Explain the trends depicted in Fig. 2 from Hunter et al. Nature Geo. (2017). (b) Write a

general, qualitative rule of thumb for predicting whether an organic compound will be important

for ozone formation or secondary organic aerosol production.

11 10 9 8 Formic acid Acetic acid Methanol Carbon mass concentration (ugC m ) Oxalic acid Ethanol Benzene Toluene Methyl ethyl ketone Xylenes Isoprene CH,0 Other terpenolds Methylglyoxal 6 FOOD ontMNO 2-methyl- 3-buten-2-ol (MBO) Acetone Monoterpenes Methacrolein/MVK Acetaldehyde OVOC S/IVOC VOC LL-VOC b 5.5 5.0+ TD-AMS SV-TAG 4.5 TD-EIMS CIMS (g) PTR-MS 4.0- 3.5 3.0 2.5 - Isoprene OH reactivity (s) 2.0 - 2-methyl- 3-buten-2-ol (MBO) CH, T 1.5 1.0 0.5 0.0 Methacrolein/MV Acetaldehyde OVOC Monoterpenes VOC OA S/IVOC LL-VOC 1.8 1.6 1.4 1.2 1.0 Max. SOA formation (ugCm 0.8 0.6 0.4 0.2 0.0 Toluene Monoterpenes VOC (OA) S/IVOC OVOC LL-VOC Figure 2 | Total observed organic carbon concentrations, calculated OH reactivity (OHR), and SOA formation, coloured by instrument and organized into major classes of organic species. Gas-phase species are classified into four categories: LL-VOCs (t > 1d), S/IVOCs (t 10'ugm-3, and OSC 10?ug m-3, and OSC 2-1); error bars, summed for each category, are lo. Unlabelled boxes indicate unidentified species or ensemble measurements. It is assumed there is no overlap in measurements by different instruments, so all values represent upper limits. Values of OHR and SOA are calculated as described in the Methods