Jingqiu Mao, Daniel Jacob Harvard University Jennifer Olson(NASA Langley), Xinrong Ren(U Miami), Bill Brune(Penn State), Paul Wennberg(Caltech), Mike Cubison(U Colorado), Jose Jimenez(U Colorado), Ron Cohen(UC Berkeley), Andy Weinheimer(NCAR), Alan Fried(NCAR), Greg Huey (Gatech)

Tropospheric HO x chemistry GHGs, SOA O3O3 CO from combustion NMHCs from biogenic, fuel emission NO x from anthropogenic, lightning, and soil HO x lifetime ~ 10 mins HO y lifetime ~ 2-6 days

Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) Phase I: April 1 st ~ April 20 th NO,O 3 : Andy Weinheimer(NCAR) NO2, PAN: Ron Cohen(UC Berkeley) OH & HO 2 : Bill Brune(Penn State) H 2 O 2 & MHP: Paul Wennberg(Caltech) Aerosol composition: Jose Jimenez(CU) HCHO: Alan Fried(NCAR) Box modeling: Jennifer Olson (Langley) BrO: Greg Huey(Georgia Tech) ARCTAS

GEOS-Chem V GEOS-5 assimilated met field FLAMBE emission Updated reaction rates with JPL06 and IUPAC06 Updated photolysis cross sections and quantum yield with Fast-JX New acetone photolysis and HNO4 IR photolysis 1 year spin up at 2x2.5 degree Use daily OMI ozone column to calculate photolysis module

Vertical Profile(Observation vs. GEOS-Chem) W. Brune(PSU), P. Wennberg(Caltech), R. Cohen(UCB), A. Weinheimer(NCAR), A. Fried(NCAR)

OH HO 2 Box model results during ARCTAS-A (Olson et al., 2009) Box model uses all in situ measurements as input, including CO, O3, VOCs, photolysis rates, T, RH, …(except OH, HO2 and HCHO), to calculate OH, HO2 and HCHO. Box model presents same problem for HO2 and HCHO.

TOPSE Similar results found in TOPSE Peroxy radicals in TOPSE (Cantrell, 2003) TOPSE was the only aircraft campaign for photochemistry in Arctic before ARCTAS. Discrepancy in H 2 O 2 cannot be explained by wet scavenging considering its chemical lifetime (1~2 days ). H 2 O 2 in TOPSE (Wang, 2003) Obs/Model Model Obs

Reconciling the discrepancy for HO 2 1. BrO? (No) ~5 ppt only changes OH.HO2 is highly buffered. 2. NO x ? (No) 1 molecule BrO = 3 molecule NO, 10ppt NO is not enough. 3. HO 2 uptake to aerosol? Cold temperature High aerosol loading (Arctic haze) (Huey) (Courtesy of J. Olson) HO 2 aerosol With/Without BrO Calculated impact of BrO on OH and HO 2 Altitude, km OHHO

Gas phase species uptake process ① gas to aerosol surface diffusion -gas molecule diffusion coefficient (Dg) and particle size (a). ② gas molecules accommodated on the aerosol surface -mass accommodation coefficient ③ mass transfer between aerosol surface and bulk -particle size and the liquid diffusion coefficient ④ reactions in aerosol bulk -its aqueous concentration and chemical reactions. HO 2 aerosol HO 2 (aq) NH 4 + SO 4 2- HSO 4 - Aqueous reactions NH 4 + HSO 4 - ④①②③ C is gas-phase concentration ν is mean molecular speed A is surface area γ is reactive uptake coefficient D g is gas phase diffusion coefficient a is particle size (important for cloud)

Literature data of γ (HO 2 ) To ensure effective HO2 uptake (γ>0.1): 1.aqueous 2.Cold or Cu-doped H is Henry’s law constant α is mass accommodation coefficient k is the first-order reaction coefficient v is the mean gas molecular speed R is the ideal gas constant T is temperature D is the aqueous-phase diffusion coefficient. If α is close to 1, γ (HO 2 ) is driven by H. H HO2 =9.4x10 -6 *exp (5920/T) M atm -1 If aerosol is aqueous:

Composition of submicron aerosols in Arctic We only focus on submicron aerosols since 95% surface area is contributed by submicron aerosols. We only focus on non-refractory aerosols since refractory aerosols contribute less than 10% of surface area. The majority is OC and Sulfate. The majority form of sulfate is bisulfate, which means generally acidic (pKa (HSO 4 - )= 2.0). Mass fraction

Phase of submicron aerosols in Arctic- (NH 4 ) 2 SO 4 Crystallization RH (CRH) Deliquescence RH (DRH) For Ammonium sulfate (NH4)2SO4, Arctic condition: RH range 40-80%, T- 230~270K. (Onasch et al., 1999) T=295K Weak temperature dependence for CRH and DRH

(Martin et al., 2003) CRH decreases with acidity Phase of submicron aerosols in Arctic- NH4HSO4 No CRH is found when T=260K. Bisulfate is more likely to be aqueous than ammonium sulfate. (Colberg et al., 2004)

OC fraction CRH DRH (NH 4 ) 2 SO 4 +OC (Parsons et al., 2004) Phase of submicron aerosols in Arctic- (NH 4 ) 2 SO 4 + OC Both DRH and CRH decreases when OC fraction increases in aerosols. So the mixture of ammonium sulfate and OC is more likely aqueous than ammonium sulfate itself. This behavior should also be applicable to ammonium bisulfate. So surface area for uptake should be mainly contributed by aqueous aerosols.

Vertical Profile

Calculated γ (HO 2 ) γ (HO2) is about 0.1 at surface and 0.4 in upper troposphere, consistent with literature data.

Fate of HO 2 in aerosol HO 2 HO 2 (aq)+O 2 - (aq) → H 2 O 2 (aq) HSO 4 - SO 5 - H SO 5 - HCOO -, HSO 3 - OH(aq) HSO 3 - SO H + H 2 SO 4 HO 2 -H 2 SO 4 complex ? H 2 O 2 (g) Cycling between HO x and peroxides Pure HO y sink HO 2 is weak acid (pKa ~ 4.7), not much O 2 - (aq) in acidic aerososl

Vertical Profile(Observation vs. GEOSChem)

Scatterplots of GEOS-Chem vs. Observations Good correlation for HO x. Weaker for peroxides due to the low dynamic range. Slight improvement from gas-phase only simulations.

HO x sources and sinks ? HO 2 uptake One way to diagnose in situ measurements based on source= sinks (τ ~ mins). H 2 O 2 + hv is dominant HO x source above 4 km (first time). Sink is only 20% of source in upper troposphere, indicating a large missing sink for HO x (70% with HO 2 uptake). HO 2 +HO 2 /RO 2 dominates below 5km (no OH+HNO 4 or OH + NO 2 as in UT of mid-lat). Sink is only 50% of source in lower troposphere due to the underestimated HCHO.

HO y sources and sinks One way to diagnose in situ measurements (τ ~ 2-6 days). Still missing a sink in upper troposphere if transport is taken into account. Discrepancy in lower troposphere is again due to the underestimated HCHO. HO 2 uptake dominates HO y sinks above 4km. OH+CH 3 OOH dominates HO y sinks below 4km(first time). ? HO 2 uptake

Circumpolar budget analysis for HO y Design regional domain 60N~90N, 30 vertical layers Includes gas phase and aqueous chemical production and loss Aqueous loss - H 2 O 2 + SO 2 (aq) Transport for H 2 O 2 and CH 3 OOH Only matters for these two species due to their lifetimes Wet scavenging is calculated by large scale and convective precipitation fluxes for the specified species (co- condensation for H 2 O 2 ). Dry deposition for H 2 O 2 and CH 3 OOH

HO y budget at different altitude bands Transport is the main supply for HO y in upper troposphere. H 2 O 2 +SO 2 (aq) dominates sink in lower troposphere. OH + peroxides makes a big contribution to HO y budget.

Chain Length of HO x and HO y where L(HO 2 ), L(HO x ) and L(HO y ) respresent the loss rate of HO 2, HO x family and HO y family. O3 Maintain oxidizing power (HO x level)

HO y budget at different altitude bands Inefficient HO x cycling due to low NO x. Inefficient HO y cycling in UT due to HO 2 uptake. Efficient cycling in LT due to low level OH + HO 2. HO y lifetime decrease with altitude, shorter than SONEX due to HO 2 uptake and high level of peroxides.

Main driver of this chemistry is by O ( 1 D)+H 2 O(70%) and transport (30%). HCHO is comparable to O ( 1 D)+H 2 O as an amplifier. Cycling between OH an HO 2 is inefficient due to low NO x (chain length=3.4). Cycling between HO x and peroxides is inefficient due to HO 2 uptake(chain length=1.8). Schematic diagram of HO x -HO y chemistry in Arctic spring Masses (in parentheses) are in units of Mmol. Rates are in units of Mmol d -1.

Sensitivity for oxidizing power– Aerosol perturbations (1) Neutralized aerosol Acidic aerosol Increase OH by 33%, H 2 O 2 by more than 200%. Less SO 2 emission or more NH 3 emission

Surface Area in the model(um 2 cm -3 ) during ARCTAS-A Surface area in the model is mainly from sulfate and OC.

Sensitivity for oxidizing power– Aerosol perturbations (2) If there is no aerosol in Arctic, OH will only increase 12%, but H 2 O 2 decrease by 30%. Oxidizing power is more sensitive to acidity (anthropogenic influence) than surface area change (natural activity). This might help to understand ice core data of H 2 O 2.

Do we need more oxidizing power?

Shut off Anthropogenic emissions

Shut off Biomass burning

Circumpolar budget from April 1 st to 20 th Avg(Gmol /day) H2O2MHP ChemP ChemL(g) Chem(aq)-0.115N/A WetDep DryDep-0.042N/A Transport Net They are in steady state! Chemical lifetime: H 2 O 2 :1~2 days MHP: 1~2 days HCHO: 3~6 hrs