundreds of parts per trillion by volume (pptv) in many regions, implying a strong source of HONO (g) (up to 2 ppbv h…”

“…The production of HONO (g) at night occurs dominantly by the reaction of NO 2 on wet surfaces (R3), although the mechanism relevant to the atmosphere likely differs from that derived in laboratory investigations [Barney and Finlayson-Pitts, 2000;Finlayson-Pitts, 2009;Finlayson-Pitts et al, 2003;Kamboures et al, 2008;Miller et al, 2009]. The ground surface has been suggested as the primary source of nighttime HONO (g) from vertically resolved data VandenBoer et al, 2013;Villena et al, 2011;Vogel et al, 2003;Wong et al, 2011;Young et al, 2012;Zhang et al, 2009], but HONO (g) production on ground surface proxies has not been investigated extensively. Measurements of particulate nitrite (NO 2 À (p) ) may help to understand HONO (g) formation and loss processes.…”

A broadband cavity enhanced absorption spectrometer for aircraft measurements of glyoxal, methylglyoxal, nitrous acid, nitrogen dioxide, and water vapor

“…Groundand ship-based BBCEAS field instruments have been used to measure CHOCHO and NO 2 (Washenfelder et al, 2011a;Coburn et al, 2014), HONO and NO 2 (Washenfelder et al, 2011a;Young et al, 2012), NO 3 , and N 2 O 5 (Benton et al, 2010). Kennedy et al (2011) have recently described an aircraft BBCEAS instrument to measure NO 2 , NO 3 , and N 2 O 5 .…”

“…Scientific questions remain about its sources, sinks, and vertical profile (Zhou et al, 1999;Su et al, 2008;Young et al, 2012;VandenBoer et al, 2013). Existing detection methods can be categorized as wet chemistry, mass spectrometry, and optical spectroscopy.…”