Abstract. Isoprene hydroxynitrates (IN) are tracers of the photochemical oxidation of isoprene in high NOx environments. Production and loss of IN have a significant influence on the NOx cycle and tropospheric O3 chemistry. To better understand IN chemistry, a series of photochemical reaction chamber experiments was conducted to determine the IN yield from isoprene photooxidation at high NO concentrations (> 100 ppt). By combining experimental data and calculated isomer distributions, a total IN yield of 9(+4/−3) % was derived. The result was applied in a zero-dimensional model to simulate production and loss of ambient IN observed in a temperate forest atmosphere, during the Southern Oxidant and Aerosol Study (SOAS) field campaign, from 27 May to 11 July 2013. The 9 % yield was consistent with the observed IN/(MVK+MACR) ratios observed during SOAS. By comparing field observations with model simulations, we identified NO as the limiting factor for ambient IN production during SOAS, but vertical mixing at dawn might also contribute (~ 27 %) to IN dynamics. A close examination of isoprene's oxidation products indicates that its oxidation transitioned from a high-NO dominant chemical regime in the morning into a low-NO dominant regime in the afternoon. A significant amount of IN produced in the morning high NO regime could be oxidized in the low NO regime, and a possible reaction scheme was proposed.

Abstract. Isoprene hydroxynitrates (IN) are tracers of the photochemical oxidation of isoprene in high NOx environments. Production and loss of IN have a significant influence on the NOx cycle and tropospheric O3 chemistry. To better understand IN chemistry, a series of photochemical reaction chamber experiments were conducted to determine the IN yield from isoprene photooxidation at high NO concentrations (> 100 ppt). By combining experimental data and calculated isomer distributions, a total IN yield of 9(+4/−3) % was derived. The result was applied in a zero-dimensional model to simulate production and loss of ambient IN observed in a temperate forest atmosphere, during the Southern Oxidant and Aerosol Study (SOAS) field campaign, from 27 May to 11 July 2013. By comparing field observations with model simulations, we identified NO as the limiting factor for ambient IN production during SOAS, but vertical mixing at dawn might also contribute (~ 27 %) to IN dynamics. A close examination of isoprene's oxidation products indicates that its oxidation transitioned from a high-NO dominant chemical regime in the morning into a low-NO dominant regime in the afternoon. A significant amount of IN produced in the morning high NO regime could be oxidized in the low NO regime, and a possible reaction scheme was proposed.

Gas-phase atmospheric concentrations of peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), and peroxymethacryloyl nitrate (MPAN) were measured on the ground using a gas chromatograph electron capture detector (GC-ECD) during the Southern Oxidants and Aerosols Study (SOAS) 2013 campaign (1 June to 15 July 2013) in Centreville, Alabama, in order to study biosphere-atmosphere interactions. Average levels of PAN, PPN, and MPAN were 169, 5, and 9 pptv, respectively, and the sum accounts for an average of 16 % of NO y during the daytime (10:00 to 16:00 local time). Higher concentrations were seen on average in air that came to the site from the urban NO x sources to the north. PAN levels were the lowest observed in ground measurements over the past two decades in the southeastern US. A multiple regression analysis indicates that biogenic volatile organic compounds (VOCs) account for 66 % of PAN formation during this study. Comparison of this value with a 0-D model simulation of peroxyacetyl radical production indicates that at least 50 % of PAN formation is due to isoprene oxidation. MPAN has a statistical correlation with isoprene hydroxynitrates (IN). Organic aerosol mass increases with gas-phase MPAN and IN concentrations, but the mass of organic nitrates in particles is largely unrelated to MPAN.

24Gas-phase atmospheric concentrations of PAN, PPN, and MPAN were measured at the ground using GC-25 ECD during the SOAS 2013 campaign (1 June to 15 July 2013) in Centerville, Alabama in order to study 26 biosphere-atmosphere interactions. Average levels of PAN, PPN and MPAN were 169, 5, and 9 pptv 27 respectively, and the sum accounts for an average of 15% of NO y during the daytime (10 am to 4 pm local 28 time). Higher concentrations were seen on average in air that came to the site from the urban NO x sources 29 to the north. PAN levels were the lowest observed in ground measurements over the past two decades in 30 the Southeastern U.S. Analysis of PAN/NO x indicates PAN production in this region was sensitive to NO x . 31A multiple regression analysis indicates that biogenic VOCs account for 66% of PAN formation in this 32 region of the Southeastern U.S. Comparison of this value with a 0-D model simulation of peroxyacetyl 33 radical production indicates that at least 50% of PAN formation is due to isoprene oxidation. MPAN has a 34 statistical correlation with isoprene hydroxynitrates (IN) with an average MPAN ppb /IN ppb ratio of 0.3. 35Organic aerosol mass increases with gas-phase MPAN and IN concentrations, but the mass of organic 36Atmos. Chem. Phys. Discuss., https://doi

Figure S1. Relationship of total PANs with other research groups during SOAS 2013 campaign. The means with standard deviation of PANs from ARA, UC, and WMU were 0.129±0.092, 0.245±0.194, and 0.138±0.119 ppb respectively. The medians of PANs from ARA, UC, and WMU were 0.111, 0.204, and 0.103 ppb respectively.