Urban photochemistry in central Tokyo: 1. Observed and modeled OH and HO₂ radical concentrations during the winter and summer of 2004

Abstract

[1] We used laser-induced fluorescence to measure the concentrations of OH and HO 2 radicals in central Tokyo during two intensive campaigns (IMPACT IVand IMPACT L) in January-February and July-August 2004. The estimated detection limit for the 10-min data was 1.3 Â 10 5 cm À3 for the nighttime and 5.2 Â 10 5 cm À3 for the daytime. The median values of the daytime peak concentrations of HO 2 were 1.1 and 5.7 pptv for the winter and summer periods, respectively, while the values for OH were 1.5 Â 10 6 and 6.3 Â 10 6 cm À3 . High HO 2 mixing ratios (>50 pptv) were observed on a day in summer when O 3 mixing ratios exceeded 100 ppbv. The average nighttime concentrations of HO 2 were 0.7 and 2.6 pptv for the winter and summer periods, respectively, while the values for OH were 1.8 Â 10 5 and 3.7 Â 10 5 cm À3 . A photochemical box model constrained by ancillary observations was able to reproduce daytime OH concentrations reasonably well for both periods, although daytime HO 2 concentrations were underestimated in winter and overestimated in summer. Increasing the wintertime hydrocarbon concentrations in the model led to an increase in daytime HO 2 concentrations, thereby showing better agreement with observations; however, the model continued to underestimate HO 2 concentrations at high NO mixing ratios. This underestimate was most pronounced in the mornings of both periods and during the daytime in winter. We studied processes that are capable of explaining this discrepancy, including unknown reactions of HNO 4 or an unidentified HO x source that is linearly scalable to the NO mixing ratio. The important processes in terms of producing radicals were the olefin + O 3 reactions in the nighttime of both periods and during the daytime in winter, the photolysis of carbonyls in the daytime for both periods, and the photolysis of HONO during the daytime in winter (using measured HONO concentrations) and during mornings in summer (using estimated HONO concentrations).

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

“…They obtained a mean concentration of 0.5 ppb at noontime (11:00-14:00 LT). The observed HONO daytime concentration fell within the range of the previous observations in urban areas and megacities worldwide, 0.05-2.0 ppb (Alicke et al 2002;Acker et al 2006;Hao et al 2006;Kanaya et al 2007;Elshorbany et al 2009;Qin et al 2009). …”

“…For comparison, the field measurement conducted by Hao et al (2006) in October 2004 to January 2005 at the same location led to daytime and nighttime HONO/NO 2 ratios of 0.022 and 0.058, respectively. The obtained HONO/NO 2 ratios were in line with those observed in many urban areas in the world during daytime (HONO/NO 2 = 0.003-0.075) and nighttime (HONO/NO 2 = 0.025-0.175) measurements (Alicke et al 2002;Acker et al 2006;Hao et al 2006;Kanaya et al 2007;Elshorbany et al 2009;Qin et al 2009;Tong et al 2015).…”

Understanding in situ ozone production in the summertime through radical observations and modelling studies during the Clean air for London project (ClearfLo)

“…Observations of OH and HO2 in the urban atmosphere have primarily been 5 made using fluorescence assay by gas expansion (FAGE), and comparisons with predicted radical concentrations using chemistry box models constrained with co-located radical precursor measurements have revealed varying levels of success in replicating observations. Radical concentrations have been reported to be under-predicted by models (Ren et al, 2003;Martinez et al, 2003;Emmerson et al, 2005a;Chen et al, 2010;Lu et al, 2012;Lu et al, 2013), over-predicted (George et al, 1999;Konrad et al, 2003;Dusanter et al, 2009) and, at times, models and measurements have been reported to be in 10 reasonable agreement, to within 40%, (Shirley et al, 2006;Emmerson et al, 2007;Kanaya et al, 2007;Sheehy et al, 2010;Elshorbany et al, 2012;Ren et al, 2013;Griffith et al, 2016). Often the level of agreement observed was found to be dependent on time of day (Brune et al, 2016); with poorest agreement between modelled and measured OH concentrations generally observed during the night.…”

“…Ozonolysis reactions have been reported as important primary radical sources during a number of studies, for example 25 these reactions accounting for 67% of the OH initiation in Birmingham during the PUMA campaign in winter (Emmerson et al, 2005b) whilst in Tokyo during the IMPACT campaign ozonolysis reactions were the dominant radical source during the night-time in winter (Kanaya et al, 2007).…”

“…Urban radical measurements can be used to estimate local ozone production (Kanaya et al, 2007;Ren et al, 2013;Brune et al, 2016) by approximating the rate of ozone production to the production rate of NO2 from the reaction of NO with HO2 and RO2 radicals, and assuming instantaneous O3 production following photolysis of NO2 at wavelengths <400 nm. Any loss of NO2 which does not yield O3, for example the reaction of OH or RO2 radicals with NO2, and also deposition, should also be 25 considered:…”