Experimental budgets of OH, HO2, and RO2 radicals and implications for ozone formation in the Pearl River Delta in China 2014
Authors: Tan Zhaofeng , Lu Keding , Hofzumahaus Andreas , Fuchs Hendrik , Bohn Birger , Holland Frank , Liu Yuhan , Rohrer Franz , Shao Min , Sun Kang , Wu Yusheng , Zeng Limin , Zhang Yinsong , Zou Qi , Kiendler-Scharr Astrid , Wahner Andreas , Zhang Yuanhang 
Published: May 2019

Abstract. Hydroxyl (OH) and peroxy radicals (HO2 and RO2)
were measured in the Pearl River Delta, which is one of the most polluted
areas in China, in autumn 2014. The radical observations were complemented
by measurements of OH reactivity (inverse OH lifetime) and a comprehensive
set of trace gases including carbon monoxide (CO), nitrogen oxides (NOx=NO, NO2) and volatile organic compounds (VOCs). OH reactivity was in
the range from 15 to 80 s−1, of which about 50 % was
unexplained by the measured OH reactants. In the 3 weeks of the
campaign, maximum median radical concentrations were 4.5×106 cm−3
for OH at noon and 3×108 and
2.0×108 cm−3 for HO2 and RO2, respectively, in
the early afternoon. The completeness of the daytime radical measurements
made it possible to carry out experimental budget analyses for all radicals
(OH, HO2, and RO2) and their sum (ROx). The maximum loss rates for
OH, HO2, and RO2 reached values between 10 and 15 ppbv h−1
during the daytime. The largest fraction of this can be attributed to radical
interconversion reactions while the real loss rate of ROx remained below
3 ppbv h−1. Within experimental uncertainties, the destruction rates of HO2
and the sum of OH, HO2, and RO2 are balanced by their respective
production rates. In case of RO2, the budget could be closed by
attributing the missing OH reactivity to unmeasured VOCs. Thus, the
presumption of the existence of unmeasured VOCs is supported by RO2
measurements. Although the closure of the RO2 budget is greatly
improved by the additional unmeasured VOCs, a significant imbalance in the
afternoon remains, indicating a missing RO2 sink. In case of OH, the
destruction in the morning is compensated by the quantified OH sources from
photolysis (HONO and O3), ozonolysis of alkenes, and OH recycling
(HO2+NO). In the afternoon, however, the OH budget indicates a
missing OH source of 4 to 6 ppbv h−1. The diurnal variation of the missing OH
source shows a similar pattern to that of the missing RO2 sink so that
both largely compensate each other in the ROx budget. These observations
suggest the existence of a chemical mechanism that converts RO2 to OH
without the involvement of NO, increasing the RO2 loss rate during the daytime
from 5.3 to 7.4 ppbv h−1 on average. The photochemical net ozone
production rate calculated from the reaction of HO2 and RO2 with
NO yields a daily integrated amount of 102 ppbv ozone, with daily integrated
ROx primary sources being 22 ppbv in this campaign. The produced ozone can
be attributed to the oxidation of measured (18 %) and unmeasured (60 %)
hydrocarbons, formaldehyde (14 %), and CO (8 %). An even larger
integrated net ozone production of 140 ppbv would be calculated from the
oxidation rate of VOCs with OH if HO2 and all RO2 radicals
react with NO. However, the unknown RO2 loss (evident in the RO2
budget) causes 30 ppbv less ozone production than would be expected from the
VOC oxidation rate.