Oxidative capacity of the Mexico City atmosphere – Part 1: A radical source perspective

Abstract

Abstract.A detailed analysis of OH, HO 2 and RO 2 radical sources is presented for the near field photochemical regime inside the Mexico City Metropolitan Area (MCMA). During spring of 2003 (MCMA-2003 field campaign) an extensive set of measurements was collected to quantify timeresolved RO x (sum of OH, HO 2 , RO 2 ) radical production rates from day-and nighttime radical sources. The Master Chemical Mechanism (MCMv3.1) was constrained by measurements of (1) concentration time-profiles of photosensitive radical precursors, i.e., nitrous acid (HONO), formaldehyde (HCHO), ozone (O 3 ), glyoxal (CHOCHO), and other oxygenated volatile organic compounds (OVOCs); (2) respective photolysis-frequencies (J-values); (3) concentration time-profiles of alkanes, alkenes, and aromatic VOCs (103 compound are treated) and oxidants, i.e., OH-and NO 3 radicals, O 3 ; and (4) NO, NO 2 , meteorological and other parameters. The RO x production rate was calculated directly from these observations; the MCM was used to estimate further RO x production from unconstrained sources, and express overall RO x production as OH-equivalents (i.e., taking into account the propagation efficiencies of RO 2 and HO 2 radicals into OH radicals).Daytime radical production is found to be about 10-25 times higher than at night; it does not track the abundance of sunlight. 12-h average daytime contributions of individual sources are: Oxygenated VOC other than HCHO about 33%;Correspondence to: R. Volkamer (rainer.volkamer@colorado.edu) HCHO and O 3 photolysis each about 20%; O 3 /alkene reactions and HONO photolysis each about 12%, other sources <3%. Nitryl chloride photolysis could potentially contribute ∼15% additional radicals, while NO 2 * + water makes -if any -a very small contribution (∼2%). The peak radical production of ∼7.5 10 7 molec cm −3 s −1 is found already at 10:00 a.m., i.e., more than 2.5 h before solar noon. O 3 /alkene reactions are indirectly responsible for ∼33% of these radicals. Our measurements and analysis comprise a database that enables testing of the representation of radical sources and radical chain reactions in photochemical models.Since the photochemical processing of pollutants in the MCMA is radical limited, our analysis identifies the drivers for ozone and SOA formation. We conclude that reductions in VOC emissions provide an efficient opportunity to reduce peak concentrations of these secondary pollutants, because (1) about 70% of radical production is linked to VOC precursors; (2) lowering the VOC/NO x ratio has the further benefit of reducing the radical re-cycling efficiency from radical chain reactions (chemical amplification of radical sources); (3) a positive feedback is identified: lowering the rate of radical production from organic precursors also reduces that from inorganic precursors, like ozone, as pollution export from the MCMA caps the amount of ozone that accumulates at a lower rate inside the MCMA. Continued VOC reductions will in the future result in decreasing peak concentrations of ozone and SOA in...

Modeling of HCHO and CHOCHO at a semi-rural site in southern China during the PRIDE-PRD2006 campaign

“…The top-10 precursor NMHCs of HCHO and CHOCHO in terms of their production rate are listed in Table 2. Compared to similar studies (e.g., Volkamer et al, 2010;Huisman et al, 2011;Washenfelder et al, 2011;MacDonald et al, 2012;Parrish et al, 2012), anthropogenic and biogenic sources contribute almost equally to the chemical formation of HCHO and CHOCHO at the BG site. However, production of HCHO and CHOCHO from anthropogenic precursors is larger than from isoprene before noon; in the afternoon, the contribution of isoprene to HCHO and CHOCHO production becomes higher than from anthropogenic NMHCs.…”

Measurements of nitrous acid (HONO) in urban area of Shanghai, China

“…The fast photolysis of nitrous acid (HONO) plays an important role in the tropospheric photochemistry, especially in the polluted urban areas, leading to the production of OH radicals and subsequently to ozone formation (Aumont et al 2003;Stemmler et al 2006;Volkamer et al 2010). HONO photolysis could account for up to 30-60 % of the morning OH production in urban areas (Alicke et al 2003;Elshorbany et al 2009).…”

Radical chemistry at night: comparisons between observed and modelled HO&lt;sub&gt;x&lt;/sub&gt;, NO&lt;sub&gt;3&lt;/sub&gt; and N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt; during the RONOCO project

“…Measurements of NO 3 have been overestimated by model calculations in several studies (Mihelcic et al, 1993;Sommariva et al, , 2007, with those of nighttime OH and HO 2 radicals typically underestimated, indicating poor understanding of nighttime tropospheric oxidation processes (Kanaya et al, 1999(Kanaya et al, , 2002(Kanaya et al, , 2007aEmmerson and Carslaw, 2009;Geyer et al, 2003;Faloona et al, 2001;Martinez et al, 2003;Ren et al, 2006). While a number of nighttime studies at ground level close to local sources of NO have observed a limited role of NO 3 in nighttime radical production owing to surface losses of NO 3 and the rapid reaction between NO 3 and NO (Salisbury et al, 2001;Fleming et al, 2006;Sommariva et al, 2007;Kanaya et al, 1999Kanaya et al, , 2002Kanaya et al, , 2007aEmmerson and Carslaw, 2009;Faloona et al, 2001;Martinez et al, 2003;Ren et al, 2003Ren et al, , 2005Ren et al, , 2006Volkamer et al, 2010), several studies of NO 3 and N 2 O 5 above ground level and in more remote regions have indicated a more significant role for NO 3 in nighttime radical production and tropospheric oxidation (Platt et al, 1980;Povey et al, 1998;South et al, 1998;Aliwell et al, 1998;Allan et al, 2002;Stutz et al, 2004;Brown et al, 2003Brown et al, , 2004Brown et al, , 2006Brown et al, , 2007Brown et al, , 2009...…”

“…Ozonolysis has been investigated in a range of studies in laboratory, chamber and field studies (Salisbury et al, 2001;Fleming et al, 2006;Sommariva et al, 2007;Kanaya et al, 1999Kanaya et al, , 2002Kanaya et al, , 2007aGeyer et al, 2003;Malkin et al, 2010;Johnson and Marston, 2008), and has been shown to be responsible for production of OH and HO 2 radicals at night (Salisbury et al, 2001;Fleming et al, 2006;Sommariva et al, 2007;Kanaya et al, 1999Kanaya et al, , 2002Kanaya et al, , 2007aEmmerson and Carslaw, 2009;Ren et al, 2003Ren et al, , 2006Volkamer et al, 2010).…”