Isoprene photochemistry over the Amazon rainforest

Abstract

Isoprene photooxidation is a major driver of atmospheric chemistry over forested regions. Isoprene reacts with hydroxyl radicals (OH) and molecular oxygen to produce isoprene peroxy radicals (ISOPOO). These radicals can react with hydroperoxyl radicals (HO 2 ) to dominantly produce hydroxyhydroperoxides (ISOPOOH). They can also react with nitric oxide (NO) to largely produce methyl vinyl ketone (MVK) and methacrolein (MACR). Unimolecular isomerization and bimolecular reactions with organic peroxy radicals are also possible. There is uncertainty about the relative importance of each of these pathways in the atmosphere and possible changes because of anthropogenic pollution. Herein, measurements of ISOPOOH and MVK + MACR concentrations are reported over the central region of the Amazon basin during the wet season. The research site, downwind of an urban region, intercepted both background and polluted air masses during the GoAmazon2014/5 Experiment. Under background conditions, the confidence interval for the ratio of the ISOPOOH concentration to that of MVK + MACR spanned 0.4-0.6. This result implies a ratio of the reaction rate of ISOPOO with HO 2 to that with NO of approximately unity. A value of unity is significantly smaller than simulated at present by global chemical transport models for this important, nominally low-NO, forested region of Earth. Under polluted conditions, when the concentrations of reactive nitrogen compounds were high (>1 ppb), ISOPOOH concentrations dropped below the instrumental detection limit (<60 ppt). This abrupt shift in isoprene photooxidation, sparked by human activities, speaks to ongoing and possible future changes in the photochemistry active over the Amazon rainforest.isoprene photochemistry | Amazon | organic hydroperoxides

Constraining nucleation, condensation, and chemistry in oxidation flow reactors using size-distribution measurements and aerosol microphysical modelling

“…The GoAmazon2014/5 field campaign (referred to as GoAmazon hereafter) took place in the area surrounding Manaus, Brazil, in central Amazonia (Martin et al, 2016;, investigating the complex interactions between urban, biomass burning, and biogenic emissions. OFR measurements of SOA formation from OH oxidation of ambient air (and also O 3 -only 25 oxidation, not investigated here) were taken at the "T3" site downwind of Manaus during two intensive operating periods (IOP1 during the wet season and IOP2 in the dry season) to study the contributions of the various emission sources to potential SOA formation.…”

“…The dry season results were chosen for investigation in this study due to the generally larger concentrations of gases, particles, and potential SOA formation than during the wet season. Whereas SOA formation at the BEACHON site was dominated by a single source type (biogenic gases, related to MT), the "T3" site was influenced by a 30 complex mixture of biogenic and anthropogenic emissions (Martin et al 2016, Palm et al 2018 and SOA formation after OH oxidation in the OFR were sampled by an AMS and an SMPS. Ambient SO 2 concentrations were sampled using a Thermo Fisher Model 43i-TLE SO 2 Analyzer.…”

“…day of aging. During IOP2, it was observed that isoprene would peak during the day around 3-4 pm local time and MT would peak later, around 6 pm local time (Liu et al, 2016;Martin et al, 2016). 10…”

“…Traditionally, SOA was viewed as a liquid mixture; however, SOA have been observed in solid and amorphous phases in both laboratory and field studies (Virtanen et al, 2010;. Measurements taken in 2013 and during the GoAmazon2014/5 campaign (Martin et al, 2016; found that SOA produced from oxidation products from the Amazonian rainforest tended to be primarily liquid 25 whereas SOA influenced by anthropogenic emissions (both from the Manaus pollution plume and biomass burning) tended to have higher fractions of semisolid and solid aerosol (Bateman et al, 2015;. Mixing in these solid or amorphous phases could decrease (Cappa et al, 2011;Vaden et al, 2011), leading to decreases in gas-particle partitioning rates (Shiraiwa and Seinfeld, 2012).…”

“…In the first, an OFR was deployed during the BEACHON-RoMBAS field campaign (Ortega et al, 2014) that 15 took place in a montane ponderosa pine forest in Colorado, USA during July-August 2011. The second is the GoAmazon2014/5 field campaign (Martin et al, 2016; that occurred from January 2014-December 2015 in the State of Amazonia, Brazil, in the central Amazon basin. OFR data from each of these two campaigns have been analyzed in previous work 2017a;Hunter et al, 2017) to understand the bulk OA mass and chemical evolution in the OFR.…”