OH and HO₂ concentrations, sources, and loss rates during the Southern Oxidants Study in Nashville, Tennessee, summer 1999

Abstract

[1] OH and HO 2 mixing ratios and total OH reactivity were measured together with photolysis frequencies, NO x , O 3 , many VOCs, and other trace gases during the midsummer 1999 SOS campaign in Nashville, Tennessee. These measurements provided an excellent opportunity to study OH and HO 2 (collectively called HO x ), and their sources and sinks in a polluted metropolitan environment. HO x generally showed the expected diurnal evolution, with maxima around noon of up to about 0.8 pptv of OH and 80 pptv of HO 2 during sunny days. Overall, daytime observed OH and HO 2 were a factor of 1.33 and 1.56 times modeled values, within the combined 2s instrument and model uncertainties. The chain length of HO x , which is determined from the ratio of the measured total OH reactivity that cycles OH to the total HO x loss, was on average 3-8 during daytime and up to 3 during nighttime, in general agreement with expectations. However, differences occurred between observed HO x behavior and expectations from theory and models. First, HO 2 was greater than expected during daytime when NO mixing ratios were high; ozone production did not decrease as expected when NO was greater than 2 ppbv. Ozone production determined by the imbalance of the NO x photostationary state, which was almost twice that from HO 2 , also shows this dependence on NO. Second, the calculated OH production rate, which should equal the measured OH loss rate because OH is in steady state, is instead less than the measured OH loss rate by (1-2) Â 10 7 molecules cm -3 s -1 , with low statistical significance during the day and high statistical significance at night. Third, surprisingly high OH and HO 2 mixing ratios were often observed during nighttime. The nighttime OH mixing ratio and the HO 2 /OH ratio cannot be explained by known reaction mechanisms, even those involving O 3 and alkenes. Because instrument tests have failed to reveal any instrument artifacts, more exotic chemicals or chemistry, such as OH adducts or other radicals that fall apart into OH inside the instrument, are suspected.

Hydroxyl radicals in the tropical troposphere over the Suriname rainforest: airborne measurements

“…Observations by Tan et al (2001) in a deciduous forest with low NO x and high isoprene emissions were a factor 2.7 in average higher than modelled, while HO 2 was in good agreement. Thornton et al (2002) suggested a reduction of the HO x chain termination reaction of HO 2 with RO 2 yielding ROOH by about a factor of 10 in order to explain measurements in Nashville, Tennessee, in a suburban environment with a high biogenic VOC load, where measured OH and HO 2 were in average 36% and 55% higher than simulated by a constrained box-model (Martinez et al, 2003). Kuhn et al (2007) indirectly inferred a range of OH concentrations during daytime of 3-8×10 6 cm −3 from vertical gradients of isoprene, methyl vinyl ketone and methacrolein in Brazil.…”

Constraints on instantaneous ozone production rates and regimes during DOMINO derived using in-situ OH reactivity measurements

“…Amongst the available direct OH reactivity measurement dataset in the literature, the sites that have been studied encompass urban, suburban and forested sites in the United States (e.g. Di Carlo et al, 2004;Martinez et al, 2003;Ren et al, 2006), Japan (e.g. Sadanaga et al, 2004a;Yoshino et al, 2006), China (e.g.…”

Radical chemistry at night: comparisons between observed and modelled HO<sub>x</sub>, NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub> 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…”