Gridview Column Sort Direction

How can change the sort direction on a GridView column?  I want to be able to just click on a gridview column header to change the sort direction.  Right now it will only sort asc.  

Col1  Col2  Col3       ---> I want to be able to sort the column.
  2      331  900.00
  3      333  940.00


<asp:GridView ID="dgd_Alerts" runat="server"
        AllowPaging="True"
        AllowSorting=true
        AutoGenerateColumns="False"
        datakeynames="USER_ACCT_ALERT_ID,ALERT_TYPE_ID,ALERT_OPER_ID"  
        BackColor="#CCCCCC"  
        OnRowCancelingEdit="Row_Cancel"
        OnPageIndexChanging="gridView_PageIndexChanging"  
        OnRowCommand="Row_Selected"
        onrowupdated="Row_Updated"
         OnRowEditing="Row_Editing"
        OnRowUpdating="Row_Updating"
        OnRowDeleting="RowDeleting"
        onsorting="GridView_Sorting"

        PagerStyle-CssClass="PagerRowStyle"  
        PagerSettings-Mode="NumericFirstLast"  
        PagerSettings-PageButtonCount="5"
        Width="476px"
        AutoGenerateEditButton="True"
        AutoGenerateDeleteButton="True"  >
        <Columns>
            <asp:BoundField DataField="ALERT_TYPE_TEXT" SortExpression="ALERT_TYPE_TEXT" HeaderText="Type" ReadOnly=True/>
            <asp:BoundField DataField="ALERT_OPER_TEXT"  SortExpression="ALERT_OPER_TEXT" HeaderText="Operator"  ReadOnly=True/>
            <asp:TemplateField HeaderText="Threshold" SortExpression="ALERT_THRESHOLD_AMT">
                <EditItemTemplate>
                    <asp:TextBox ID="TextBox1" runat="server" Text='<%# Bind("ALERT_THRESHOLD_AMT") %>'></asp:TextBox>
                </EditItemTemplate>
                <ItemTemplate>
                    <asp:Label ID="Label1" runat="server" Text='<%# Bind("ALERT_THRESHOLD_AMT") %>'></asp:Label>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:BoundField DataField="USER_ACCT_ALERT_ID"  Visible="False" />
            <asp:BoundField DataField="ALERT_TYPE_ID" Visible="False"  />
            <asp:BoundField DataField="ALERT_OPER_ID" Visible="False" />
         
        </Columns>
        <HeaderStyle BackColor=Black />
            <PagerSettings Mode="NumericFirstLast" PageButtonCount="5" />
            <PagerStyle CssClass="PagerRowStyle" />
    </asp:GridView>

protected void GridView_Sorting(object sender, GridViewSortEventArgs e)
        {
            GetViewState();


            if (dsAlertCriteria != null)
            {
                DataView dataView = new DataView(dsAlertCriteria);
                dataView.Sort = e.SortExpression + " " + ConvertSortDirectionToSql(e.SortDirection);

                dgd_Alerts.DataSource = dataView;
                dgd_Alerts.DataBind();

            }
        }
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