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<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2021-08-09T21:25:30.5129875+00:00 -->
<!-- Version            : 5.0.1.648 -->
<!-- Package ID         : 54f7a76f3eba4cc1bf89213e5b9a1e06 -->
<!-- Copyright (c) 2021 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="arct_OctoberTwoThousandAndSeventeenLeaseAmendmentMember" name="OctoberTwoThousandAndSeventeenLeaseAmendmentMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_GrantReceivableNumberOfInstallments" name="GrantReceivableNumberOfInstallments" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_PaymentReceivedFirstInstallment" name="PaymentReceivedFirstInstallment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="arct_PaymentsForMatchingFundsForRemainingBudgetedCosts" name="PaymentsForMatchingFundsForRemainingBudgetedCosts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_DisbursedAmountUponExecutionOfAmendment" name="DisbursedAmountUponExecutionOfAmendment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_DisbursementAmountPayableUponAchievementOfProjectGoal" name="DisbursementAmountPayableUponAchievementOfProjectGoal" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_FinalPaymentOfDisbursementAmount" name="FinalPaymentOfDisbursementAmount" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_DisbursementPaymentUponAchievementOfRequiredManufacturingPracticesAndINDApplication" name="DisbursementPaymentUponAchievementOfRequiredManufacturingPracticesAndINDApplication" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="arct_ContraExpenseIncludedInResearchAndDevelopmentExpense" name="ContraExpenseIncludedInResearchAndDevelopmentExpense" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="arct_MonthlyBaseRent" name="MonthlyBaseRent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="arct_CommonStockSharesRestrictedFromSellingPeriodSubsequentToIssuanceDate" name="CommonStockSharesRestrictedFromSellingPeriodSubsequentToIssuanceDate" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="arct_ADAIRTechnologyMember" name="ADAIRTechnologyMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="arct_NonRefundableDepositAmount" name="NonRefundableDepositAmount" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="arct_UpfrontPaymentReceivable" name="UpfrontPaymentReceivable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit" />
  <xsd:element id="arct_VingroupAgreementMember" name="VingroupAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
</xsd:schema>
